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River-Sediment Inputs to Major Deltas
Abstract
ABSTRACf: River sediments are distributed unevenly in space and time, and they are markedly susceptible to human influences. Half the world's river sediment is derived from the Himalayan region and its environs. Most of the remainder is derived from other tectonically active regions such as the western Pacific islands, the Andes, and southern Alaska. River-sediment loads are variable at many time scales: seasonal, annual, decadal, and longer. The storage of sediment in river systems confounds our ability to predict the delivery of sediment to coastal zones. Natural river-sediment loads are increased by deforestation and crop farming, and decreased by dams and reservoirs.
... Ces apports induisent de fortes productions biologiques qui font des RiOMar des zones de pêche Introduction 8 importantes (Lesueur 1992, Salen-Picard et al. 2002, Robert 2007, Ferré et al. 2008, Palanques et al. 2014, Mengual et al. 2016. D'un point de vue biogéochimique, on estime que moins de 5 % des sédiments fluviaux apportés aux zones côtières atteignent l'océan profond (Meade 1996). Ce faible pourcentage s'explique par le fait que les compartiments benthiques des RiOMar constituent des zone actives de dépôt de matériel particulaire et de transformations biogéochimiques ultérieures de la matière organique associée . ...
... McKee BA, Aller RC, Allison MA, Bianchi TS, Kineke GC (2004) Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: Benthic boundary layer and seabed processes. Cont Shelf Res 24:899-926.MeadeRH (1996) Chapter 3: River-Sediment Inputs to Major Deltas. In: Sea level-rise and Coastal subsidence. ...
Les grands fleuves influencent fortement certaines régions côtières (i.e., les Riverdominated Ocean Margins, ou RiOMar), qui présentent de forts taux de sédimentation et auxquelles sont associés des communautés benthiques et des processus biogéochimiques dont le fonctionnement varie en fonction de la dynamique temporelle des flux particulaires et de leur interaction avec l'hydrodynamisme. La Vasière Ouest-Gironde (VOG) constitue un modèle pertinent pour ce type de systèmes puisqu’elle constitue la principale zone de dépôt primaire des particules issues de l'estuaire de la Gironde, et qu’elle est située dans un environnement hautement énergétique. Bien que sa dynamique sédimentaire ait fait l’objet de nombreux travaux, l’étude des caractéristiques (dont la matière organique associée) des sédiments superficiels, et de la macrofaune benthique y a été jusqu'à présent négligée. L’objectif de cette thèse consiste à mieux décrire la structuration spatio-temporelle de l’écosystème benthique de la VOG, via l’étude de la matière organique particulaire (MOP) sédimentée ainsi que de la composition de la macrofaune benthique et de son activité. Une comparaison a par ailleurs été effectuée avec le prodelta du Rhône qui a déjà beaucoup été étudié. Bien qu’également situé en zone tempérée, celui-ci diffère en effet de la VOG à la fois par la saisonnalité plus marquée des apports fluviaux ainsi que par la plus faible intensité de l'hydrodynamisme dans la zone réceptrice. Une campagne synoptique (juin 2018, 32 stations) et 4 campagnes saisonnières (5 stations le long d’un gradient côte-large, octobre 2016-avril 2018) ont été réalisées sur la VOG dans des conditions de débits et d’hydrodynamisme contrastées. Une large gamme de paramètres a été mesurée : (1) caractéristiques des sédiments superficiels (granulométrie, surfaces spécifiques, descripteurs quantitatifs et qualitatifs de la MOP), (2) composition de la macrofaune, et (3) traces d’activité biologique (imagerie de profils sédimentaires). Sur la base de l’analyse de la distribution spatiale de ces paramètres, les résultats obtenus confirment la subdivision de la VOG en une zone proximale et une zone distale qui avait déjà été mise en évidence par des études sédimentologiques. Ils montrent l’existence de gradients de profondeur (i.e., entre zones proximale et distale et à l’intérieur de la zone distale) marqués pour la plupart de ces paramètres. L’analyse des corrélations entre ces variations spatiales et celles de plusieurs facteurs de contrôle potentiels suggère le rôle prédominant de l’hydrodynamisme comparé à celui du débit de la Gironde et du chalutage de fond. Mes résultats montrent également l’existence de variations temporelles dont la composante saisonnière est liée à l’efflorescence printanière, et à laquelle se superpose une tendance interannuelle entre 2016 et 2018 pour la composition de la macrofaune benthique. Dans le cas de cette dernière, et pour les 3 stations déjà échantillonnées en 2010, mes résultats montrent enfin l’existence d’importants changements temporels entre 2010 et 2016-2018. Ces changements sont attribués à la succession de tempêtes exceptionnelles intervenues durant l’hiver 2013/2014, qui aurait profondément perturbé l’écosystème benthique de la VOG et initié une séquence de cicatrisation. De manière générale, une différence importante avec le prodelta du Rhône réside dans le rôle majeur joué par l’hydrodynamisme (i.e., par rapport aux apports fluviaux) dans le contrôle de la structuration spatio-temporelle des paramètres étudiés. Cette différence tend à valider la transposition aux zones tempérées de la typologie des RiOMar jusqu’ici établie sur des bases biogéochimiques et principalement à partir d’exemples tropicaux et subtropicaux.
... Shelf sediment sourceto-sink processes indicate great diversity in different sedimentary environments (Li et al., 2014a;Liu et al., 2016), especially in river sediment inputs and ocean dynamic conditions, e.g., high energy, low sediment inputs in west European shelf (Stride, 1982) and on the contrary, low energy, high inputs in the Gulf of Pupua shelf (Harris et al., 1996;Howell et al., 2014). Differently, it is well known that rivers draining the subtropical-tropical regions (most in East and South Asia) supply the largest loads of sediment discharge to the oceans (Meade, 1996;Milliman and Farnsworth, 2011), such as the Yangtze River (0.48 bt y − 1 ), Irrawaddy River (0.26 bt y − 1 ) and Mekong River (0.11 bt y − 1 ) (Fig. 1A). The Taiwan Strait, located on the southern East China Sea shelf (Fig. 1B), provides a significant link between the ECS and the South China Sea (SCS) and accumulates abundant land-derived sediments from mainland China and Taiwan island (Liu et al., 2008a;Xu et al., 2009Xu et al., , 2012 Milliman and Meade, 1983;Meade, 1996;Xu et al., 2009). ...
... Differently, it is well known that rivers draining the subtropical-tropical regions (most in East and South Asia) supply the largest loads of sediment discharge to the oceans (Meade, 1996;Milliman and Farnsworth, 2011), such as the Yangtze River (0.48 bt y − 1 ), Irrawaddy River (0.26 bt y − 1 ) and Mekong River (0.11 bt y − 1 ) (Fig. 1A). The Taiwan Strait, located on the southern East China Sea shelf (Fig. 1B), provides a significant link between the ECS and the South China Sea (SCS) and accumulates abundant land-derived sediments from mainland China and Taiwan island (Liu et al., 2008a;Xu et al., 2009Xu et al., , 2012 Milliman and Meade, 1983;Meade, 1996;Xu et al., 2009). Dominated currents in Taiwan Strait and ECS and East Asian monsoon winds direction are marked. ...
The Taiwan Strait serves as a link between the East China Sea and South China Sea in East Asia. Complex ocean dynamics, huge sediment inputs and distinct tectonic, climatic and bedrock lithological settings of the two sides of the strait make it ideal for sediment source-to-sink studying. While mud sediments in the strait have been well investigated, sand composition and provenance remain understudied. Here, we present framework petrography and heavy mineral data of sands from the southern Taiwan Strait and the adjacent representative rivers to characterize sand provenance and depositional mechanisms. As expected, the SE China river sands are dominated by quartz and feldspar, whereas sands from the westward-flowing mountainous rivers in Taiwan are rich in lithic fragments and heavy minerals of metamorphic origin. The southern Taiwan Strait sands show significant spatial variations in composition and texture of the framework grains and heavy minerals. Framework grain-based provenance modeling results show that sands in the southwest margin of Taiwan Strait (water depth of 30–60 m) are mainly supplied by SE China rivers. Taiwan mountainous rivers made prominent contributions to the central-western Taiwan Strait (40–60 m) and the south of Taiwan Shoal (below 50 m), both of which are far away from the Taiwan island (ca. 100–300 km away). Furthermore, sands from the Taiwan Shoal (20–30 m) show extremely high compositional maturity and are mainly composed of coarse, rounded quartz. These sands, previously proposed as relict sediments, have been intensely altered by modern high-energy hydrodynamic conditions and can also be interpreted as palimpsest sediments. These results demonstrate that modern river-derived sands are eventually deposited in relatively deep-water regions in the strait, rather than the shallow regions (Taiwan Shoal). We propose that the sand composition and distribution are closely related to the submarine topography of the Taiwan Strait. Combining previous mud belt investigations, we suggest that sands and muds tend to have different fates in shallow continental shelves with complex climate, ocean current and seafloor landform conditions. This study also highlights the importance of both modern and relict sands in the strait and our findings are important to better understanding of shelf sedimentary systems with huge river-sediment-input and high wave/tidal-current-energy.
... As the major link between continents and oceans, global rivers play a dominant role in delivering huge amounts of particulate and dissolved materials to the oceans (Milliman and Meade, 1983;Meade, 1996;Meybeck and Vörösmarty, 2005). Approximately 20 gigatonnes (Gt) of terrestrial sediment is delivered annually to the oceans from global rivers, of which > 70% is contributed by Asian rivers (Milliman and Meade, 1983;Milliman and Syvitski, 1992). ...
... Approximately 20 gigatonnes (Gt) of terrestrial sediment is delivered annually to the oceans from global rivers, of which > 70% is contributed by Asian rivers (Milliman and Meade, 1983;Milliman and Syvitski, 1992). As the carriers of massive amounts of nutrients and pollutants, these river-borne sediments are of importance not only to geomorphological changes in coastal areas but also to marine productivity of the continental margins (Martin and Meybeck, 1979;Meade, 1996;Seitzinger et al., 2010;Cohen et al., 2014). Understanding the flux, transport processes, and depositional fate of these terrestrial sediments therefore has particular relevance as we seek to study global biogeochemical cycles, and this has been set as a goal of the International Geosphere Biosphere Programme (Syvitski et al., 2005). ...
Understanding the flux of riverine sediment is crucial not only to document the global terrestrial material budget but also to understand erosion patterns within the drainage basin under the impact of natural and anthropogenic forces. Here we reconstructed changes of sediment discharge in the Yellow River over the last 7000 years based on a large number of ¹⁴C dates obtained from literature and discussed the impact of climate change and human activities from the mid-Holocene to the Anthropocene. From 7000 to about 3000 cal yr BP, the Yellow River's sediment discharge was lower than 0.09 gigatonnes per year (Gt/yr). Due to low population and limited ability to conquer nature, climatic conditions almost completely controlled the sediment discharge over this period. After 3000 cal yr BP, the sediment discharge increased gently to 0.23 Gt/yr at 1400 cal yr BP. Since then, the sediment discharge increased abruptly to 1.56 Gt/yr in 1855 CE and subsequently remained at a fairly high level of 1.31 Gt/yr with fluctuations for nearly a century. Human activities gradually enhanced the increasing trend of sediment discharge and began to affect the environment at a scale comparable with powerful natural forcing due to population growth and technique innovation. Since the late 1950s, the terrestrial sediment from the Yellow River to the sea has experienced a cliff-like drop, which appears to be reverting to the pristine levels of the middle Holocene. With the explosive growth of the population, technological innovation and economic development, human activities in the drainage basin intensified continuously and greatly regulated the water and land resources.
... More than 40 years ago, the delta was still expanding, moving into the sea from 60 -70 meters to nearly 100 meters per year, especially in the Ca Mau Peninsula region. Therefore, the Mekong River is ranked 10th in the world in terms of sediment load (Meade, 1996). More than two decades ago, many studies estimated the amount of alluvium from the Mekong River pouring into the coastal plain at about 145 -160 million tons/year (Liu et al., 2013;Dang et al., 2018;Kondolf et al., 2022), depending on years of high or low river water flows. ...
Coastal erosion is one of the types of geological disasters that is occurring quite commonly and seriously in the Mekong Delta. The problem of landslides and erosions is one of the major concerns for coastal stability. Currently, along 9 coastal provinces, there are over 115 serious erosion and landslide points; each year, a total of 300 –500 hectares of coastal land can be lost. Erosion and landslides narrow the area of mangrove forests, residential land, and aquaculture land of local people. The Mekong Delta has to spend billions of VND yearly to build breakwaters and sea dikes and restore mangrove forests. Many structural and non-structural measures have been implemented to reduce the risk of erosion over the past two decades. However, there is not much assessment of the sustainability of landslide prevention projects. The research question is what are the challenges and difficulties in the ongoing fight against erosion in the coastal plain. Through a practical approach from surveying works in coastal areas, the results show that choosing an effective solution depends mainly on the cost factor and the terrain of the ground. Water resource management policy on the Mekong River system is difficult to find a satisfactory answer.
... Annually, the Mekong discharges approximately 300e550 km 3 of water (Darby et al., 2016;Milliman and Fransworth, 2011) and 40e160 Mt sediment (Binh et al., 2020a;Kondolf et al., 2014a) into the East Vietnam Sea through eight distributaries. With such excessive water and sediment delivered, the Mekong is ranked 8th and 10th, respectively, of the world's largest rivers (Grumbine et al., 2012;Meade, 1996). ...
In the Mekong Basin, rice plants have low yields because of degraded soils, fresh water is becoming scarce, and rice cultivation consumes a lot of water. Beijing-directed rules to govern the river and its plan to build cascade dams upstream have weakened the Mekong River. The Mekong River Commission was established in 1957, but for over 50 years, the Commission still has only Laos, Cambodia, Thailand, and Vietnam as members. To confront all these troubling trends and challenges and the sustainability of the region, riparian countries in the lower Mekong areas have sought partnerships with support from the USA, Japan, Australia, and Korea. Many water development projects in the Mekong Basin have been researched and implemented by bilateral, transboundary, regional, and international collaborations. Although the collaboration is still not holistic as expected, increasing regional and international collaboration projects conducted recently have provided the bright potential for integrated sustainable water resources development and management in the Mekong Basin. A synthetic review of previous water development projects within the Mekong Basin in this chapter introduces an overview of opportunities and challenges and implies lessons for further water development projects in the future among riparian countries in the Mekong Basin and other transboundary river basins in the world.
... The Mekong River has the tenth largest sediment discharge load in the world at 160 Mt./yr before any dams were built (Meade, 1996). Sand, clay and silt are typically transported down the river as bedload or suspended sediment and deposited in lateral and median bars that have built up over rocky islands along the length of the river from Stung Treng to the upper delta (Bravard et al., 2014). ...
... The Amur River is the largest river entering the Okhotsk Sea, with an average annual runoff of ~7-14 × 10 3 m 3 /s. The sediment flux of the Amur River (5.2 × 10 7 ton/a; Meade, 1996) is 2-3 times higher than those of all other Siberian rivers. Therefore, sediment input from the Amur River has a large impact on sedimentation in the Okhotsk Sea. ...
The Okhotsk Sea, located between the Asian continent and the western Pacific, is a natural laboratory for investigating sediments source-to-sink influenced by atmosphere–ocean–land–sea ice interactions. However, despite their paleoenvironmental significance, changes in sediment provenance within the Okhotsk Sea are still debated due to the diversity of sediment sources and transport processes. Here, we investigate Sr and Nd isotope compositions in surface sediment samples from regions across the Okhotsk Sea, as well as Sr and Nd isotope compositions and elemental abundances in the sediments of core LV55-40-1, collected from the southwestern Okhotsk Sea. Our results reveal an increasing trend of εNd values over the past ~110 kyr, with an abrupt increase during the transition into the last deglacial/early Holocene (MIS 1). We also observed minor variations characterized by less radiogenic εNd during the glacial/stadial (G/S) periods (MIS 5d, MIS 5b, MIS 4, MIS 2), and more radiogenic εNd during the interglacial/interstadial (I/I) periods (MIS 5c, MIS 5a, MIS 3). The major terrigenous sediment sources in the northwestern Okhotsk Sea are via the Amur River and from Sakhalin Island (~60–80%), with a minor volcanic contribution (~20–40%) from the Okhotsk-Chukotka volcanic belt and the southern Kuril Islands/Hokkaido. Provenance variations are primarily controlled by the interplay of sea-ice drifting and ocean circulation, reflecting a dominant transport mode shift from sea ice to surface current dominance since the early Holocene. During the G/S periods with severe sea-ice condition, terrigenous materials from the Amur River and Sakhalin Island were transported to the study site, due to the southeastward drift of sea ice driven by the prevailing northwesterly winds and enhanced Okhotsk Sea Intermediate Water circulation. In contrast, during the I/I periods with moderate sea-ice conditions, an increased contribution from the Okhotsk-Chukotka volcanic belt was caused by sea-ice drifting southwestwards under the prevailing northerly and northeasterly winds, supplemented by surface currents. This study provides the first Sr-Nd isotopes record spanning the last glacial cycle in the Okhotsk Sea and it offers new insights into the relationship between sediment provenance and the interplay of sea ice and ocean currents in this region.
... The combined discharge per year at the two outlets averages about 18,320 m 3 /s but is highly variable depending on time of year. In high water months discharge often exceeds 28,000 m 3 /s (and was more than 64,000 m 3 /s in April and May 2011), or about three times the discharge typical for low water months (Meade, 1996). ...
... 12-14 × 10 9 t y − 1 (Mulder and Syvitski, 1995;Mouyen et al., 2018). The 25 largest rivers in the world discharge 40% of those fluvial sediments, and 50% of the freshwater that is drained into the oceans (Meade, 1996;Skliris and Lascaratos, 2004). The dispersal patterns of large river plumes are relatively well known, but information about the dynamics of small stream discharge is limited (Gaston et al., 2006;Romero et al., 2016), and patterns may differ from those of large rivers. ...
... Upstream dams and levees capture sediment, reducing downstream sediment loads by 50-100% on many regulated rivers and resulting in erosion and subsidence of coastal habitats (Tessler et al., 2018;Besset et al., 2019). Over longer timescales (i.e., centuries or millennia), deforestation and cropping also have increased river sediment loads and transport to coastal habitats, but often not enough to offset recent and rapid effects of dams and channelization on coastal erosion and subsidence (Meade, 1996). ...
Introduction
Large dam removals provide a restoration opportunity for shrinking coastal wetland habitats. Dam removal can increase sediment delivery to sediment-starved river deltas and estuaries by restoring natural sediment transport and mobilizing reservoir-impounded sediment. However, rapid mobilization of massive quantities of sediment stored behind large dams also constitutes a major ecological perturbation. Information is lacking on coastal habitat responses to sediment pulses of this magnitude.
Methods
Removal of two large dams along the Elwha River (Washington, USA) in 2011–2014 released ~20.5 Mt of impounded sediment, ~5.4 Mt of which were deposited in the delta and estuary (hereafter, delta). We used time series of aerial imagery, digital elevation models, and vegetation field sampling to examine plant community responses to this sediment pulse across seven years during and after dam removal.
Results
Between 2011 and 2018, the Elwha River delta increased by ~26.8 ha. Vegetation colonized ~16.4 ha of new surfaces, with mixed pioneer vegetation on supratidal beach, river bars, and river mouth bars and emergent marsh vegetation in intertidal aquatic habitats. Colonization occurred on surfaces that were higher and more stable in elevation and farther from the shoreline. Compared to established delta plant communities, vegetation on new surfaces had lower cover of dominant species and functional groups, with very low woody cover, and lower graminoid cover than dunegrass and emergent marsh communities. Over time following surface stabilization, however, vegetation on new surfaces increased in species richness, cover, and similarity to established communities. By 2018, ~1.0 ha of vegetation on new surfaces had developed into dunegrass or willow–alder communities and ~5.9 ha had developed into emergent marsh. At the same time, dam removal had few discernible effects on established delta plant communities.
Discussion
Together, these results suggest that rapid sediment mobilization during large dam removal has potential to expand coastal wetland habitat without negatively affecting established plant communities. However, as sediment loads declined in 2016–2018, new delta surfaces decreased by ~4.5 ha, and ~1.6 ha of new vegetation reverted to no vegetation. Long-term persistence of the expanded coastal habitat will depend on ongoing erosional and depositional processes under the restored natural sediment regime.
... Irrawaddy and Salween river system contributes 640 km 3 /year of fresh water (Milliman and Farnsworth 2011). Irrawaddy contributes 360 million tonnes of sediments annually (Meade 1996;Robinson et al. 2007) and is the chief sedimentary source to the Andaman Sea (Rodolfo 1969a;Ramaswamy et al. 2004;Rao et al. 2005). The catchment and drainage of Irrawaddy consist of the western Indo-Burman Ranges, Myanmar central basin, and eastern syntaxis of the Himalayas. ...
The Andaman Sea is a unique deep basin with limited connectivity with the global ocean. The riverine influx from the major rivers draining from the surrounding Asian landmass combined with a large amount of rainfall during the summer monsoon season strongly affects the Andaman Sea. The biogenic carbonate-rich sediments with ample terrigenous material ensure the potential applicability of a wide range of proxies to reconstruct the past. Thus, the Andaman Sea is an excellent archive to reconstruct the past monsoon and associated processes. The isolated nature of the deeper regions provides a unique opportunity to understand deep water changes, without the usual influence of the Antarctica and North Atlantic Ocean. The presence of both the aerial and sub-marine volcanoes in the Andaman Sea makes it an interesting region to understand the temporal changes in the climate-tectonics interaction. Here, we synthesize the paleomonsoon and associated studies from the Andaman Sea. A majority of the paleoclimatic studies from this region are restricted to the last glacial-interglacial transition, with limited attention being paid to the long-term changes. The provenance of fresh water and sediments reaching the Andaman Sea varied during glacial-interglacial times. A few coral and speleothem-based studies provide high-resolution records of the past monsoon from the Andaman region. The limited drilling during the last expedition to the Andaman Sea (Site U1447 and U1448) was restricted to a relatively shallow depth. The region requires both high-resolution and long-term paleoclimatic studies based on several more drill holes covering a range of depths and latitudinal zones.
... In this part, we elaborate the location of study area, and details about the data, which were used in this study. The MRB is located in the mainland Southeast Asia and is ranked the 10 th largest river in the world in terms of mean water discharge [32] It covers an area of 795,000 km 2 , and having 4,800 km in length [33]. There are two main sub-basins, namely the Upper MRB or Lancang River (i.e., the name of the Mekong in China), and the lower MRB (including Myanmar, Laos, Thailand, Cambodia, and Vietnam). ...
In recent years, the Mekong River Basin (MRB), one of the largest river basins in Southeast Asia, has experienced severe impacts from extreme droughts and floods. Streamflow forecasting has become crucial for effective risk management strategies in the region. However, this task presents significant challenges due to rapid climate changes and the presence of numerous newly constructed upstream dams, which disrupt the natural flow. In this paper, we develop multiple deep learning models (incl. Multi-Layer Perceptron (MLP), Convolutional Neural Network (CNN), Long short‐term Memory (LSTM), and Transformer) to predict streamflow with different lead time forecasts based on observed meteorological variables and climatic indices (i.e., discharge, water level, precipitation, and temperature) from 1979 to 2019. The results indicate that LSTM obtains high performance for streamflow prediction in both dry and wet seasons while Transformer is not recommended for long-term prediction, especially in the dry season. The proposed deep learning models capture well the fluctuation of river flow in the MRB during the period of high-dam development, especially LSTM (NSE ≥ 0.8). The models’ performances are enhanced with the adding of temperature for short-term prediction while precipitation was the most sensitive variable for long-term one. Such proposed models are essential for government agencies to plan mitigation and adaptation strategies at different periods, which can range from days to years.
... Even though rivers act as conduits transporting freshwater, particulate and dissolved OM to the oceans (McKee et al., 2004), only about 5 % of sediment delivered by rivers reaches the deep sea (Meade, 1996). Thus, the discovery of hyperpycnal flow deposits and intrabasinal turbidites with interbedded plant fragments in the deep sea provides a good opportunity to study sediment transport mechanisms along with terrestrial OM deposition and preservation in deep-sea settings, helping us understand more about terrigenous carbon burial in deep marine environments. ...
Extrabasinal turbidity currents, also known as hyperpycnal flows, can deposit vast amounts of organic carbon to the marine realm. On the other hand, intrabasinal turbidity currents are commonly thought to resuspend and further oxidize organic matter (OM), creating deposits with poor preservation of terrestrial OM. However, recent studies have shown that intrabasinal turbidity currents can still preserve terrigenous OM after reworking previously deposited OM-rich sediments. As such, both types of turbidity currents may act as unconventional source rocks. Considering that there is a discrepancy between global carbon burial fluxes in the marine environment and the amount of organic carbon delivered by rivers, OM preservation within such sandy deposits should not be overlooked. We investigated both plant fragment-rich extrabasinal and intrabasinal turbidites of the Miocene Kawabata Formation (Hokkaido, Japan) by using biomarker distributions within different facies formed by changes in sediment concentration due to progressive deposition during waning flow. Clear distinctions in terms of sedimentological features and organic geochemical characteristics between different facies were found and are interpreted to be caused by different OM sources and sedimentological processes. The sandy extrabasinal turbidites have high amounts of organic carbon, high pristane/phytane ratios, and fewer amounts of marine algal biomarkers due to direct transport from the terrestrial environment without mixing with marine waters. Intrabasinal, Bouma-type turbidites generally have poor OM, and low pristane/phytane values but high concentrations of algal biomarkers since sediments are sourced from the marine environment. In both extrabasinal and intrabasinal turbidites of this study, the plant fragment-rich sandy layers deposited by flows with lower sediment concentrations preserve high amounts of terrigenous OM, possibly due to having densities allowing woody plant fragments to remain within the flow and corresponding deposit, while basal Ta sandstones and Te hemipelagic deposits contain more marine OM due to erosion plus incorporation of marine sediments and mixing with marine waters respectively. Mud clasts formed in basal flows such as surge-like intrabasinal turbidity currents may also enhance OM preservation in plant fragment-poor sandstones.
... The morphological characteristics of the Ganga-Padma channel form a complex interaction of channel dynamics and sediment characteristics (Md and Islam, 2016). According to Meade (1996, as cited by Bhuiyan et al., 2017) about 600 million tonnes of sediments are deposited annually in the Bengal delta. Over-siltation on the river bed decreases the channel depth and thus, in the rainy season or at the time of over-discharge, the river's capacity to transport the excess discharge and sediments is impeded; this causes the river to become unstable and to adjust its channel configuration depending upon the varying nature of morphometric variables. ...
Th e present paper is a micro-level study aimed at analyzing the impacts of riverbank erosion on riparian society. To fulfi ll the objective, the study has been carried out from two distinctive perspectives; viz. general outlooks and specifi c inquiry. For the general outlook, the Ganga-Padma river in its lower reach downstream of Farakka Barrage in Samserganj Community Development Block has been explored in terms of channel shift ing using multi-dated satellite images. For specifi c inquiry, the eff ects of riverbank erosion on society have been studied using primary data in two selected active bank-erosion-prone cadastral units. Th e results show that recurrent bank erosion has brought a drastic change in the social fabric and caused huge losses rendering riparian inhabitants quite vulnerable, since most of the people in the study units are dependent on land-based economy and belong to poor economic groups. Th ese situations necessitate signifi cant outside assistance. However, there is a signifi cant diff erence (χ2 = 10.85 at p<0.01) in terms of assistance received during the previous year between the two selected study units.
... Bed sediments in estuaries can be affected by four factors: (1) upstream river sediment supply, (2) sediment resuspension in estuaries, (3) estuarine circulation, and (4) tidal asymmetry (Dyer, 1997;Burchard and Baumert, 1998;Zhu et al., 2021). First, fluvial sediment supply directly determines the balance of estuarine sediments, which changes regional sediment erosion and deposition patterns (Milliman and Meade, 1983;Meade, 1996;Poulenard et al., 2009;Rickson, 2014). Heavy rainfalls and extreme flood events will substantially increase the river sediment supply, which alters the estuarine sediment transport (Croke et al., 2013;Koiter et al., 2013;Bracken et al., 2015). ...
River discharge is a controlling factor in estuarine morphological changes; estuarine topography can be significantly altered even by only one flooding event. However, the mechanism of morphological changes in mountain-stream macro-tidal estuaries during wet seasons is not fully understood. Taking the Yalu River Estuary (YRE), China, as an example, this study aims to explore the effects of extreme flooding events on estuarine morphology. An improved Finite Volume Coastal Ocean Model (FVCOM) was applied in the YRE to reproduce the distribution of bed sediment erosion and deposition during dry years, normal years, and wet years. Sensitivity tests were conducted to assess the responses of the estuarine system to river discharge. The influence of the Yalu River on the magnitude of estuarine bed change was examined. With an increase in river discharge, the bed thickness in the main estuarine channel first increased and then decreased after reaching a threshold of 0.4 × 10⁴m³/s. Simultaneously, density stratification became stronger with the increase in runoff. Subsequently, vertical mixing of water was weakened after a certain threshold, which in turn, enhanced the density stratification and changed the location of the Estuarine Turbidity Maxima (ETM). In addition, river effects contributed to nearly half of the bed erosion under 0.6 × 10⁴m³/s (flooding with 50-year return period). The ETM extended and moved approximately 8 km seaward during flooding events. The West River channel experienced strong siltation, which was more significant during flood events. The results of this study demonstrate that estuarine circulation plays a key role in morphology change. The magnitude of bed thickness erosion can reach 22% of the total initial thickness during one flooding event. This rapid change is a threat to the stability of port infrastructures, and actions should be taken to maintain the safety of coastal construction.
... Once in the coastal seas, this fluvial contribution can influence sedimentation in the inner shelf due its interaction with coastland shelf dynamics, and can be trapped in deltas, deposited on the adjacent nearshores, distributed in the continental shelf, or delivered to the deep sea over the shelf break or via submarine canyons (Meade, 1996). Only the four world's largest rivers in terms of suspended sediment load (Amazon, Ganges-Brahmaputra, Yangtze and Yellow before damming) contribute together with about 4 billion tons of suspended sediment into the oceans, accounting for 20% of the total global discharge (Milliman and Syvitski, 1992). ...
Global coastal zones are constantly changing due to the actions of various physical forces. Recent studies show that the supply of suspended sediment of continental origin plays an important role in these changes. Once in the coastal region, this sediment significantly influences the sedimentation process on the internal platform. Knowing this, understanding the transport and destination of these sediments is crucial to interpreting the morphodynamic evolution of the seabed and biogeochemical processes in the ocean. The Southwest Atlantic Shelf is the largest continental shelf in the Southern Hemisphere and one of the most important in biological production, because of the great continental contribution exercised by both the Río de la Plata and the Patos Lagoon. Studies in the region show that these effluents are significantly affected by the El Niño–Southern Oscillation (ENSO) effect, which can significantly interfere with the availability of suspended material inserted into the coastal region. Despite the great efforts of several authors to understand these changes and their interactions with the environment, some questions remain unanswered. Thus, this work aims to fill this gap by answering questions related to the contribution of sediment of continental origin and its behaviour on the Southwest Atlantic Inner Shelf. The behaviour of suspended sediment was investigated in two distinct periods using the hydromorphodynamic model TELEMAC-3D: one representing normal years, that is, without the effect of ENSO (2005–2006) and the other experiencing the effect of ENSO (2008–2009).The model was calibrated and validated using field data for both studied periods and for other periods. The results were based on statistical analyses, such as wavelet and empirical orthogonal function (EOF) analyses, in addition to time series. The results show that the Southwest Atlantic Inner Shelf is mainly influenced by the fluvial discharge of the Río de la Plata and the Patos Lagoon, and by the local wind. The Río de la Plata being the largest exporter of suspended sediment in the region, with an approximate rate of 1.2 × 10⁸ tons/year in neutral (normal) years and 3.0 × 10⁸ tons/year in years under the influence of ENSO. The Patos Lagoon, on the other hand, exports approximately 1.25 × 10⁷ tons/year in the period without the ENSO effect and 1.35 × 10⁷ tons/year in the period influenced by ENSO. The results also show that the fluvial discharge interacts with the suspended sediment in seasonal to interannual scales, while the wind contributes to the concentration of suspended sediment on synoptic scales. Still on the local wind regime, the results show that the local wind regime gains importance particularly when the suspended sediment reaches the coastal region.
... The SPM from river to estuary and then to continental shelves, heavy metals constantly migrate and transform in water, SPM and sediment, which is a complex dynamic process (Bhosale and Sahu, 1991;Kassim et al., 1997;Beltrame et al., 2009;Nouri et al., 2011;Feng et al., 2017;Zeng et al., 2020). In addition, 90 % of the land-based solids are transported to the sea in the form of SPM (Zhang et al., 2012), most of which are deposited in estuary and adjacent shelves (Meade, 1996). Consequently, the study of heavy metals in SPM is of great significance for understanding the transport of heavy metals in water environment and estimating the buried fluxes of heavy metals in estuaries and continental shelves. ...
The suspended particulate matter (SPM) is an important carrier of heavy metals transportation from land to sea, so it is significant to study the heavy metal pollution in SPM. The distribution and assessment of five heavy metals (Mn, Cr, Ni, Cu, and Pb) in SPM collected from Passur River and its estuary in Sundarban were studied in combination with water temperature, salinity, and turbidity. The results show that the heavy metal content and distribution in SPM are mainly controlled by runoff input, hydrodynamic process and the interaction process of salt and fresh water in estuaries. The quality evaluation results of heavy metals in SPM show that pollution degree is light. Studies on the heavy metals in SPM are of great significance to comprehensively evaluate regional pollution status and carry out early warning.
... The Mekong is among the largest rivers worldwide, the eighth largest river in terms of its discharge, and the tenth most important for its sediment load (Li et al., 2017;Meade, 1996). The average annual discharge of the Mekong River is approximately 12,500 m 3 /s, although it experiences strong seasonal variations, with a ratio of maximum to minimum discharge in a year of around six to eight. ...
The delta of the Mekong River is one of the largest in the world, with the Mekong River carrying a large amount of sediments in its Region of Freshwater Influence (ROFI). This study investigates the flow structure and movement of both suspended and bedload sediments in the ROFI of the Lower Mekong Delta (LMD) in order to identify areas prone to sediment accretion and erosion. This is accomplished by applying the three-dimensional Coastal and Regional Ocean COmmunity (CROCO) model and then calculating the sediment budget of different stretches of the coastline. The model outputs, depicting areas experiencing sediment accretion and erosion along the coastline of the LMD, are then compared against observations obtained during the period 1990–2015 and demonstrate the ability of the model to identify areas particularly prone to erosion and where preventive actions against coastal erosion should focus.
... Riverine discharge accounts for the majority of sediment input to the ocean, with the largest 25 rivers accounting for 40% of this sediment (see Milliman and Meade, 1983;Meade, 1996;McKee et al., 2004). Once delivered to oceanic environments, a number of transport processes operate on, and transform sediment and associated nutrients such as organic carbon, before their eventual deposition and burial (McKee et al., 2004). ...
The Ayeyarwady and Thanlwin Rivers, which drain Myanmar, together form one of the largest point sources of freshwater and sediment to the global ocean. Combined, these rivers annually deliver an estimated 485 Mt of sediment to the northern Andaman Sea. This sediment contributes to a perennially muddy zone within the macro-tidal Gulf of Martaban, but little is known about the processes that dominate dispersal and trapping of sediment there, as very few water column observations are available. A research cruise in December 2017 provided a rare opportunity to obtain Acoustic Doppler Current Profiler (ADCP) data along transects from the Gulf of Martaban and adjacent continental shelf. Two transects were obtained from the outer portion of the Gulf of Martaban in water depths that ranged from about 20–35 m. These showed very fast currents, especially during flood tide conditions, exceeding 1.5 m/s. The backscatter record from the ADCP indicated asymmetries in distribution of suspended sediment during the ebb versus flood phase of the tide. During ebb tidal conditions, the backscatter record indicated that sediment was transported in either a surface advected layer, or fairly well-mixed throughout the water column. In contrast, during flood tidal conditions, sediment was confined to the bottom boundary layer, even though the velocities were faster during flood than the ebb conditions. The vertical structure of the currents during flood tide conditions indicated the presence of sediment–induced stratification because currents within the near-bed turbid layers were relatively slow, but speeds increased markedly above these layers. This albeit limited dataset provides an exciting glimpse into the dynamics of sediment transport within the muddy, macrotidal Gulf of Martaban, and implies the importance of tidal straining and bottom nepheloid layer formation there.
... Several studies have shown that coupling of sealevel oscillations, coastline evolution, base-level adjustments, vertical ground motions, natural riversediment loads, basin runoff and erosional processes, coastal sediment budget, and Holocene climate dynamics increases the complexity of geomorphological development of delta systems (Bird, 2008;Meade, 1996;Rempe and Dietrich, 2014;Tejedor et al., 2017). Moreover, Blum and Tornqvist (2000) highlight the variations in the base level caused by eustatic fluctuations that may result in downcutting, aggradations, and lateral displacement of fluvial channels, which incise coastal geomorphic systems. ...
The Paraiba do Sul River delta, a South America east coast system, presents deposits recording complex sedimentary dynamics in the deltaic plain during the Little Ice Age (LIA). The use of optically stimulated luminescence (OSL) dating, sediment grain-size characterization, and morpho-chronological analysis showed that from ∼450 to 100years (LIA period), both in lower to upper deltaic plain, remodeling occurred in conjunction with the coastline regression. The freshwater and sediment influx increases due to the moisture enhancements during the LIA resulted in the intensification of the fluvial dynamic, channel downcutting on the marine terrace, and sedimentary environments remodeling. Further, the base-level changes occasioned by the coastline regression strengthen the geomorphological dynamics associated with the recent changes in the Paraiba do Sul delta. Currently, the coastal erosion problem found in the area seems to reflect the coupling between climate dynamic (rainfall rates decrease) and multiple-scale anthropic intervention. Results show a high sensitivity of humid tropical deltaic systems to short-time climate events, further contribute to broadening discussions on the complexity of this delta system’s morphosedimentary evolution during the Late Holocene.
... Draining a basin of 1.8 × 10 6 km 2 , the Yangtze River is the largest in southeastern Asia, and one of the world's largest rivers in terms of both water (5th; 900 km 3 yr −1 ) and sediment discharges (4th; 480 million tonnes per year, mt yr −1 ) (Milliman and Meade 1983;Milliman and Syvitski 1992;Meade 1996;Milliman and Farnsworth 2011). With headwaters in the Qinghai-Tibet Plateau (5400 m elevation, Saito et al. 2001), the Yangtze flows through steep valleys in its upper reaches, meanders across low-gradient alluvial plains in its middle and lower reaches, and merges with numerous northern and southern tributaries before debouching into the East China Sea (Figure 31.1). ...
Temporal variations in water and sediment discharges in the Yangtze basin have been controlled by various forcings, both climatic (precipitation and evapotranspiration) and anthropogenic (water diversions, dam construction, and lake reclamation). Superimposed on the spatial variations, various temporal changes of both water and sediment discharge have occurred in the Yangtze drainage basin since 1950. As a result of the severe decline in sediment flux relative to water discharge, suspended sediment concentrations (SSCs) in the middle and lower reaches of the Yangtze River have decreased dramatically. While low‐runoff rivers seem more vulnerable to climatic and anthropogenic change, high‐runoff rivers also can be affected, as shown by the Yangtze in this chapter. The collective erosion of Dongting Lake, the mainstem channel and the Yangtze subaqueous delta have changed the Yangtze sediment dispersal system dramatically.
... As global sea-level rise slowed around 8 kya, the accumulation of large volumes of fluvial sediment on river-dominated ocean margins began to outpace the creation of accommodation, initiating the formation of large river deltas (Gibbs, 1981;Stanley and Warne, 1994;Meade, 1996;Saito, 2021). These deltaic deposits are repositories for terrestrial and shallow marine sediment, and serve as archives of continentalmargin biogeochemical signatures due to their high preservation potential. ...
Large river deltas serve as globally important archives of terrestrial and shallow marine biogeochemical signatures and because of rapid sedimentation have the potential to impact global biogeochemical cycling. The Ayeyarwady Delta in Myanmar ranks as the world's third largest river delta in terms of sediment supply; however, modern increases in regional anthropogenic impacts risk severe alteration to sediment and TerrOC loads within this major system. By investigating modern sediment and terrestrial organic carbon (TerrOC) accumulation within the offshore Ayeyarwady Delta this study estimates baseline sediment and TerrOC budgets for this understudied mega-delta. Using ²¹⁰Pb geochronology of 27 sediment cores collected from the continental shelf, we estimate that 405 ⁺⁵² Mt of sediment, or ~70–80% of fluvial sediment discharged from the Ayeyarwady and Thanlwin rivers (the main inputs to the delta), accumulates there annually. Sediment not retained on the shelf is likely partitioned between the Ayeyarwady floodplain, shoreline accretion, and minor deep-sea export. Estimates of TerrOC (based on δ¹³C mixing models) were coupled with modern sediment accumulation rates to determine an annual burial of 1.93 +1.09 Mt C on the shelf, with TerrOC burial fluxes being highest in the foreset beds of the subaqueous delta, coincident with the area of highest sediment accumulation rate. Based on estimates of the Ayeyarwady and Thanlwin rivers' TerrOC delivery, an apparent ~100% of TerrOC input is preserved on the continental shelf. However, an across shelf trend of increasing TerrOC degradation with distance offshore is also observed, indicating that while the shelf has high apparent TerrOC sequestration, carbon remineralization is also occurring prior to deposition within the subaqueous delta. Based on these conflicting outcomes, we suggest that input of TerrOC from additional sources other than the Ayeyarwady and Thanlwin rivers roughly balance the observed carbon remineralization. Main additional sources of TerrOC include the Sittang and several smaller rivers, and the Ayeyarwady delta plain below the river gauging station. As anthropogenic development within the Ayeyarwady and Thanlwin watersheds continues to increase, these sediment and TerrOC budgets provide a baseline from which future changes within the offshore Ayeyarwady Delta can be monitored.
... It eventually discharges into the South China Sea. The Lancang-Mekong River, with a total length of about 4,909 km and an average annual flow of 14,500 m 3 /s (Wang et al. 2017), is the seventh longest and the eighth largest river in the world with reference to mean annual discharge (Meade 1996). It is also the third longest river in Asia. ...
With the operation of six cascade reservoirs, the flow regime and sediment discharge of the Lancang River have changed greatly. The changes of runoff and suspended load have attracted extensive attention. The hydrological data of Gajiu and Yunjinghong stations in Lancang River from 1964 to 2019 were analyzed by using wavelet analysis, double mass curve and abrupt change analysis. The temporal trends in runoff and suspended load were evaluated. Results revealed that the reduction of suspended load was much more profound than the change of runoff. There was a slight downward trend in annual runoff due to climate change. After the completion of Xiaowan and Nuozhadu reservoirs, the proportion of runoff in flood season decreased by 22.64 and 30.75%, respectively. Wavelet analysis was used to reveal the characteristics of runoff evolution. With the operation of reservoirs, suspended load showed abrupt changes in 1993 and 2008. The amount of suspended load during 2009–2019 decreased by 95.47–98.78% compared with that before the reservoir construction. This paper presents the latest quantitative study on the temporal variation of runoff and suspended load since the completion of Xiaowan and Nuozhadu reservoirs, which is of great importance for guiding the operation of reservoirs and maximizing the value of the whole Lancang-Mekong River basin. HIGHLIGHTS
Variations of runoff and suspended load during 1964–2019 in Lancang River were first quantified in detail after Xiaowan and Nuozhadu reservoirs were completed.;
The total amount of runoff was dominated by climate change, while reservoir construction mainly influenced the annual process of runoff.;
Suspended load exhibited abrupt changes in 1993 and 2008 and the reduction reached 95–99% after the reservoir construction.;
... This estimate is significantly larger than the sediment load quoted by other authors, for example: 520 Mt/year (Milliman & Syvitski, 1992), 586 Mt/year (Narayana & Babu, 1983), 800 Mt/year (Subramanian, 1993), 729 Mt/year (Wasson, 2003), and 524 Mt/year (Latrubesse et al., 2005). Such large variability in the reported T A B L E 1 Compilation of hydrological and sediment transport characteristics of major rivers (in terms of sediment load > 100 Mt/year) draining through the east, southern, southeast (ESSE) Asian region from various sources Subramanian (1978); d Milliman & Meade (1983); e Narayana & Babu (1983); f Abbas & Subramanian (1984); g Milliman & Syvitski (1992); h Dai et al. (2009); i Subramanian (1993); j Latrubesse et al. (2005); k Rice (2010); l Khan et al. (2018); m Islam et al. (1999); n Tandon & Sinha (2007); o Meade (1996); p Inam et al. (2007); q Dang et al. (2010 The Ganga River rises along the southwest side of the Himalayan mountains and is subjected to intense erosion owing to the high relief and tropical climate regime (Holeman, 1968;Tandon et al., 2008;Swarnkar, Tripathi, & Sinha, 2021). The Ganga and its tributaries are also among the world's worst affected rivers in terms of flood hazard (Agarwal & Narain, 1991), which has been attributed to the high monsoonal discharge and sediment load of these rivers (Sinha, 1998;Sinha & Jain, 1998;Sinha et al., 2005;Singh et al., 2007;Sinha, Sripriyanka, et al., 2014). ...
The Ganga River is one of the largest river systems in the world that has built extensive alluvial plains in northern India. The stretch of the Lower Ganga River is vulnerable to siltation because of: (a) naturally low slope in the alluvial stretch; (b) confluence of several highly sediment-charged rivers such as the Ghaghra, Gandak, and Kosi, and (c) reduction in non-monsoon flows because of upstream abstractions of both surface and groundwater. Additionally, the Farakka barrage has impacted the morphology of the Ganga River significantly both upstream and downstream of the barrage. Large-scale siltation in several reaches has reduced the channel capacity leading to catastrophic floods in this region even at low discharges. This work has utilized historical remote sensing data and UAV surveys to reconstruct channel morphodynamics and to compute sediment volumes accumulated in the channel belt along the Lower Ganga River between Buxar and Farakka. The work was carried out by dividing the total length of the river into four continuous stretches: (a) Buxar-Gandhighat (GW1; 160 km), (b) Gandhighat-Hathidah (GW2; 106 km), (c) Hathidah-Azmabad (GW3; 182 km), and (d) Azmabad-Farakka (GW4; 132 km). We document that major ‘hotspots’ of siltation have developed in several reaches of the Lower Ganga during the last 4-5 decades. Sediment budgeting using planform maps provide estimates of ‘extractable’ volumes of sediments in GW1, GW2, GW3, and GW4 as 656±48, 706±52, 876±71 and 200±85 Mm3 respectively. These estimates are considerably lower than those computed from the hydrological approach using observed suspended sediment load data which assumes uniform sedimentation between two stations. Further, our approach provides reach-scale hotspots of aggradation and estimates of extractable sediment volumes, and this can be very useful for river managers to develop strategic sediment management plan for the given stretch of the Ganga River.
... Mississippi River, the seventh largest river in the world in terms of discharge and suspended sediment load (Milliman and Meade, 1983), drains 47% of the conterminous U.S. and carries 66% of the sediment load (~210 × 10 6 tons/yr) from the continental U.S. to the Gulf of Mexico (Meade, 1996;Allison et al., 2012). However, anthropogenic influence on the Mississippi River, such as damming and land use change, has reduced the sediment load to about 50% over the past century (Corbett et al., 2004;Blum and Roberts, 2009). ...
The Mississippi River Delta consists of six major passes and numerous small crevasses through which water and sediments are transported to the continental shelf of northern Gulf of Mexico. The water and sediment fluxes through individual passes are not routinely monitored due to the complex morphology and challenging environment. However, they are critical factors in determining the highly nonlinear near-field characteristics of freshwater plumes, fronts, possible internal hydraulic jumps, nutrient dispersal and rapid initial deposition of fine sand and mud. The flux partitioning and flow/sediment dispersal characteristics near the modern Mississippi River Delta were investigated using a high-resolution (~ 100 m horizontally) unstructured-grid, three-dimensional coupled hydrodynamic-wave-sediment numerical model. Model result analysis for the period of April–June 2010 revealed that Southwest Pass is the main conduit among the six major passes, through which 64% and 32% of the Mississippi River water and sediment, respectively, are transported to the coastal ocean. In contrast, South Pass has the lowest water and sediment fluxes among the tri-furcation channels downstream of the Head of Passes. Due to the more energetic flow at Southwest Pass compared to other passes, an elongated freshwater/sediment jet is typically present in the near-field. The average width of the plume originating from Southwest Pass, 5 km away from the mouth, is approximately 8 km, while the high sediment concentration within the plume (SSC > 10⁻² kg/m³) is maintained in the upper 4 m of the water column for more than 8 km. The pattern of the plume in the far-field varies with wind direction, and the buoyant plume bends towards the coast/mid-shelf in response to downwelling/upwelling favorable winds.
... The Mississippi River and its tributaries drain approximately 41% of the conterminous United States, with a drainage area of $3.22 million km 2 (Meade, 1996;Milliman & Meade, 1983;Mossa, 1996). Besides its role as the largest river in North America, the Mississippi River has been a backbone of commerce and industry. ...
Riverbed changes are important to applications in engineering in the environment, such as navigation, salt-water intrusion and habitat quality. This study employed decadal hydrographic data over the past 50 years (1963 to 2013), as well as daily water stages and discharges to outline the long-term channel morphological changes and bed-load sediments deposited by floods in the lowermost Mississippi River (LMR). To generate accurate bathymetric digital elevation models (DEMs), we interpolated the hydrographic survey in a channel-centered coordinate system by employing Ordinary Kriging with Anisotropy (OKA). Riverbed deformations were estimated by subtracting the elevations between DEMs at two different times to generate a series of DoDs (DEM of Difference). The results from DoDs exhibit temporal riverbed change patterns and also reveal net degradation after the completion of the Old River Control Structure in 1963, except for the period 1992–2004. Spatially, over 32% of the total area experienced continuous erosion, with only 0.4% having continuous aggradation. Historical erosion over the past 50 years could be removing the alluvial veneer in sections of the river, and exposing more non-alluvial sediments, particularly between RK (river kilometer) 230 and 40. Our analysis also shows riverbed deformation patterns are mainly controlled by flood events, backwater effects, as well as drawdown effect near BCS (Bonnet Carré spillway). In this study, we also examine whether the LMR traps bed-load sediment under flood conditions. The probability distribution estimated by Monte Carlo simulation suggests only low amounts of bed-load sediments are deposited on the channel bed during floods. Spatially, the short-term bed-load erosion/deposition pattern roughly matches the decadal-scale riverbed deformation estimated from DoDs. Moreover, this study finds that the trends between the thalweg changes and riverbed deformation are similar, suggesting that both of these patterns are controlled by same factors.
... The Mississippi River drains ~ 40% of the conterminous United States and carries approximately 65% of all suspended solids and dissolved solutes that enter the ocean from it (Milliman and Meade 1983;Meade 1996;Milliman and Farnsworth 2011). Deltaic Mississippi River estuaries historically received Mississippi River water but are now largely devoid of its influence because of closure of active distributaries and artificial flood control levees constructed along the Mississippi River over the last 100 years (Day et al. 1997;Turner 1997). ...
Estuarine and coastal geomorphology, biogeochemistry, water quality, and coastal food webs in river-dominated shelves of the Gulf of Mexico (GoM) are modulated by transport processes associated with river inputs, winds, waves, tides, and deepocean/continental shelf interactions. For instance, transport processes control the fate of river-borne sediments, which in turn affect coastal land loss. Similarly, transport of freshwater, nutrients, and carbon control the dynamics of eutrophication, hypoxia, harmful algal blooms, and coastal acidification. Further, freshwater inflow transports pesticides, herbicides, heavy metals, and oil into receiving estuaries and coastal systems. Lastly, transport processes along the continuum from the rivers and estuaries to coastal and shelf areas and adjacent open ocean (abbreviated herein as “river-estuary-shelf-ocean”) regulate the movements of organisms, including the spatial distributions of individuals and the exchange of genetic information between distinct subpopulations. The Gulf of Mexico Research Initiative (GoMRI) provided unprecedented opportunities to study transport processes along the river-estuary-shelf-ocean continuum in the GoM. The understanding of transport at multiple spatial and temporal scales in this topographically and dynamically complex marginal sea was improved, allowing for more accurate forecasting of the fate of oil and other constituents. For this review, we focus on five specific transport themes: (i) wetland, estuary, and shelf exchanges; (ii) river-estuary coupling; (iii) nearshore and inlet processes; (iv) open ocean transport processes; and (v) river-induced fronts and cross-basin transport. We then discuss the relevancy of GoMRI
findings on the transport processes for ecological connectivity and oil transport and fate. We also examine the implications of new findings for informing the response to future oil spills, and the management of coastal resources and ecosystems. Lastly, we summarize the research gaps identified in the many studies and offer recommendations for continuing the momentum of the research provided by the GoMRI effort. A number of uncertainties were identified that occurred in multiple settings. These include the quantification of sediment, carbon, dissolved gasses and nutrient fluxes during storms, consistent specification of the various external forcings used in analyses, methods for smooth integration of multiscale advection mechanisms across different flow regimes, dynamic coupling of the atmosphere with sub-mesoscale and mesoscale phenomena, and methods for simulating finer-scale dynamics over long time periods. Addressing these uncertainties would allow the scientific community to be better prepared to predict the fate of hydrocarbons and their impacts to the coastal ocean, rivers, and marshes
in the event of another spill in the GoM.
... The Mekong is one of the world's great rivers, draining a basin of 795,000 km 2 from the Tibet Plateau to the South China Sea, ranking 8th globally in annual discharge, and 12th in overall length (Meade 1996;Gupta and Liew 2007 (2015)(2016)2017) and damaging floods (2011,2013,2018). For the period 1995-2015, CRED and UNISDR (2015) estimate that an annual average of 8.4% of the Cambodian population was directly affected by weather-related hazards, placing the country among the top 10 in the world in terms of the proportion of population affected by such hazards. ...
Integrating local knowledge and scientific information can aid in co-developing locally relevant approaches for climate change adaptation and disaster risk reduction. Communities along the Mekong River have adapted to variability in temperature, rainfall, and flooding patterns over time. Rapid environmental change in the Mekong Basin presents a new set of challenges related to drought, altered seasonal rainfall, more frequent high-flow flood events, and water withdrawals for hydropower and irrigation. We present a multi-method approach to understand how local knowledge of the spatial and temporal patterns of floods, droughts, and rainfall can be integrated with scientific information along a flood-prone section of the lower Mekong River in Kratie Province, Cambodia. Participatory hazard mapping of community members’ knowledge of the movement of floodwaters through the landscape enabled interpretation of flood extent mapping using Synthetic Aperture Radar images from the Sentinel-1A satellite. Seasonal calendars of weather patterns and livelihood activities, together with local indicators of flooding, rainfall, and drought were compared with trends in 35 years of rainfall data, and highlighted “pressure points” at the beginning and end of the rainy season where agriculture may be particularly impacted by climate change. We discuss potential applications of our findings for adaptation and hazard planning.
... Continental margins are the interface between land and the open ocean, where 50 to 80% of continental Particulate Organic Carbon (POC) inputs are mineralized [1][2][3]. Less than 5% of these inputs are transferred to the deep ocean [4], the remaining part being buried in continental margin sediments. River-dominated ocean margins (RiOMars), defined as margins impacted by major rivers freshwater and/or sediment discharges (e.g., plumes of Amazon, Yangtze or Mississippi Rivers), are the main marine primary depositional areas of riverine particulate continental inputs [2,5]. ...
The spatial distributions of (1) surface sediment characteristics (D0.5, Sediment Surface Area (SSA), Particulate Organic Carbon (POC), Chlorophyll-a (Chl-a), Phaeophytin-a (Phaeo-a), Total and Enzymatically Hydrolyzable Amino Acids (THAA, EHAA), δ13C) and (2) sediment profile image (apparent Redox Potential Discontinuity (aRPD), numbers and depths of biological traces) characteristics were quantified based on the sampling of 32 stations located within the West Gironde Mud Patch (Bay of Biscay, NE Atlantic) in view of (1) assessing the spatial structuration of a temperate river-dominated ocean margin located in a high-energy area, (2) disentangling the impacts of hydrodynamics and bottom trawling on this structuration, and (3) comparing the West Gironde Mud Patch with the Rhône River Prodelta (located in a low-energy area). Results support the subdivision of the West Gironde Mud Patch in a proximal and a distal part and show (1) the existence of depth gradients in surface sedimentary organics characteristics and bioturbation within the distal part; (2) no evidence for a significant effect of bottom trawling, as opposed to Bottom Shear Stress, on the West Gironde Mud Patch spatial structuration; and (3) major discrepancies between spatial structuration in the West Gironde Mud Patch and the Rhône River Prodelta, which were attributed to differences in tidal regimes, sedimentation processes, and local hydrodynamics, which is in agreement with current river-dominated ocean margin typologies.
... According to Meade (1996), less than 5 % of the river sediment delivered to the global coastal ocean reach the deep sea; the vast majority becomes trapped in estuaries, floodplains and on the continental shelf. The exact apportionment of the proximally trapped sediment is not known, as most sampling surveys and monitoring stations of rivers are located considerably far upstream of the rivers' mouths. ...
Mud depocenters (MDCs) represent major proximal-marine sinks for fine-grained terrigenous material, carbon, and contaminants on modern continental shelves. Throughout the past decades, several studies have shed light on the physical processes controlling MDC development at various timescales, ranging from controlled flume experiments and in-situ oceanographic monitoring, to stratigraphic analyses of recent and ancient deposits based on seismo-acoustic and sediment-core data. Thereby, key mechanisms related to the formation and maintenance dynamics of MDCs have been discovered: a) cross-shore bottom transport of suspended mud through gravity flows, b) interaction of mud with density gradients associated with oceanic fronts, c) resuspension and dispersal control of mud by internal waves, d) bedload deposition of mud forming laminated bedding under energetic flow conditions, and e) mud resuspension resulting from chronic bottom trawling.
Among the physical processes identified or proposed, three conceptual paradigms for MDC development can be distinguished: 1. continuous supply, associated with a steady sediment supply and hemipelagic settling in relatively calm conditions; 2. continual resuspension-deposition cycles, wherein parts of an MDC area are subject to multiple cycles of resuspension, redeposition and reworking before ultimate burial; and 3. episodic sedimentation and erosion, in which extreme events such as riverine floods and atmospheric storms dominate the total, long-term sediment flux. Although the predominance of each of these paradigms within a single MDC depends to a large degree on the timescales considered, case studies tend to emphasize processes associated with only one of these three paradigms. As a result, the relative, long-term contribution of individual processes remains largely uncertain for many MDCs.
The ability of numerical models to accurately predict medium to long-term mud accumulation is restricted not only by computational costs, but also by insufficient parametrizations of the muddy sedimentation process. These remain challenging to constrain due to the multiplicity and complexity of factors affecting the cohesive properties of mud, including its state of consolidation, and the amount and type of organic matter present. Bridging the gap between individual events and long-term accumulation is the key to a more complete understanding of sedimentation processes in MDCs.
... Large rivers play a dominant role worldwide in transferring both particulate and dissolved materials from the land to the sea (Liu et al., 2007). Most river-derived sediments are trapped in estuaries, leading to the formation of a delta (Meade, 1996;Liu et al., 2007). Through the rapid accumulation of deposits, estuaries record environmental changes, such as continental-scale trends in climate and land-use in watersheds, frequency and magnitude of cyclonic storms, and sea-level change (Bianchi and Allison, 2009). ...
This study investigates Holocene sedimentary evolution and hypoxia development using borehole cores CJK06 and CJK09, in combination with other published core data. Based on lithology and microfossil (benthic foraminifera) characteristics, seven types of sedimentary facies were identified from the base upward: river, tidal flat, tide-influenced river, transgressive lag, estuary, inner-shelf, and prodelta. Isochronous correlation among the cores was established relying on accelerator mass spectrometry ¹⁴C dates. Facies succession indicates that a weak progradation occurred in coastal environments (12–10 ka BP) due to the Younger Dryas-induced deceleration of sea-level rise; rapid deposition locally occurred in the southern marginal area of tide-dominated estuary environments (10–8 ka BP), likely due to the junction of the Yangtze and Qiantangjiang River currents; and marine current-induced fluid mudflows prevailed in the shallow marine environments (8–1 ka BP), with the cooling climates of 5–1 ka BP. Furthermore, prodelta and shallow marine environments co-occurred with an uneven progradation of the delta within the last 1 ka, while deposition occurred just inside the more southern, delta-influenced area. The occurrence of euryhaline benthic foraminifera suggests that an enhanced freshwater discharge of the mid-Holocene (8–5 ka BP) was followed by a sharp decline in the late Holocene (5–1 ka BP) with climate change. The occurrence of cold-water benthic foraminifera indicates a severe cold-water condition during the mid-Holocene due to the intrusion of upwelling currents triggered by the propelling force of warm currents and summer monsoon winds. In addition, the occurrence of low-oxygen foraminiferal assemblages reveals that hypoxia has developed since 10 ka BP in stages consistent with the sedimentary evolution: localized hypoxia formation coincident with the southern depocenter during the early Holocene (10–8 ka BP); severe hypoxia with enhanced freshwater discharge and upwelling current intrusion during the mid-Holocene; and weakened hypoxia of the late Holocene, mainly due to the sharp decline in freshwater discharge. Within 1 ka BP, freshwater discharge from the approaching river mouth and increased nutrient inputs from enhanced human activities on land have contributed to the prevalence of hypoxia, especially in the southern deltaic area. Overall, it was revealed that the freshwater discharge modulated with climate variations and delta progradation plays a primary role in controlling the sedimentary evolution and hypoxia development during the mid-late Holocene.
... The annual mean of sediment yield of Mekong River with other major rivers in Asia and continents. Note: The value from[23,58,[73][74][75][76]. ...
The Mekong River Basin (MRB) in Southeast Asia is among the world's ten largest rivers, both in terms of its discharge and sediment load. The spatial and temporal resolution to accurately determine the sediment load/yield from tributaries and sub-basin that enters the Mekong mainstream still lacks from the large-scale model. In this study, the SWAT model was applied to the MRB to assess long-term basin hydrology and to quantify the sediment load and spatial sediment yield in the MRB. The model was calibrated and validated (1985-2016) at a monthly time step. The overall proportions of streamflow in the Mekong River were 34% from surface runoff, 21% from lateral flow, 45% from groundwater contribution. The average annual sediments yield presented 1,295 t/km 2 /year in the upper part of the basin, 218 t/km 2 /year in the middle, 78 t/km 2 /year in the intensive agricultural area and 138 t/km 2 /year in the highland area in the lower part. The annual average sediment yield for the Mekong River was 310 t/km²/year from upper 80% of the total MRB before entering the delta. The derived sediment yield and a spatial soil erosion map can explicitly illustrate the identification and prioritization of the critical soil erosion-prone areas of the MR sub-basins.
... Numerous studies have documented the fate of these river-derived sediments after delivery to the ocean (Sternberg, 1986;Nittrouer and DeMaster, 1996;Liu et al., 2007). A large portion of the river derived sediments are either trapped in estuaries (Gao and Collins, 2014), or deposited on adjacent continental shelves (Liu et al., 2007;Gao and Collins, 2014) or outer shelf areas (Gao et al., 2015), while a relatively small amount reach the deep sea (Meade, 1996). Furthermore, small rivers are also important for transferring terrestrial sediments and nutrients to global oceans (Milliman and Syvitski, 1992), especially during extreme rainfall and flood events (Liu et al., 2013). ...
Tropical mountain river systems that drain rainforests and experience tropical monsoons typically demonstrate high runoff and discharge a disproportionate amount of particulate load to the world''s oceans compared with other more temperate river systems. As a typical tropical small mountainous river with an broad continental shelf system and wave dominated environment, the Kelantan River is one of the few north-flowing streams in Southeast Asia that discharges into the broad and shallow continental shelf of South China Sea. The existence of seasonal variations of salinity, turbidity, and sediment composition indicate that coarse-grained sediments are deposited rapidly when terrestrial sediments delivered by the Kelantan River leave the river mouth. They are then reworked by wave action during the rainy season, leading to the development and rapid migration of barrier spits near the west side of the river mouth. Under the influence of the seasonal variations in the river plume, fine-grained sediments discharged into the sea by the Kelantan River are transported both along-shore and cross-shore, and most of them are deposited in the continental shelf area within 10 km of the river mouth, because of rapid removal of sediments from the river plume. The spatial distribution patterns of sediment composition, C/N ratio, terrestrial organic carbon content, and Sr/Al ratio also support our conclusion.
... The discharges of large tropical rivers have a huge influence on adjacent coastal waters, mainly due to the transfer of sediments from the continent, changes in nutrient dynamics and the input of organic matter. All these mechanisms together can change the physical-chemical properties of coastal marine environments and consequently affect their biological communities (Nixon et al., 1986;Meade, 1996;Dagg et al., 2004;Wu et al., 2017). ...
The western tropical Atlantic is strongly influenced by the Amazon, receiving the full discharge from the largest river basin of the world. In order to ascertain the coastal-oceanic gradient in abundance and composition of planktonic decapod communities along the Amazon River Plume (ARP) and its retroflection, 33 plankton samples were obtained along three sampling transects: Coastal and oceanic area influenced by the ARP (Coastal IARP and Ocean IARP, respectively), and oceanic waters without ARP influence (Ocean). A total of 13,117 specimens, belonging to 33 taxa, were analyzed. The distribution of planktonic decapod communities was strongly influenced by the Amazon discharge, showing the following patterns: (1) The coastal-oceanic gradients in decapod abundance are similar to other shelf areas of eastern South America, with higher abundance in the area under ARP influence, (2) meroplanktonic decapods dominate in coastal and some oceanic areas due to the offshore transport of coastal organisms along the ARP retroflection, and (3) the ARP is clearly responsible for the observed differences in community structure between the three transects. Furthermore, this study highlights the importance of coastal and shelf environments as main sources of planktonic decapods for pelagic ecosystems in the tropical Atlantic during the period of strong North Brazil Current retroflection.
... The Texas-Louisiana continental shelf is located in the northern Gulf of Mexico (Fig. 1). Every year the Mississippi and Atchafalaya River system delivers about 5.3 � 10 2 km 3 of fresh water, 2.1 � 10 8 tons of sediment, and 0.95 � 10 6 tons of nitrogen to this region (Milliman and Meade, 1983;Meade, 1996;Bianchi et al., 2010). The large flux of nutrients leads to eutrophication within the Mississippi/Atchafalaya plume (Lohrenz et al., 1990;Turner and Rabalais, 1994). ...
Stratification has been proved to be essential to the development and sustainment of bottom hypoxia off the Texas-Louisiana shelf. Observations indicate that bottom hypoxia does not always arise under strong stratification, especially in spring/winter season when the strong stratification is transient. A simple oxygen model is coupled with a high-resolution hydrodynamic model to further investigate linkage between stratification and oxygen dynamics. We put forward a definition of stratification when vertical stratification maxima exceeds some critical value. This duration time is longest in summer and shortest in winter, and is identified regulating the horizontal and vertical extensions of bottom hypoxia. A long duration of strong stratification expands over the shelf, defining a stratification envelope within which bottom hypoxia occurs with high probability. Persistent strong stratification also acts to reserve local oxygen depletion, facilitating the growth of the hypoxic layer thickness. The main pycnocline is identified to be the upper limit of the top reach of bottom hypoxia.
... The major gravity-driven systems tend to be linked to major sedimentary systems in particular river deltas (e.g., Niger, Mississippi, Orange rivers), and the timing of major episodes of movement are probably linked to major episodes of sediment flux (e.g. Meade, 1996;Simoes et al., 2010;Grimaud et al., 2018). Rates and magnitude of sedimentation are primarily functions of climate change (e.g. ...
... Main sediment production areas are loosely based on Diaconu 1971. Today, less than 5-10% of all the fluvial sediments reach the deep sea (Meade, 1996), but different conditions during interglacials when deltas and river mouths were closer to the shelf edge resulted in higher sediment transport to the deep basin (Hampson and Storms, 2003). ...
This thesis started from the observation that river mouths play a central role in the development of the Danube Delta and from an intuition that river mouths, as suppliers of sediment and water to coastal areas are quintesential in understanding river-influenced coasts. Contribution of rivers and deltas to biodiversity and human society is immeasurable, as deltas contain some of the world’s largest cities, and natural wetlands with important biodiversity hotspots. River mouths are one of the Earth’s greatest transitional environments where riverine flows coexist with wave, tidal and wind driven currents, and fresh waters mix with salty marine waters. The present thesis is based on field data which comprises bathymetric surveys, measured river water and solid discharges, wave and wind data, ADCP data, topographic profiles, satellite imagery. Also, a big part of the thesis is dedicated to using the Mike 21/3 by DHI (Danish Hydraulic Institute), a coupled hydrodynamic and wave model, which is used to characterize water circulation, waves and the bed shear stresses at the complex river-mouth systems. Although starting with examples from the Danube Delta, the thesis expands with global examples so that the findings here can have a wider relevance.
Floods and storms are common phenomena at river mouths and can have a considerable impact on river-mouth sedimentation and morphological change. Results show a linear response in bathymetric change in relation to the relative influence of floods and storms and that change can be predicted by a Flood/Storm index based on river water or sediment discharge and wave height proxies (R2=0.84). Field data are integrated with the Mike 21/3 model results to derive a conceptual hydro-morphodynamic model of an asymmetric wave influenced river-mouth bar during the two antagonistic phases of flood-driven and storm-driven dynamics. Furthermore, the analysis is extended using exploratory modelling to a full range of conditions on 5 idealized bathymetries which resulted in ~650 different hydro-sedimentary snapshot conditions as a function of discharge (0 to 8000 m3/s), wave height (0.75 to 6 m) and direction(0 to 90*). Jet deflection is predicted by a balance of river jet discharge and longshore current discharge, such that the jet is deflected (undeflected) when the longshore current is much higher (much lower) than the river jet, and a ~45* deflection is predicted when the discharge of jet equals the discharge of the longshore current. The effects of the mouth bar on the hydrodynamics is examined, as well as highlighting the differences between symmetric, asymmetric and mouth bars with different volumes. Finally, predictive relationships are proposed for emergent morpho-hydrodynamic interactions such as the “dynamic diversion”.
Next, the event of an exceptionally large natural breaching of a sandy spit (Sacalin spit) is used to have an insight into the sediment transport processes downdrift of the Sf. Gheorghe river mouth. The large breach quickly reached 3.4 km in May 2014, is attributed to morphological preconditioning of the narrow (50–150 m) barrier, which was susceptible to breaching even during moderate storm conditions. The event switched the barrier’s decadal evolution from low cross-shore transport to high cross-shore transport over the barrier, which is an order of magnitude larger than during the non-breach period. Finally, the thesis sketches a preliminary scheme for the sediment transport at the wave-influenced Sf. Gheorghe lobe, taking into account the known sources and sinks, based on bathymetric measurements and numerical modelling results; and outlines the actions needed to envisage a full sediment transport scheme which is needed for a successful management of the deltaic coast.
... It is estimated that 12 Tg OC yr −1 were buried in the hadal trenches based on DSDP and ODP cores surrounding the subduction zones (Clift, 2017), and over 1 Tg OC could be transported into the hadal depth by single megathrust earthquake event based on the study for Japan Trench (Kioka et al., 2019). Considering that many trenches such as Japan Trench, Philippine Trench and Atacama Trench, like the NBT, are also close to continent landmasses, and can receive significant terrigenous input, we propose that the amount of fluvially-derived OC terr ultimately reaching the hadal zone can't be ignored in estimating global OC terr burial in the ocean (Meade, 1996). Over geological timescales, the role of hadal trenches in burying OC terr and removing atmospheric CO 2 may be important in glacial time due to lower sea level and closer distance to land. ...
The fate of terrigenous organic carbon (OCterr) in the ocean remains an enigma for four decades. Hadal trenches, the deepest ocean realm (6 – 11 km deep), were recently proposed to be OC depocenters, but whether and how much OCterr was sequestrated there remain elusive. Here we conducted comprehensive analyses for four sediment cores from the New Britain Trench (NBT) close to Papua New Guinea to assess the source, translocation and burial of OC. The bulk and molecular radiocarbon data suggest that the NBT landward slope and axis sediments mainly receive young and biogenic rather than petrogenic OC. The three-endmember mixing model based on Δ14C, δ13C and OC contents reveals that sediments of the NBT axis (8,225 m) comprise relatively high OC contents (0.66 ± 0.08%), of which biogenic OCterr accounts for 62 ± 10%. The high proportion of biogenic OCterr was attributed to the selective translocation of OCterr-enriched coarse particles and rapid delivery of sediments supported by unique V-shape feature of the trench. In contrast, the sediment OC at the oceanward slope is primarily of a marine origin, suggesting that OCterr was efficiently trapped in the trench bottom. It is estimated that the burial rate is 2.75±0.32 g C m-2 yr-1 for OC and 1.69±0.41 g C m-2 yr-1 for OCterr in the NBT. Given a fact that many trenches are close to the landmasses, we propose that the hadal trenches may contribute significantly to the burial of OCterr in the ocean.
A global network of subsea telecommunications cables underpins our daily digital lives, enabling >95% of global digital data transfer, $trillions/day in financial trading, and providing critical communications links, particularly to remote, low-income countries. Despite their importance, subsea cables and their landing stations are vulnerable to damage by natural hazards, including storm surges, waves, cyclones, earthquakes, floods, volcanic eruptions, submarine landslides and ice scour. However, the likelihood or recurrence interval of these types of events will likely change under future projected climate change scenarios, compounded by sea-level rise, potentially increasing hazard severity, creating previously unanticipated hazards, or hazards may shift to new locations during the 20–30-year operational life of cable systems. To date, no study has assessed the wide-reaching impacts of future climate change on subsea cables and landing stations on a global scale. Here, for the first time we synthesize the current evidence base, based on published peer-reviewed datasets, to fill this crucial knowledge gap, specifically to assess how and where future climate change is likely to impact subsea cables and their shore-based infrastructure. We find that ocean conditions are highly likely to change on a global basis as a result of climate change, but the feedbacks and links between climate change, natural processes and human activities are often complicated, resulting in a high degree of geographic variability. We identify climate change ‘hotspots’ (regions and locations likely to experience the greatest impacts) but find that not all areas will be affected in the same manner, nor synchronously by the same processes. We conclude that cable routes should carefully consider locally-variable drivers of hazard frequency and magnitude. Consideration should be given both to instantaneous events (e.g. landslides, tropical cyclones) as well as longer-term, sustained impacts (e.g. seabed currents that circulate even in deep water). Multiple factors can combine to increase the risk posed to subsea cables, hence a holistic approach is essential to assess the compounded effects of both natural processes and human activities in the future.
This volume provides a state-of-the-art summary of biogeochemical dynamics at major river-coastal interfaces for advanced students and researchers. River systems play an important role (via the carbon cycle) in the natural self-regulation of Earth's surface conditions by serving as a major sink for anthropogenic CO2. Approximately 90 percent of global carbon burial occurs in ocean margins, with the majority of this thought to be buried in large delta-front estuaries (LDEs). This book provides information on how humans have altered carbon cycling, sediment dynamics, CO2 budgets, wetland dynamics, and nutrients and trace element cycling at the land-margin interface. Many of the globally important LDEs are discussed across a range of latitudes, elevation and climate in the drainage basin, coastal oceanographic setting, and nature and degree of human alteration. It is this breadth of examination that provides the reader with a comprehensive understanding of the overarching controls on major river biogeochemistry.
Large rivers are massive conveyance systems for moving detrital sediment and dissolved matter across transcontinental distances. In the post‐Pleistocene Orinoco and Amazon River basins, the original sources have been the eastern slopes of the northern half of the Andean mountain ranges. The extensive alluvial plains of the Andean foreland basin and the large tracts of floodplain along the Amazon mainstem and some of its major tributaries are the depositories in which much of the Andean‐derived sediment is stored – for periods of centuries, millennia, or longer – during its eastward journey. This chapter recounts the major components of the story in downstream sequence, from west to east across the northern half of South America. The mountainous terranes of the Andes are the principal sources of the fluvial sediment carried by the Orinoco and Amazon Rivers.
The Mekong is an unusual river due to its morphology and behaviour. A study of the Mekong River and its basin should explore its source‐to‐the sea physical characteristics, examine the different expectation of a number of stakeholders, and discuss the potential of adapting suitable management techniques which, as described by the MRC, are politically correct, socially just, and environmentally sound. The Mekong is called Dza Chu (River of Rocks) in Tibet and Lancang Jiang (Turbulent River) in Yunnan, both names aptly describing the physical condition of the river. Sediment stored in the channel and the valleyflat of the Mekong tends to move downstream during high flows. The basin of the Mekong is primarily rural, and until recently, the river used to be little utilized for navigation, irrigation, or power generation. Mekong River, morphology, physical characteristics, management techniques, physical condition, power generation.
This chapter discusses the factors that have led to an increasing interest in the human impact in geomorphology, and then discusses the literature that appeared between c 1960 and 2000. These developments were in four main areas: (i) intellectual and policy-related (ii) technological developments that alter geomorphological processes (iii) demographic trends, and (iv) proliferation of techniques for the study of landform and process change. Much work was undertaken on landforms produced by construction and excavation. Interest also developed in accelerating ground subsidence, which is a widespread phenomenon that creates engineering problems. Indeed, with increasing exploitation of tundra areas for such activities as oil exploitation, there was an increasing interest in the problems associated with permafrost. Rivers have also been greatly impacted. Humans have modified sediment transport by rivers in two ways. First, as a result of accelerated soil erosion, the delivery of sediment to rivers has increased. Secondly, burgeoning dam construction has caused sediment to be trapped in reservoirs. Far-reaching changes in channel form have been produced by land-use and land-cover changes. In addition to non-deliberate changes to river systems, there have been a whole range of deliberate modifications (e.g. channelization). Some valley bottoms areas have suffered from accelerated sedimentation while others have become incised with gullies ( arroyos ). Studies have indicated an increasing incidence of mass movements. These have been attributed to such factors as deforestation, road cuts, changes in slope drainage and irrigation of farm land. Much work has also been undertaken on wind erosion of dryland surfaces. Human activities, most notably air pollution, have changed the nature and rate of weathering, though enhanced weathering by salt can also be accelerated by irrigation. Large numbers of people live in coastal zones and have had a major impact on coastal landforms and processes. Many of the world's shorelines have been eroding and the complex mix of causes, natural and anthropogenic, that could be responsible have been analysed. Finally, since the 1980s there has been a growing realisation of the importance of global heating for geomorphological phenomena.
Whether the Indian Summer Monsoon (ISM) Maximum started in the early Holocene or middle Holocene remains a controversial issue. Grain-size measurements were conducted on 157-cm-long core ADM-C1 from the Andaman Sea; the core spanned the last 11.2 kyr BP. Four end-members (EMs) were unmixed using lognormal parametric end-member modeling. Genetic analyses of the grain-size EMs suggested the EM1 and EM2 may correlate with suspension in the upper layers and transportation in the benthic nepheloid layer, respectively, while the EM3 and EM4 were tied with deposition affected by monsoonal currents and sedimentation under extreme events, respectively. The EM3 was dominated by ISM intensity and transported by summer monsoon currents, with increase in the proportion of EM3 reflecting increasing ISM intensity, and vice versa. Variations in EM3 were used to define three stages of ISM evolution, as follows: (1) During 11.2–9.1 kyr BP, EM3 was relatively low, indicating a weak ISM; (2) during 9.1–4.5 kyr BP, EM3 increased substantially, indicating a strengthened ISM;
(3) during 4.5-0 kyr BP, EM3 decreased gradually, suggesting a gradual weakening of the ISM. The Holocene ISM Maximum started at ~9.1 kyr BP rather than in the early Holocene, which may have been partly due to the slowdown of the Atlantic meridional overturning circulation (AMOC) during the early Holocene that resulted in a decreased land-sea thermal contrast between the landmass and the Indian Ocean. An additional cause may have been the remnant Northern Hemisphere ice sheets that impeded the northward shift of the Intertropical Convergence Zone (ITCZ). Seven relatively brief decreases in the ISM intensity (events 7, 6, 5, 4, 3, 2, 1) occurred at ca. 10.3, 9.7, 7.3, 5.7, 4.2, 2.4, and 0.8 kyr BP, respectively. They corresponded, within the age uncertainties, to the increased supplies of ice-rafted detritus to North Atlantic sediments, implying that changes in the ISM intensity on millennial scale were dominated by climatic processes in northern high latitudes.
In the last few decades, riverine sediment flux to the coastal zones has been decreasing globally. The sediment flux from rivers to seas is usually estimated based on the lowermost gauging stations free of tides in the rivers, for example the Yangtze River. Knowledge of decadal sediment budget and the morphological evolution in the tidal reach is still limited. Here, using historical bathymetry of the tidal reach of the Yangtze, the changes in the sediment budget and local morphology in this reach for three periods between 1970 and 2008 were investigated. In terms of input sediment flux, Period I 1970–1992 is a relatively steady period, Period II 1992–2003 is a decline period with a significant decrease in the sediment flux, and Period III 2003–2008 is a decline period after the closure of the Three Gorges Dam (TGD). The reach was nearly in equilibrium during Period I. Net erosion occurred in the reach during both the pre- and post-TGD decline periods 1992–2003 and 2003–2008, with annual erosion of approximately 65 and 37 Mt/yr, respectively. The contribution of the annual erosion in the reach to the annual sediment flux at the downstream end of the reach increased from around 16.9% during 1992–2003 to 19.1% during 2003–2008. These changes in the sediment budget are closely related to the decrease in the input sediment flux, coarsening of the sediment grain size after the closure of the TGD, and changes in the riverine water flux and tidal conditions. Characterized by a length scale of 4–32 km, local deposition/erosion patterns were quite complex, which relates to the complexity of the local geometry, bathymetry and flow condition. The horizontal location of the thalweg of the reach had limited changes during 1992–2008, likely owing to the fixed riverbank due to human intervention.
Five major factors affect erosion and transportation processes by water: climate, relief, soil, vegetation and man's activities. In order to discern climatic differences in erosion, soil loss models and erosion maps are analysed. Global and regional maps of sediment yield are reproduced and the construction methods and the reliability of the maps are considered. Fournier's and Strakhov's maps are compared with regional maps in order to evaluate the reliability of the global maps.
The main conclusions of this study are that rainfall erosivity is great in Af, Am, Aw and Ca climates and lower in other climates. Low sediment yields are found in Db, Dc, and Dd cclimates. High values occur in Cs, Ca and, in many cases, in Da climates. Graphs of relations between precipitation and sediment yield indicate high sediment yield in climates which experience seasonal rainfall and in BS climates. The map analysis indicates both high and low sediment yields in BS climates. Graph and map analysis indicates increased erosion in mountains.
Changes in sediment loads in the Atlantic drainage can be related to construction of reservoirs and changes in land use. Sediment loads have decreased immediately downstream from reservoirs but the persistence of large loads at points farther downstream indicates that the river channels themselves are now being degraded. The decline of crop farming and the improve-ment of soil-conservation practices has also decreased the sediment yields, but this is reflected mainly in the sediment loads of the tributary streams; as yet, there has been no correspondingly marked decrease in the sediment loads in most of the main-stem rivers. Les différences des charges sédimentaires des fleuves dans l'écoulement vers l'Atlantique en les Etats-Unis, depuis 1900 Résumé. Les différences des charges sédimentaires dans l'écoulement vers l'Atlantique peuvent être liées à la construction des réservoirs et aux changements dans l'utilisation de la terre. Les charges sédimentaires ont précisément diminué en aval des réservoirs mais la persistance de grandes charges plus en aval encore indique que les lits eux-mêmes sont en cours de dégradation. La diminution des surfaces cultivées et les progrès dans la conservation du sol se traduisent égale-ment par une décroissance des charges sédimentaires, mais ce changement apparaît surtout dans les rivières tributaires. Jusqu'à présent, il n'existait pas une diminution similaire dans les charges sédimentaires de la plupart des fleuves.
During the first 40 years of the twentieth century, erosion was the dominant geomorphic process affecting the morphology of the Little Colorado River channel. The discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation. In the 1940s and early 1950s, average annual discharge to about 57% of that of the preceding period as well as reducing the frequency of large floods. The channel adjusted to the new hydrologic regimen by reducing its width. Events of 1980 suggest that the flood plain has ceased to accrete, although climate has not fluctuated. The flood plain has probably reached a critical height above the channel, beyond which further accretion is unlikely under the existing discharge regimen. The recent history of the Little Colorado broadly suggests that flood-plain development was initiated by climatically induced hydrologic fluctuations. Floor-plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions. Refs.
Analysis of data from 280 rivers discharging to the ocean indicates that sediment loads/yields are a log-linear function of basin area and maximum elevation of the river basin. Other factors controlling sediment discharge (e.g., climate, runoff) appear to have secondary importance. A notable exception is the influence of human activity, climate, and geology on the rivers draining southern Asia and Oceania. Sediment fluxes from small mountainous rivers, many of which discharge directly onto active margins (e.g., western South and North America and most high-standing oceanic islands), have been greatly underestimated in previous global sediment budgets, perhaps by as much as a factor of three. In contrast, sediment fluxes to the ocean from large rivers (nearly all of which discharge onto passive margins or marginal seas) have been overestimated, as some of the sediment load is subaerially sequestered in subsiding deltas. Before the proliferation of dam construction in the latter half of this century, riv
During the Holocene sediment input from the Yellow River to the Yellow Sea and adjacent areas has amounted to about 3000 km3. Detailed records from river gauging stations over the past 70 yrs, together with estimates based on earlier historic records, suggest that the river has contributed 2300 km3 of sediment during the last 2300 yrs, an average discharge of 1 km3/yr (1 x 109 t/yr). Prior to extensive agricultural use of the loess plateau in northern China (beginning about 200 BC), therefore, the river's load must have been about 0.1 km3/yr, an order of magnitude lower than that at present. A broad delta 20 m below sea level suggests that early Holocene rates were higher than the pre-agricultural average and therefore that middle Holocene loads were low.
It has been claimed, on the basis of relatively shortterm gage observations and the perceptions of riverbank dwellers, that height and duration of Amazon floods are increasing. The paper discusses these assertions and some of the processes that might lead to the predicated trends. Two such processes are: an increase in peak discharge and a decrease in channel cross-section. Both could be triggered or reinforced by human-induced changes in the environment. By altering the hydrologic relations of plant, soil and water, deforestation in the headwaters can enhance runoff. It can also increase the sediment load, which, if the carrying capacity of the river is exceeded, may be deposited in and aggrade the channel. That deforestation in the Amazon drainage, now affecting small affluents, will eventually bear upon the regime of major tributaries and of the main stem itself can hardly be disputed. However, given the vastness of the basin, and the proportionately small and concentrated areas impacted by settlement, reports that such effects are already perceptible thousands of kilometers from the cleared areas require careful scrutiny.
A detailed statistical analysis, combining gage readings and adequate precipitation data, may, of course, reveal the existence, in one or more sections of the river, of long-term changes in flow that cannot be explained in terms of rainfall variability. Even then, one should not exclude the possibility that such trends might be elucidated without invoking human intervention. They might, indeed, be caused by stream adjustment to neotectonic influences.
In relation to the testimony of riverine populations, major inundations are likely to polarize concerns and dim the memory of moderate floods.
Suspended sediment was collected during 1982-84 from the Rio Solimoes-Amazonas (Amazon River) and its major tributaries on eight downriver sampling cruises between Rio Ica and the town of Obidos, Brazil, and more frequently at a fixed time-series station near the city of Manaus. Data are presented which may serve as a baseline for the assessment of the effects of future changes.-from Author
The Soil Conserevation Service, US Department of Agriculture, has completed the second phase of a national erosion inventory designed to provide data reliable to the state level that illuminate the relative importance of various erosion processs and origins. Phase I dealt with sheet and rill erosion on cropland, range, pasture, and forest lands and was completed in 1977. Phase II inventoried erosion rates on streambanks, gullies, roads, and construction areas. Data collection was completed in October 1979. The data are from some 72,500 randomly selected primary sample units on non-federal land in all states except Alaska and including Puerto Rico. (A)
Evolution of the estuary reflects the comprehensive interaction of two dynamic factors, the river and the sea. Its capricious characteristic is caused by the huge sediment load of the Yellow River, the shallow depth and the weak tidal currents in the gulf area.
Discusses preliminary findings in a study of water and sediment discharge in the Rio Orinoco, Venezuela. The river bed is mainly sand median diameter near 0.4 mm and the typical bedform large dunes (75-100 m long, 2-3 m high). Presents data on mean monthly discharges (water) for 1970-1976, discharge characteristics sand plots sediment discharge vs. water discharge. Discusses the latter relationship in more detail, noting greater sediment discharge, at the rising stage. Considers deposition in the delta area, and deposition/storage pattern, between triburary confluences. Depth integrated samples show cross channel variations in suspended sediment concentrations. Inputs of tributary sediment may remain laterally unmixed for about 200 km downstream. (C.J.U.)
This book deals with the rapid erosion of deep stream gullies (arroyos) which have become a feature of many valleys of America's South-western states since the middle of the 19th century. The nature and causes of entrenchment have been a matter of interest and practical concern to research workers in the environmental sciences and some fields of terrain management for many years, but the abundant records of environmental change in California and Arizona relevant to the study of historical arroyos have never before been systematically collated and analysed. Those concerned with environmental change or with an interest in fragile, semi-arid environments.
It has been claimed, on the basis of relatively short-term gage observations and the perceptions of riverbank dwellers, that height and duration of Amazon floods are increasing. The paper discusses these assertions and some of the processes that might lead to the predicated trends. Two such processes are: an increase in peak discharge and a decrease in channel cross-section. Both could be triggered or reinforced by human-induced changes in the environment. By altering the hydrologic relations of plant, soil and water, deforestation in the headwaters can enhance runoff. It can also increase the sediment load, which, if the carrying capacity of the river is exceeded, may be deposited in and aggrade the channel. That deforestation in the Amazon drainage, now affecting small affluents, will eventually bear upon the regime of major tributaries and of the main stem itself can hardly be disputed. However, given the vastness of the basin, and the proportionately small and concentrated areas impacted by settlement, reports that such effects are already perceptible thousands of kilometers from the cleared areas require careful scrutiny. A detailed statistical analysis, combining gage readings and adequate precipitation data, may, of course, reveal the existence, in one or more sections of the river, of long-term changes in flow that cannot be explained in terms of rainfall variability. Even then, one should not exclude the possibility that such trends might be elucidated without invoking human intervention. They might, indeed, be caused by stream adjustment to neotectonic influences. In relation to the testimony of riverine populations, major inundations are likely to polarize concerns and dim the memory of moderate floods.
Almost 50% of human-induced sediment has historically gone into flood plain storage and less than 7% has left the basin. However, some of the stored sediment is now becoming mobile, and the present sediment yield per unit area may actually be increasing downstream with the augmentation coming from storage loss. In general, sediment yield from a basin is limited by the conveyance capacity of the streams and floodplains. -from Author
During the last 1800 years there have been eight major periods of erosion and alluvial sedimentation in New Zealand. These and their probable times of occurrence are: Taupo (1764 yr BP), Post-Taupo (1600-1500 yr BP), Pre-Kaharoa (1300-900 yr BP), Waihirere (680-600 yr BP), Matawhero (450-330 yr BP), Wakarara (180-150 yr BP), Tamaki (1870-1900 AD) and Waipawa (1950 to present). The Taupo period, which is identified only in North Island, possibly resulted from heavy rainfalls induced by the Taupo Pumice eruption. The other seven periods, which probably occurred universally in both main islands of New Zealand, were almost certainly caused by increased northerly airflow and atmospheric warming over New Zealand, and the associated increased magnitude of major rainstorms and floods, producing increased rates of erosion and channel sediment transport. -from Author
Prior to cultural development in the Mississippi River Basin, the main stem was a heavy sediment carrier due to the character of the climate and soils in the basin. The placement of flood control structures and other channel improvement features and the implementation of improved land management practices have significantly changed the suspended-sediment flow regime of the main stem. The estimated annual sediment yield of the basin is 900 million (817 million metric) tons. That portion of the yield passing into the Gulf of Mexico, as monitored at suspended-sediment sample collection stations near the mouths of the Mississippi and Atchafalaya rivers, totaled 434 million (394 million metric) tons prior to 1963; however, this value has now declined to 225 million (204 million metric) tons. Long-term trends in the bed material gradation of the Lower Mississippi downstream from the mouth of the Arkansas River have remained relatively constant; however, from Donaldsonville, Louisiana, downstream to Head of Passes, there has been a pronounced shift from the sand to silt fraction.
Owing to different physiographic conditions in its watershed, southern tributaries of the Changjiang flood in April to June, whereas the upper Changjiang and Hanjiang flood in July and August. Southern tributaries have greater water discharge but significantly smaller sediment loads than the northern tributaries and upper reaches of the river. Discharge-sediment concentration correlation diagrams for the lower portion of the river (Luoshan, Hankou, and Datong) show higher concentrations during falling stages of river flow. This unique situation results from the combining of the different hydrological regimes in the middle and lower Changjiang. Correlation curves for the first six months (January to June) largely reflect the influence of southern tributaries (high discharge but low silt content) whereas the second six months (July to December) show the effect of highly silt-laden waters from the upper Changjiang.
The mean annual water flow and sediment transport of the Godavari River are estimated to be 92 km3 and 170 × 106 ton (t), respectively. In terms of sediment transport and the rate of physical erosion (555 t km−2yr−1), the position of the Godavari would be ninth and fifth respectively among the world rivers. Geology of the basin is the main controlling factor of the sediment transport. The sedimentary rocks located in the lower part of the basin and constituting 7% of the total basin area are responsible for 33% of the sediment load. More than 67% of sediment load is silt and clay. Annual, monthly and daily variations in sediment transport indicate that a few selected days in the hydrological year control the annual sediment budget. Our latest estimates are nearly twice that of our earlier values. The presently available global estimates on sediment transport need to be re-evaluated.
Is it possible to reconstruct erosion rates for past epochs when modern rates of erosion all reflect the hand of man to a greater or lesser extent ?
The Helley-Smith bed load sampler is a direct measuring, pressure differential sampler designed for use with sediment ranging in size from coarse sand to medium gravel. For sediment particle sizes between 0.50 and 16 mm, the Helley-Smith bed load sampler has a near- perfect sediment trapping efficiency. For particle sizes smaller than 0.50 mm or larger than about 16 mm, inadequate calibration data exist to establish valid sampling efficiencies. An adequate sampling procedure for many rivers consists of sampling bed load at about 20 equally-spaced transverse locations on each of two traverses across the river. This procedure enables determination of mean bed load transport rate as well as providing insight to spatial and temporal variations in transport rate. Mesure du charriage de fond des cours d'eau RESUME L'appareil à prélever les transports de fond de Helley-Smith effectue une mesure directe, utilisant la pression différentielle. Il a été conçu pour les sédiments dont la granulométrie varie depuis le sable grossier jusqu'au gravier moyen. Pour les classes de particules de sédiment comprises entre 0.50 et 16 mm, l'appareil Helley-Smith a une efficacité de prélèvement presque parfaite. Pour les classes de particules plus petites que 0.50 mm ou plus grandes que 16 mm environs, les données d'étalonnage sont insuffisantes pour déter miner valablement l'efficacité de l'appareil. Une méthode d'échantillonnage suffisante pour beaucoup de rivières consiste à prendre des échantillons des sédiments de fond à environs 20 emplacements également espacés sur un profil en travers et à répéter cette mesure sur un second profil en travers. Ce procédé permet de déterminer la vitesse de transport moyenne du sédiment de fond en même temps elle fournit un aperçu sur les variations spatiales et temporelles des transports de fond.
The Brahmaputra River in Assam, India, characterized by high seasonal variability in flow, sediment transport, and channel configuration, experienced a secular period of aggradation from 1971 to 1979. The suspended load budget indicates an overall aggradation of the 607-km Assam reach of the Brahmaputra by about 16 cm during that period, with about 70% of the suspended sediment inflow into the reach being retained in the channel. Expressed as a percentage of the change in storage for the different reaches, computed errors due to sampling variability in sediment discharge generally lie within about 5–15% and do not appear to be large enough to affect the conclusions drawn from the suspended load budget. For a 145-km reach of the Brahmaputra, an alternative method based on measurement of channel cross sections suggests 21 cm of aggradation, somewhat more than estimated by the suspended load budget. Based on the suspended load carried by trans-Himalayan rivers, the present rate of denudation of the eastern Himalayas is estimated to be 73–157 cm/103 years. The average rate for the last 2–3 million years, estimated from the volume of alluvial fill in the Brahmaputra valley in Assam, the sediment yield of the Himalayan rivers, and assuming a total yield to deposition ratio of 1.4 (present study), is 3 cm/103 years. The current high rate of denudation of the Himalayas may be attributed mainly to the rapid uplift of the mountain system, recent earthquake activity, and high susceptibility of geologic formations to erosion by running water coupled with the effectiveness of the monsoon rainfall regime.
The history of sediment and its movement in the Atlantic drainage demonstrate some of the difficulties of modeling sediment on a river-basin scale. Soil erosion was accelerated by a factor of at least 10 when European settlers cleared forests and planted crops. Although increasing soil-conservation practice and decreasing crop farming have since reduced the rates of erosion, large quantities of eroded material are still stored on hillslopes and in stream valleys where they continue to augment the sediment loads of the rivers. The sediment from this episode of erosion that is largely past can be expected to emerge from storage for many decades and perhaps even several centuries to come. The reservoirs that have been built on many of the major rivers trap significant portions of the moving sediment which, in some places, may be remobilized by large floods. Essentially all the river sediment that reaches the Atlantic coastal zone is trapped in estuaries and coastal marshlands. Probably less than 5% is deposi...
Forty‐four records of reservoir trap efficiency and the factors affecting trap efficiency are analyzed. The capacity‐inflow (C/I) ratio is found to offer a much closer correlation with trap efficiency than the capacity‐watershed (C/W) ratio heretofore widely used. It appears likely from the cases studied that accurate timing of venting or sluicing operations to intercept gravity underflows can treble or quadruple the amount of sediment discharged from a reservoir. Desilting basins, because of their shape and method of operation, may have trap efficiencies above 90 pct even with very low C/I ratios.
Semi‐dry reservoirs with high C/I ratios, like John Martin Reservoir, may have trap efficiencies as low as 60 pct. Truly “dry” reservoirs, such as those in the Miami Conservancy District, probably have trap efficiencies in the 10 to 40 pct range, depending upon C/I ratio
The nature of shelf sedimentation seaward of the world's largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples. Sediments from the Ganges-Brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m, forming a clinoform-like deposit similar to subaqueous deltas found off other major river systems. The highest sediment accumulation rates on the shelf occur near the head of the Swatch of No Ground, a major submarine canyon that indents the shelf west of the present river mouths. This observation, together with textural data, suggests that river sediments are transported seaward and westward and that the Swatch of No Ground is currently a major conduit for the transport of sediments from the Bengal shelf.
The practice of calculating natural rates of denudation from routinely collected data on the loads of suspended and dissolved matter in modern rivers is subject to several significant errors. The sources of these errors are demonstrated by examples from the Atlantic drainage of the United States, where their total effect has apparently doubled the natural rate of erosion. The largest error is caused by assuming that modern sediment loads in populated areas represent natural erosion, whereas in fact they mainly reflect the influence of man. Conversion of forests to croplands in the middle Atlantic states causes about a tenfold increase in sediment yield. Coal mining, urbanization, and highway construction have added extra loads of sediment to the streams. Modern sediment loads in the Atlantic-draining rivers are probably 4 to 5 times greater than they would be if the area had remained undisturbed by man. Errors in calculating the chemical denudation are caused by atmospheric contributions to the dissolved loads of streams and by pollutants that are added directly to stream waters. About one-quarter of the salts in Atlantic-draining streams were contributed from the atmosphere, either as recycled sea salts or as pollutants and soil dust that originally became airborne as a result of the activities of man. Perhaps another one-tenth of the dissolved load consists of industrial and agricultural wastes or acid mine waters that have been added directly to the streams.
Comparison of the sediment discharge records at the Grand Canyon gauging station on the Colorado River for the two periods, 1926-1941 and 1942-1960 show that the suspended- sediment loads for the latter period are only 50 per cent of the earlier period. These data combined with studies of erosion on the Colorado Plateau rangelands indicate that changes in land use probably are responsible for much of the reduction in the sediment yield. Livestock numbers have been greatly reduced on the arid and semiarid rangelands in the past 20 years and erosion control practices have been initiated in many critically eroding areas. Experimental studies by the Geological Survey indicate that grazing control alone can reduce runoff by as much as 30 per cent and sediment yield by 40 per cent. Le débit solide et l'utilisation des terres dans le sud-ouest des Etats-Unis
Based on data from our own and from other sources, the origin, magnitude, annual behaviour and granulometry of suspended sediments in the Middle Parana River, were investigated. It was concluded that the River Bermejo through the Lower Paraguay supplies more than 60% of the suspended material carried by the Middle Parana, and that the concentration peaks coming from that river, lag behind the highest Middle Parana streamflows. The suspended sediments are composed largely of wash load, with high percentages of particles within the fine and very fine silts and clay ranges (
New data and new estimates from old data show that rivers with large sediment loads (annual discharges greater than about 15 x 106 tons) contribute about 7 x 109 tons of suspended sediment to the ocean yearly. Extrapolating available data for all drainage basins, the total suspended sediment delivered by all rivers to the oceans is about 13.5 x 109 tons annually; bedload and flood discharges may account for an additional 1-2 x 109 tons. About 70% of this total is derived from southern Asia and the larger islands in the Pacific and Indian Oceans, where sediment yields are much greater than for other drainage basins.-Authors
On the basis of wet- and dry-season sampling along the Amazon River, at its mouth, and from its 16 large tributaries, the factors that control the salinity and the composition and concentration of the suspended solids of the Amazon River were determined. The multiple regression analysis of the analytical results of 74 river-water samples and numerous in situ measurements revealed: (1) of the 92 percent of the variability of the salinity accounted for, 85 percent was accounted for by relief - 86 percent of the total dissolved salts discharged by the Amazon was supplied from the 12 percent of the total area of the Basin comprising the mountainous-environment type; (2) of the 62 percent of the variability of the concentration of the suspended solids accounted for, 43 percent was accounted for by relief - 82 percent of the total suspended solids discharged by the Amazon was supplied from the mountainous environment-type. The multiple regression analysis of the analytical results obtained by X-ray diffraction from 50 suspended solids samples revealed: (1) increased-relief physical weathering controlled the percentages of quartz, plagioclase, K-feldspar, mica, chlorite, and amphibole; (2) the percentage of "calcic" rocks in the upper portions of tributary basins controlled montmorillonite concentrations; and (3) a combination of low-relief chemical weathering and the percentage of igneous and metamorphic rocks controlled the percentages kaolinite and gibbsite. The increased-relief physical weathering dominant in the Andean mountainous environment controls the geochemistry of the Amazon River, indicated by the following observations: (1) the over-all composition of the suspended solids discharged by the Amazon differs only slightly from that of the suspended solids eroded from the mountainous environment; and (2) 84 percent of the total amount of dissolved salts and suspended solids discharged is eroded from the 12 percent of the total area of the Amazon Basin comprising the mountainous Andean environment-type.
Sustained storage and transport of hydraulic mining sediment in the Bear Basin are documented and a revised model of sediment transport is proposed. The empirical foundation of Gilberts symmetrical wave model is biased. Channel incision in the Sacramento Valley has been promoted by several factors in addition to decreased sediment loads and does not confirm the return of sediment loads to pre-mining levels. A revised, skewed sediment wave model is proposed for basins with a large component of long-term channel storage. This conceptual model is in better harmony with growing evidence of the importance of sediment storage in and near channels to long-term sediment loads. The persistence of anthropogenic sediment in fluvial systems may be much greater than implied by Gilbert's model. -from Author
Presently, the water discharge rate to the Black Sea by Turkish rivers is approximately 41 km3/yr. The sediment discharge rate of Turkish rivers to the Black Sea is 28106 t/yr. Before construction of the hydroelectric dams, the sediment discharge rate was approximately 70106 t/yr. The sharp reduction in sediment load is largely a result of the dams near the mouths of the Yesil Irmak and Kizil Irmak rivers. Before the construction of dams, Turkish rivers contributed approximately one third of the total amount of sediment received by the Black Sea from all surrounding rivers. The life-span of the major reservoirs varies from approximately only one century (Yesil Irmak river reservoirs) to several thousand years (Sakarya river reservoirs). Life-span for the large Altinkaya Dam reservoir is estimated with approximately 500 yr.
The geometry, stratigraphy, and structure of recently deposited Huanghe (Yellow River) Delta sediments were examined by high resolution subbottom profiles and medium-penetration boomer profiles. The results indicate that the active (post-1976) subaqueous delta advances as a single thin localized lobe with a maximum thickness of only 15 m. Calculations of sediment volumes indicate that 90% or more of the sediment supplied by the Huanghe remains within 30 km of the mouth. Sediment on the delta platform near the mouth is fine sand; elsewhere silts and clays prevail.[/p]
Sediment accumulation on the Brazilian continental shelf near the Amazon River is investigated using radiochemical (e.g.210Pb,14C) techniques to provide a better understanding of this major dispersal system of fine-grained sediment.210Pb profiles from 57 cores collected during 1983 reveal the distribution of modern (100-y time scale) accumulation rates on the Amazon subaqueous delta. Accumulation rates increase from <0.1 cm y1 (0.1 g cm2 y1) nearshore, to rates as high as ∼ 10 cm y−1 (6.9 g cm2 y−1) on the outer topset and the foreset regions (30–50 m water depth). Reduced upward accretion nearshore (<15 m water depth), which is reflected in the limited subaerial expression of the Amazon delta, probably results from the intense activity of surface waves and tidal currents. A thick (as much as 2 m) depth). This layer probably is reworked by waves and currents, and most of the sediment is eventually transported to other parts of the dispersal system.14C dating of an anomalous area of relict (age >100 y) sediment in the northwestern portion of the subaqueous delta indicates that this sediment was deposited <1000 y ago. The absence of modern sediment in this area is not understood. A sediment budget for the Amazon shelf indicates that 6.3 ± 2.0 × 108 tons of sediment accumulate annually. Much of the remainder of Amazon River sediment (∼6 × 108tons y−1) probably is transported northwestward beyond the Brazilian shelf and/or is accumulating landward of the shelf as coastal accretion.
Based on sampling of the entire region of the Ganges basin, chemical and sediment load supplied at various parts of the basin have been computed. Annual flux of materials from sub-basin into the main basin and input to the Hoogly estuary have been calculated and compared to major river systems of the world. The total annual load at Calcutta (mouth of the river) was calculated as 411 · 106 t (328 · 106 t sediment load + 83 · 106 t chemical load). Erosion rate (549 t km−2 yr.−1) is among the highest in this river system and controlling factors on a global scale, such as basin area, are discussed in detail. Annual decrease in basin elevation indicates a rapid process of denudation and such rates have a bearing on rates of shelf sediment accumulation.
The dispersal of Amazon mud reaches from the mouth of the Amazon river to the Orinoco delta and to the eastern Caribbean, a distance of more than 1500 km. The present dispersal system has been in existence for at least 103 years, but the deposition of mud from this system has not been constant during that period. A change from net erosion to net deposition along the Suriname coast between 1947 and 1981 was found to coincide with a shift of the trade winds from a dominantly NE to a more ENE direction from 1959 onward, accompanied by an increase in mean wind velocity. The change to net-deposition can be explained by enhanced longshore transport of suspended matter with simulataneous reduction of erosion by reduction of the onshore wave energy component. The sequence of a recent mud deposit 100–200 years old off the Amazon river mouth separated by a period of non-deposition from an older mud deposit less than 1000 years old, agrees well with indications for a wetter period in the Amazon basin and in the Colombian Andes since about 200 years BP and a wetter period between 500 and 900 y BP. This implies that during wetter periods the suspended-sediment supply from the Amazon (and the Orinoco) was (is) higher.
The Ganges and Brahmaputra rivers combined have formed one of the largest deltas in the world, comprising some 23,000 sq. miles. These rivers originate within the Himalayan Mountains and drain an enormous land area before entering East Pakistan. Individually, each of the rivers discharges in excess of 2.5 million cusecs of water during flood, and combined they carry nearly 6 million cusecs of water to the Bay of Bengal, nearly three times the amount borne by the Mississippi River. Having such a large drainage area, the rivers are also heavily charged with sediment, transporting approximately 13 million tons of suspended sediment per day during flood. The large discharge and heavy sediment load cause the rivers to be extremely unstable, and the channels are constantly migrating laterally. Within Recent times both rivers have occupied and abandoned numerous river courses. The Brahmaputra followed a route some 60 miles to the east of its present course only 200 years ago. The long-term patterns of river migration indicate that the Ganges has been migrating eastward, whereas the preferred migration of the Brahmaputra is westward. These movements are obviously controlled by major faults or fractures in the earth's crust.
The Amazon and Orinoco Rivers are massive transcontinental conveyance systems for suspended sediment. They derive about 90% of their sediment from the Andes that support their western headwaters, transport it for thousands of kilometers across the breadth of the continent and deposit it in the coastal zones of the Atlantic. At their points of maximum suspended-sediment discharge, the Amazon transports an average of 1100–1300 × 106 tons per year and the Orinoco transports about 150 × 106 tons per year. Relations of sediment discharge to water discharge are complicated by unusual patterns of seasonal storage and remobilization, increased storage and reduced transport of sediment in the middle Orinoco during periods of peak water discharge, and storage of suspended sediment in the lower Amazon during rising discharge and resuspension during falling discharge. Spatial distributions of suspended sediment in cross-sections of both rivers are typically heterogeneous, not only in the vertical sense but also in the lateral. The cross-channel mixing of tributary inputs into the mainstem waters is a slow process that requires several hundred kilometers of downriver transport to complete. Considerable fine-grained sediment is exchanged between rivers and floodplains by the combination of overbank deposition and bank erosion.