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Distributary network reorganization (ΔRQs) for displacement magnitudes (Equation 5) of Set 1 (triangles) and Set 2 (circles) simulations and field cases; multiple same‐color symbols represent displacement length and fault width pairs in Set 2 simulations (i.e., increasing displacement magnitude for a given color is due to increasing fault width). Not‐connected simulations from Set 2 were omitted, and symbols and error bars represent mean and standard deviation of replicate simulations, respectively. Gray bands in Figure 5 depict displacement magnitude range estimated for real‐world distributary systems (Supporting Information S1). Faulting‐induced subsidence scaling on the Mississippi River delta indicates likely distributary network reorganization within deltas built from sediment diversion efflux (e.g., at scale of Cubit’s Gap).
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Deltas exhibit spatially and temporally variable subsidence, including vertical displacement due to movement along fault planes. Faulting‐induced subsidence perturbs delta‐surface gradients, potentially causing distributary networks to shift sediment dispersal within the landscape. Sediment dispersal restricted to part of the landscape could hinder...
Citations
... Avulsion is a fundamental delta-building process that sustains coastal landscapes by redistributing the focus of sediment deposition over time to offset basin subsidence, sea-level rise, and localized aggradation of channel belts. The processes that govern avulsion remain debated [1][2][3][4] , and the dominant mechanisms may vary between relatively high-gradient, coarse-sediment fan settings and relatively low-gradient, fine-grain coastal settings as well by climate and sediment supply 5 . In its simplest form, avulsion results when channel belts become superelevated relative to their floodplain and shift to a new course through gradual abandonment by stream capture 6,7 . ...
Earthquakes present severe hazards for people and economies and can be primary drivers of landscape change yet their impact to river-channel networks remains poorly known. Here we show evidence for an abrupt earthquake-triggered avulsion of the Ganges River at ~2.5 ka leading to relocation of the mainstem channel belt in the Bengal delta. This is recorded in freshly discovered sedimentary archives of an immense relict channel and a paleo-earthquake of sufficient magnitude to cause major liquefaction and generate large, decimeter-scale sand dikes >180 km from the nearest seismogenic source region. Precise luminescence ages of channel sand, channel fill, and breached and partially liquefied floodplain deposits support coeval timing of the avulsion and earthquake. Evidence for reorganization of the river-channel network in the world’s largest delta broadens the risk posed by seismic events in the region and their recognition as geomorphic agents in this and other tectonically active lowlands. The recurrence of comparable earthquake-triggered ground liquefaction and a channel avulsion would be catastrophic for any of the heavily populated, large river basins and deltas along the Himalayan arc (e.g., Indus, Ganges, Brahmaputra, Ayeyarwady). The compounding effects of climate change and human impacts heighten and extend the vulnerability of many lowlands worldwide to such cascading hazards.
... In this case, subsidence rates will likely be correlated with underlying marsh deposit thickness. If subsidence rates are sufficiently large over long enough timescales, they could influence a wide range of geomorphic processes including marsh drowning, channel kinematics (Liang et al., 2016;Moodie & Passalacqua, 2021), and delta progradation (Chamberlain et al., 2021). ...
... This near surface bed lowering is continuously replenished by new marsh accretion and therefore does not contribute much to the long-term generation of accommodation space, as σ s accounts for only about one quarter of RSLR b below a channel depth (Figure 7b). Field (van Asselen, 2011) and modeling (Kim et al., 2010;Liang et al., 2016;Moodie & Passalacqua, 2021;Xotta et al., 2022) studies have repeatedly shown the potential influence of subsidence on surface morphology and kinematics by causing differential compaction and steering channels. However, the treatment experiment, along with notable examples from field-scale deltas (e.g., Keogh, 2020;Steckler et al., 2022), suggest that compactional subsidence is often too shallow to significantly impact surface processes in this way. ...
We present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory‐scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non‐negligible compactional subsidence rates across the delta‐top, comparable in magnitude to our boundary condition relative sea level rise rate of 250 μm/hr. Subsidence in the control experiment (on average 54 μm/hr) is concentrated in the lowest elevation (<10 mm above sea level) areas near the coast and is likely related to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 μm/hr) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (∼10 mm) of the sediment surface. These rates compare favorably with field and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence “hot spots” may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates over a short time frame may exceed thresholds for marsh platform drowning, even if the long term trend does not.
... These studies are consistent with a growing general perception that many of the world's major rivers have a long-term and regional-scale tectonic control (e.g., Potter, 1978;Burrato et al., 2003;Hongbo and Juntao, 2009;Menier et al., 2017;Woodbridge et al., 2019). Tectonicallyinduced Quaternary-Recent fluvial system adjustments have been well documented in the Mississippi, the classical global archetype of a meandering river (Schumm et al., 1994;Schweig and Arsdale, 1996;Moodie and Passalacqua, 2021), the Nile, the longest river in Africa (Kusák et al., 2019;Beshr et al., 2021), and the lower Rhine in northwest Europe (Woolderinka et al., 2021). Evidencing neotectonics based on river adjustments in folded reliefs has been a question of particular interest (e.g., Dolan and Avouac, 2007;Delcaillau et al., 2006), yet underexplored in many large rivers across the world, which has hindered comparisons and modeling. ...
In intracratonic South America, the origin of neotectonic activity and its impact on large Amazonian rivers have been of continuous research interest. Despite the low relief of the land surface, which has generally been attributed to tectonic stability, there is increasing evidence of Neogene fault reactivation within central Amazonia. Recent research has also reported surface folding during the Quaternary, but this record was based only on morphological analysis of remote sensing data. This investigation focuses on a megascale (~60,000 km2) domal relief from central Amazonia (the Juruá dome). It firstly aims to verify the domal relief relationship to folding. It then explores the origin of the stress field from which it has developed within the context of Andean uplift and the westward movement of the South American plate. The approach of analyzing river adjustments based on morphological and morphometric information from remote sensing imagery, integrated with subsurface data consisting of Bouguer gravity and magnetic anomaly maps, well logs, and seismic reflection sections. The results revealed that the Juruá dome coincides with the location of a basin depocenter. Subsurface evidence exists for the growth of a broad anticline producing thicker sedimentary units away from the dome core. NE- and NW-striking normal and reverse faults with flower structure geometries are abundant and suggest deformation from strike-slip tectonics. Gravity and magnetic data revealed that the fold and many of its associated faults are deep-rooted into basement rocks. Within the analyzed stratigraphic interval, faults are mostly developed into pre-Cretaceous units, but often propagate to the surface, where they define the edges of the dome as well as partly deform its overall shape. The fold and its related faults have modified the course of several rivers within the domal relief area, including the Juruá River, which is entrenched along NE-striking faults that are configured to release stress along the fold axis. The geomorphological and structural data are collectively compatible with a long-term NW-trending maximum horizontal compressive stress-field that is driving basin inversion. This neotectonic activity can be linked to far-field stresses from the pushes from the Andean orogeny and the movement of the South American plate against the North Andean and Nazca plates.
As humans continue to inhabit and modify river deltas, the natural processes governing material transport through these landscapes are altered. Two common engineering projects undertaken on deltas are the dredging of channels to enable shipping and the construction of embankments to reduce flooding. While the impact of these topographic modifications has been studied at a local level for specific sites, there is a gap in our generalized understanding of how these landscape modifications impact material transport. To narrow this gap, we conduct exploratory numerical modeling to develop deltaic landscapes with different input sediment compositions, modify their topography to mimic dredging and embankments, simulate different flow conditions, and then model the transport of passive particles. We find that human modification of topography lowers hydrological connectivity by reducing the area visited by fluvial inputs. The amount of time particles spend within the delta is reduced by the construction of polders and is lengthened by dredging. Material buoyancy has a greater impact on nourishment areas and exposure times than flow regime or topographic modification, with positively buoyant particles spending longer and visiting a greater area of the delta than neutral and negatively buoyant material. The results of this study can help guide the design of future engineering projects by providing estimates of their likely impact on transport processes.
Arctic river deltas define the interface between the terrestrial Arctic and the Arctic Ocean. They are the site of sediment, nutrient, and soil organic carbon discharge to the Arctic Ocean. Arctic deltas are unique globally because they are underlain by permafrost and acted on by river and sea ice, and many are surrounded by a broad shallow ramp. Such ramps may buffer the delta from waves, but as the climate warms and permafrost thaws, the evolution of Arctic deltas will likely take a different course, with implications for both the local scale and the wider Arctic Ocean. One important way to understand and predict the evolution of Arctic deltas is through numerical models. Here we present ArcDelRCM.jl, an improved reduced-complexity model (RCM) of arctic delta evolution based on the DeltaRCM-Arctic model (Lauzon et al., 2019), which we have reconstructed in Julia language using published information. Unlike previous models, ArcDelRCM.jl is able to replicate the ramp around the delta. We have found that the delayed breakup of the so-called “bottom-fast ice” (i.e. ice that is in direct contact with the bed of the channel or the sea, also known as “bed-fast ice”) on and around the deltas is ultimately responsible for the appearance of the ramp feature in our models. However, changes made to the modelling of permafrost erosion and the protective effects of bottom-fast ice are also important contributors. Graph analyses of the delta network performed on ensemble runs show that deltas produced by ArcDelRCM.jl have more interconnected channels and contain less abandoned subnetworks. This may suggest a more even feeding of sediments to all sections of the delta shoreline, supporting ramp growth. Moreover, we showed that the morphodynamic processes during the summer months remain active enough to contribute significant sediment input to the growth and evolution of Arctic deltas and thus should not be neglected in simulations gauging the multi-year evolution of delta features. Finally, we tested a strong climate-warming scenario on the simulated deltas of ArcDelRCM.jl, with temperature, discharge, and ice conditions consistent with RCP7–8.5. We found that the ramp features degrade on the timescale of centuries and effectively disappear in under 1 millennium. Ocean processes, which are not included in these models, may further shorten the timescale. With the degradation of the ramps, any dissipative effects on wave energy they offered would also decrease. This could expose the sub-aerial parts of the deltas to increased coastal erosion, thus impacting permafrost degradation, nutrients, and carbon releases.
450 million people live on river deltas and thus on land that is precariously low above the sea level and sinking because of human activities and natural processes. Although global debates around coastal risk typically focus on sea level rise, it is sinking lands and rising seas that together endanger lives and livelihoods in river deltas. However, the ability to quantify and address those risks in and integrated manner remains limited. Herein, we identify four priority areas where a lack of data, models, and knowledge are limiting sustainable delta management, namely (1) developing practical models for delta-scale processes and nature-based solutions, (2) coupling models for basin and delta processes, (3) closing knowledge disparities between river deltas, and (4) integrating deltas in assessments of global change and vice versa. Addressing those challenges through global scientific efforts is instrumental to identify local-to-global levers to design adaptation and mitigation measures for resilient river deltas.
As humans continue to inhabit and modify river deltas, the natural processes governing material transport through these landscapes are altered. Two common engineering projects undertaken on deltas are the dredging of channels to enable shipping and the construction of embankments to reduce flooding. While the impact of these topographic modifications has been studied at a local-level for specific sites, there is a gap in our generalized understanding of how these landscape modifications impact material transport. To narrow this gap, we conduct exploratory numerical modeling to develop deltaic landscapes with different input sediment compositions, modify their topography to mimic dredging and embankments, simulate different flow conditions, and then model the transport of passive particles. We find that human modification of topography lowers hydrological connectivity by reducing the area visited by fluvial inputs. The amount of time particles spend within the delta is reduced by the construction of polders and is lengthened by dredging. Material buoyancy has a greater impact on nourishment areas and exposure times than flow regime or topographic modification, with positively-buoyant particles spending longer and visiting a greater area of the delta than neutral and negatively-buoyant material. The results of this study can help guide the design of future engineering projects by providing estimates of their likely impact on transport processes.
Understanding the way fluvially transported materials are partitioned in river deltas is essential for predicting their morphological change and the fate of environmental constituents and contaminants. Translating water‐based partitioning estimates into fluxes of nonwater materials is often difficult to constrain because most materials are not uniformly distributed in the water column and may have characteristic transport pathways that differ from the mean flow. Here, we present a novel reduced‐complexity modeling approach for simulating the patterns of transport of a diverse range of suspended fluvial inputs influenced by vertical stratification and topographic steering. We utilize a mixed Eulerian‐Lagrangian modeling approach to estimate the patterns of nourishment and connectivity in the Wax Lake and Atchafalaya Deltas in coastal Louisiana. Using the reduced‐complexity particle routing model dorado, in conjunction with a calibrated ANUGA hydrodynamic model, we quantify how transport patterns in each system change as a function of a material's Rouse number and environmental conditions. We find that even small changes to local topographic steering lead to emergent system‐scale changes in patterns of fluvial nourishment, with greater channel‐island connectivity for positively buoyant materials than negatively buoyant materials, hydraulically sorting different materials in space. We also find that the nourishment patterns of some materials are more sensitive than others to changes in discharge, tidal conditions, and anthropogenic dredging. Our results have important implications for understanding the eco‐geomorphic evolution of deltas, and our modeling framework could have interdisciplinary implications for studying the transport of materials in other systems, including sediments, nutrients, wood, plastics, and biotic materials.
Climate change is raising sea levels across the globe. On river deltas, sea‐level rise (SLR) may result in land loss, saline intrusion into groundwater aquifers, and other problems that adversely impact coastal communities. There is significant uncertainty surrounding future SLR trajectories and magnitudes, even over decadal timescales. Given this uncertainty, numerical modeling is needed to explore how different SLR projections may impact river delta evolution. In this work, we apply the pyDeltaRCM numerical model to simulate 350 years of deltaic evolution under three different SLR trajectories: steady rise, an abrupt change in SLR rate, and a gradual acceleration of SLR. For each SLR trajectory, we test a set of six final SLR magnitudes between 5 and 40 mm/yr, in addition to control runs with no SLR. We find that both surface channel dynamics as well as aspects of the subsurface change in response to higher rates of SLR, even over centennial timescales. In particular, increased channel mobility due to SLR corresponds to higher sand connectivity in the subsurface. Both the trajectory and magnitude of SLR change influence the evolution of the delta surface, which in turn modifies the structure of the subsurface. We identify correlations between surface and subsurface properties, and find that inferences of subsurface structure from the current surface configuration should be limited to time spans over which the sea level forcing is approximately steady. As a result, this work improves our ability to predict future delta evolution and subsurface connectivity as sea levels continue to rise.
Channel avulsions on river deltas are the primary means to distribute sediment and build land at the coastline. Many studies have detailed how avulsions generate delta lobes, whereby multiple lobes amalgamate to form a fan-shaped deposit. Physical experiments demonstrated that a condition of sediment transport equilibrium can develop on the topset, characterized by neither deposition nor erosion of sediment, and material is dispersed to the foreset. This alluvial grade condition assumes steady subsidence and uniform basin depth. In nature, however, alluvial grade is disrupted by variable subsidence, and progradation of lobes into basins with variable depth: conditions that are prevalent for tectonically active margins. We explore sediment dispersal and deposition patterns across scales using measurements of delta and basin morphology compiled from field surveys and remote sensing, collected over 150 years, from the Selenga Delta (Baikal Rift Zone), Russia. Tectonic subsidence events, associated with earthquakes on normal faults crossing the delta, displace portions of the topset several meters below mean lake level. This allogenic process increases regional river gradient and triggers lobe-switching avulsions. The timescale for these episodes is shorter than the predicted autogenic lobe avulsion timescale. During quiescent periods between subsidence events, channel-scale avulsions occur relatively frequently because of in-channel sediment aggradation, dispersing sediment to regional lows of the delta. The hierarchical avulsion processes, arise for the Selenga Delta, preserves discrete stratal packages that contain predominately deep channels. Exploring the interplay between discrete subsidence and sediment accumulation patterns will improve interpretations of stratigraphy from active margins and basin models.
