Distribution Characteristics and Geogenic Mechanisms of Riverbed Overburden in Southwest China

Abstract

The distribution, scale, and engineering geological characteristics of riverbed overburden have become one of the key issues in the construction of water conservancy and hydropower engineering projects in southwest China. In this study, we summarize and discuss the (variation of thickness) distribution and the geogenic (formation) mechanisms of riverbed overburden in the associated rivers. This was done by compiling thickness data from constructed and planned dams. The results show that the overburden thickness is generally shallower in the upper Tibetan Plateau region, it is thickest in the marginal mountain region in the middle reaches, and shallower in the lower reaches of the mountain regions that are in contact with the Yunnan-Guizhou Plateau or Sichuan Basin. This holds true with the shallow-thick-shallow Distribution Law. Additionally, the river overburden has the characteristic of thickening gradually from the basin edge to the plateau slope. Through the genesis, source, and distribution of the aggradation deposits in the riverbed, the geogenic (formation) mechanisms of the river overburden layer is explored, and the coupling effect of tectonic-climatic-fluvial sedimentation processes on the variation of overburden thickness and spatial distribution is proposed. Finally, the geological problems encountered when engineering dams in thick overburden are analyzed, and common engineering measures are put forward. The results provide basic data support for water resources exploitation and further development of river engineering in Southwest China.

Full text

Distribution Characteristics and
Geogenic Mechanisms of Riverbed
Overburden in Southwest China
Zongping Yan
1
,
2
,MoXu
1
*, Xiaobing Kang
1
, Leilei Guo
2
and Shishu Zhang
3
1
State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu,
China,
2
Sichuan Water Conservancy Vocational College, Chengdu, China,
3
Power China Chengdu Engineering Corporation
Limited, Chengdu, China
The distribution, scale, and engineering geological characteristics of riverbed overburden
have become one of the key issues in the construction of water conservancy and
hydropower engineering projects in southwest China. In this study, we summarize and
discuss the (variation of thickness) distribution and the geogenic (formation) mechanisms
of riverbed overburden in the associated rivers. This was done by compiling thickness data
from constructed and planned dams. The results show that the overburden thickness is
generally shallower in the upper Tibetan Plateau region, it is thickest in the marginal
mountain region in the middle reaches, and shallower in the lower reaches of the mountain
regions that are in contact with the Yunnan-Guizhou Plateau or Sichuan Basin. This holds
true with the shallow-thick-shallow Distribution Law. Additionally, the river overburden has
the characteristic of thickening gradually from the basin edge to the plateau slope. Through
the genesis, source, and distribution of the aggradation deposits in the riverbed, the
geogenic (formation) mechanisms of the river overburden layer is explored, and the
coupling effect of tectonic-climatic-fluvial sedimentation processes on the variation of
overburden thickness and spatial distribution is proposed. Finally, the geological problems
encountered when engineering dams in thick overburden are analyzed, and common
engineering measures are put forward. The results provide basic data support for water
resources exploitation and further development of river engineering in Southwest China.
Keywords: southwest China, river, overburden, distribution, geogenic mechanisms
1 INTRODUCTION
The thickness of riverbed overburden poses great challenges to the construction of hydropower
projects in river basins, thus it is of great theoretical value and engineering significance to study the
distribution law and the geogenic (formation) mechanisms of riverbed overburden layer thickness.
As is well known, rivers transport soil from the hillside to tributaries, to the main stem, and then from
the main stem to the lowland, especially in mountainous areas (Fabrizio et al., 2018). The riverbeds
formed are a variety of loose deposits closely related to the topography and geomorphology, parent
rock lithology, geological structure, neotectonic movement, and climate environments. The
formation types are complicated and its lithofacies and thickness vary greatly.
Many researchers have analyzed the exploration and experimental data of dam sites, and used
numerical simulations and physical models to study riverbed overburden from the aspects of the
formation mechanisms, distribution law, engineering characteristics, exploration, anti-seepage
Edited by:
Xiaoyan Zhao,
Southwest Jiaotong University, China
Reviewed by:
Chengcheng Li,
China University of Geosciences
Wuhan, China
Jianping Chen,
Jilin University, China
*Correspondence:
Mo Xu
xm@cdut.edu.cn
Specialty section:
This article was submitted to
Geohazards and Georisks,
a section of the journal
Frontiers in Earth Science
Received: 14 March 2022
Accepted: 04 May 2022
Published: 23 June 2022
Citation:
Yan Z, Xu M, Kang X, Guo L and
Zhang S (2022) Distribution
Characteristics and Geogenic
Mechanisms of Riverbed Overburden
in Southwest China.
Front. Earth Sci. 10:895769.
doi: 10.3389/feart.2022.895769
Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 8957691
ORIGINAL RESEARCH
published: 23 June 2022
doi: 10.3389/feart.2022.895769

technology, and related engineering geological problems
(foundation bearing capacity, seepage deformation, uneven
settlement, sand liquefaction, etc.) (Hedayati Talouki et al.,
2015;Michael, 2018;Luo and Huang, 2020;Yu and Shao,
2020;Zhang et al., 2021). The thickness variation of riverbed
overburden has the obvious characteristics of basin and region,
and there are many new achievements in the study of riverbed
overburden from the dam site areas, especially in the section of
the riverbed engineering area to the main stem and tributaries of
rivers (Shi, 1986;Wang Y. S. et al., 2007;Xu et al., 2008;Wang,
2009;Xu et al., 2010;Yu and Shao, 2020;Yu and Shao,
2020).According to borehole data from construction sites of
hydropower stations, the thickness variation of riverbed
overburden from the top of Tiger Leaping Gorge to Xiakou is
thinner in the upper and lower reaches of the Jinsha River, and
deepest in the middle reaches (Hongyan Dam site thickness of
250 m) (Wang., 2009). The deposit layer in the Yalong river bed is
often over 30 m thick and can be up to 51 m thick (Wang L. S.
et al., 2007).The thickness of the riverbed overburden increases
from the lower reaches to the upper reaches of the Dadu River,
and suddenly decreases in the Dagangshan area of the middle
reaches, and the longitudinal section of the main stem bed cover
forms a “W”shape (Xu et al., 2010).The riverbed overburden of
the Minjiang River appears thick in the upper reaches and thin in
the lower reaches (Wang et al., 2005).Research on the distribution
and geogenic mechanisms of riverbed overburden in southwest
China is mainly focused on a single dam site or individual river
sections, meanwhile there is less systematic research on the
thickness variation of riverbed overburden in the primary
rivers of Southwest China. These rivers include the Jinsha,
Tsangpo, Lancang, Nujiang, etc.
Parts of the previously mentioned rivers are in the slope zone
transition from the Qinghai-Tibet Plateau to the Yunnan-
Guizhou Plateau and the Sichuan Basin. This is one of the
regions with the most abundant water resources and also the
most developed deep riverbed overburden. As for the origins of
the overburden, there are many opinions, but no agreement has
been reached yet. Xu et al. (2010) proposed that global climate
change and eustacy (sea level rise and fall) cause the thick
overburden of river valleys. Wang et al. (2005) proposed that
earthquakes, heavy rains, and other disasters induce mountain
collapse, landslide and debris flow, resulting in river blockage and
other phenomena where the landslide front is directly deposited
on the early overburden, resulting in local accumulation of deep
overburden. Wang et al. (2006) proposed that during the
interglacial period of the last glacial max, the river valley was
strongly eroded and cut into the basal layer below the present
riverbed, and then rapidly backfilled to form the deep
overburdenlayer. Ding et al. (2021) investigated the 26.5 m-
thick lakebed profile at the bottom of Tongzilin Dam in the
Yalong River and proposed that the formation of the overburden
was the result of the combined action of climate fluctuations and
strong tectonic activity (local tectonic subsidence). Bridgland
(2000) and Maddy et al. (2001) proposed that the reasons
affecting the development of fluvial geomorphology (including
riverbed cover) can be summarized as dynamic changes within
the fluvial system and non-fluvial origins which are based on the
geological records of the river terraces of the Thames River in the
United Kingdom during the Middle Pleistocene. By considering
ice dynamics, bedrock erosion, and sediment transport processes,
Delaney and Adhikari (2020) simulated the sediment discharge of
alpine glaciers that experienced 100 years of accelerated glacier
melting, and found that during the glacier retreat sediment
emissions are likely to increase significantly, and depending on
the amount of sediment stored under the ice, there are additional
effects based on the different glacial topography and bed
roughness. Alexey et al. (2019) studied riverbed deposition of
rivers in the Russian Far East under extreme rainfall conditions
and found that the upper reaches of secondary tributaries and
part of the estuaries of large tributaries were areas with increased
deposition. Syvitski et al. (2022) reviewed that Global warming is
also substantially affecting the global sediment cycle through
temperature impacts (sediment production and transport, sea ice
cover, glacial ice ablation and loss of permafrost), precipitation
changes, desertification and wind intensities, forest fire extent and
intensity, and acceleration of sea-level rise. Humans have
transformed the mobilization, transport and sequestration of
sediment, to the point where human action now dominates
these fluxes at the global scale as seen in data from 1950 to
2010. Summarizing previous studies, it is shown that the
formation of the riverbed overburden is related to several
factors, such as tectonic movement, climate change,
sedimentary law, geological environment, and geological
disasters. However, the origin of the regional deep overburden
in the primary rivers of southwest China may be more complex
and diverse.
As for the classification of overburden thickness, most researchers
demarcate 30 m as the boundary of thick overburden. The code for
engineering geological investigation of water resources and
hydropower in China (GB50487-2008) stipulates that the
thickness greater than 40 m of overburden layer of is considered
thick. This study adopts three categories according to the
specification, namely the shallow layer (<40 m), a deep
overburden layer (40–100 m), and giant-thick overburden layer
(>100 m). For clarity the terms “thick and thin”or “thickness”is
used when regarding the vertical height of the overburden layer(s)
andtheabovetermsareusedwhendistinguishingtheimpacton
hydrological engineering projects as shown in Table 1.
Based on previous work, this paper systematically summarizes
and explores the distribution characteristics and geogenic
mechanism of riverbed overburden in major rivers of
Southwest China by grading and analyzing many of the
boreholes related to already built or planned dams (sluices).
This will provide engineering experience, reference, and basic
data support for the hydropower cascade planning, development,
and site selection of river channel projects in Southwest China.
2 RIVERBED OVERBURDEN DEPTH
DISTRIBUTION CHARACTERISTICS
By summarizing the overburden thickness and composition data
of 632 dams (sluices) that have been built or planned in
Southwest China, the thickness variation of overburden in
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Yan et al. Riverbed Overburden in Southwest China

TABLE 1 | The riverbed overburden thickness characteristic table in the upper Reaches of the Yangtze River.
River Maximum
thickness/
m
Corresponding
position
General characteristics of overburden thickness
Mainstem Tributary
Giant-thick
Overburden
(>100 m)
Deep overburden(40–100 m) shallow layer (<40 m)
Jinsha
River
250 Hongyan dam site The main riverbed overburden is“bead-like”, the middle and upper reaches of Tiger Leaping
Gorge reach the thickest, the middle reaches are thinner, and the lower reaches are
generally thicker, The section from Xinshizhen to Yibin city presents a belt-like distribution of
deep layers
Except for Yalong River, the
overburden thickness of the
Jinsha River tributaries is
generally less than 40 m,
but the overburden
thickness of the middle and
lower reaches of the Jinsha
River tributaries is relatively
thicker, for example, the
overburden thickness of the
Shuiluo River basin is
relatively thicker, that is, it is
thicker in the upstream and
thinner in the lower
reaches. The Chongjiang
River in the Shigu water
source area has a deep
overburden, with a
maximum thickness of
132.80 m. In addition, the
tributaries are thick in local
sections and near the
confluence, but the overall
thickness is irregular
①Qizong- Longpan
section
①Lawa-Qizong section. ②Longpan-
Xiaxiakou section. ③Panzhihua-yibin
city section
①Above the Lawa dam
site. ②Xiaxiakou-
panzhihua section
Yalong
River
109.2 Jiangbian dam site The main stem is bound by the Lenggu dam site, and the thickness of overburden in the
lower reach is 30–50 m, while the thickness of overburden in the upper reach is less
than 30 m
The upper Yalong River
tributaries have shallow
overburden, such as in the
Xianshui River Basin where
the overburden is less than
40 m. The overburden of
the middle and lower
reaches of tributaries is
generally thicker, for
example, the overburden of
the Jiulong River Basin is
generally thicker, of which
the Jiangbian dam site is
109.2 m. However, the
xigeda formation exists in
many dam sites in the
Anning River Basin. The
bedrock has not been
explored in many dam sites,
and the thickness is difficult
to be determined
The river section below Lenggu dam
site
The river section above
Lenggu dam site
Dadu
River
>420 yele dam site The overburden in the headwater area above the Shuangjiangkou dam site of the main
stem is shallower, and the riverbed overburden of the lower reach is in a continuous and
stable distribution of “strip”, and the overall thickness increases from the downstream to the
upstream
The distribution of deep
overburden in the
tributaries of Dadu River is
also relatively common, for
example, the Nanya River
and Tianwan River have
giant-thick overburden, but
there are fluctuations
①Daoban- Danba
section. ②Yeba-
Yingliangbao
section
①Shuangjiangkou-Daoban section.
②Danba- Yeba section. ③Reach
below Yingliangbao (except for
Dagangshan and Angu Dam site)
①Above the
Shuangjiangkou dam site.
②Dagangshan dam site.
③Angu dam site
Minjiang
River
101.5 Shiziping dam site The riverbed overburden in the upper reaches of Minjiang River isthicker from the lower to
upper reaches, while the riverbed overburden in the middle and lower reaches is shallower
The deep overburden of the
tributaries of the upper
Minjiang River is widely
distributed. For example,
Diexi to Xuankou town The river below Xuankou
(Continued on following page)
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Yan et al. Riverbed Overburden in Southwest China

each major river basin is as follows: the Yellow River (Reach
above Lanzhou) (61), the Jialing River (69), the Fujiang River
(28), and the Tuojiang River (6), the Minjiang River (56), the
Qingyi River (22), the Dadu River (75), the Yalong River (89), the
Jinsha River (104), the Lancang River (29), the Nujiang River
(36), the Lixian River (9), the Irrawaddy River (9), the Nanpan
River (9) and the Yuanjiang River (4), the Yarlung Tsangpo river
(26), etc. The distribution is shown in Figure 2.
2.1 Main Riverbed Overburden Thickness
Variation Characteristics
2.1.1 The Upper Reaches of the Yangtze River
The upper reaches of the Yangtze River at the Jinsha River and its
important tributaries such as the Yalong, Dadu, and Min Rivers
have deep riverbed overburden (Figure 1), with diverse
lithofacies and complex structures. In the upper reaches of the
Yangtze River (from Yibin to Chongqing) and its tributaries, such
as the Tuojiang and Jialing Rivers, the riverbed overburden is
mainly composed of a single sand and gravel layer of small
thickness (Table 1).
2.1.2 Nujiang River
The riverbed overburden in the Nujiang River Basin is relatively
shallow. It is generally 20–40 m thick. However, the thickness of
the riverbed overburden at the Songta dam site is 58–70 m. Above
the Songta dam site the riverbed overburden thickness is
10–15 m. In some cases however (between the upper and
lower reaches) there are almost no deposits, and the riverbed
is bedrock. From the Songta dam site to the Saige dam site, the
TABLE 1 | (Continued) The riverbed overburden thickness characteristic table in the upper Reaches of the Yangtze River.
River Maximum
thickness/
m
Corresponding
position
General characteristics of overburden thickness
Mainstem Tributary
Giant-thick
Overburden
(>100 m)
Deep overburden(40–100 m) shallow layer (<40 m)
the overburden of the
Zagunao River, Heishui
River and Erhe River basins
is generally about 40–80 m,
and the overburden at the
Shiziping dam site of
Zagunao River is 101.5 m
Qingyi
River
104.46 Maotan dam site The overburden distribution of the mainstem of the Qingyi River is thicker in the upper and
lower reaches and shallower in the middle
The distribution of tributary
overburden is irregular, and
its thickness fluctuates
greatly
①Maotan dam site ①Qiaoqi-Xiaoguanzi dam site ①Below the Xiaoguanzi
dam site (except Maotan
dam site)
Tuojiang
River
49.9 Guankou dam site The Tuojiang main stem overburden is shallower The riverbed overburden of
the tributaries in the upper
reaches Tuojiang River is
thick and irregular
Fujiang
River
107 Yinping dam site The riverbed overburden in the main stem of Fujiang River is relatively shallow, and the
thickness is less than 40 m
The riverbed overburden of
the tributary Huoxi River is
relatively thick, but there are
fluctuations
Jialing
River
73 Xianyaba dam site The deposit layer of the main stem of Jialing River is 10–25 m. In the upper reaches of
Jialing River, the overburden is shallower, while in the middle reaches, the overburden is
thicker, and the lower reaches to the Yangtze River, the overburden is shallower
The riverbed overburden in
the upper tributaries of the
Jialing River is relatively
thick. Among them, the
Bailongjiang riverbed
overburden is thicker in the
middle reaches and
relatively thin in the upper
and lower reaches. The
majority of the deposit layer
of the Baishui River basin is
40 m thick or more. The
middle and lower reaches
of the Jialing River
tributaries have a shallower
overburden
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Yan et al. Riverbed Overburden in Southwest China

overburden layer is 25–40 m. Downstream of the Songta dam the
overburden is 10–15 m. The overall thickness is shallow in the
upstream and downstream portions and deep in the middle
(Figure 2).
2.1.3 Lancang River
The overburden in the main stem of the Lancang River
between the upper and lower reaches has a consistent
thickness of between 20 and 30 m. Analyzing the
overburden thicknesses based on longitudinal distribution;
the upstream is thinner, the middle is thicker, and the
downstream is thinner. The tributaries to the Lancang have
relatively thin overburden layers (Figure 2).
2.1.4 Yellow River (Above Lanzhou)
The Yellow River basin (above Lanzhou) has a relatively thin
overburden thickness. The main stem riverbed overburden is
2–26 m thick and mainly composed of pebbles and gravel. The
overburden of the tributary Tao and Huangshui Rivers are less
than 40 m, except at the Jiudianxia dam site in the Tao River
where it is 56 m. The overburden of the Tao River is thicker in the
upper reaches and shallower in the middle and lower reaches.
2.1.5 Yarlung Tsangpo River
The thickness of the overburden in the middle and upper reaches
of the Yarlung Tsangpo River can reach more than 120 m while
the wide valley area in the lower reaches may reach hundreds of
meters. The maximum thickness of the overburden in the Niyang
and Lhasa Rivers is more than 100 and 400 m respectively. At the
Qushui Bridge location on the Yarlung Zangbo (Tsangpo?) River
in Zedang, the thickness of riverbed overburden is more than
50 m, and in the Lhasa area, the thickness of riverbed overburden
is 123 m. The maximum thickness of the sand and gravel layers in
the adjacent ancient riverbed as measured by electromagnetic
exploration is 600 m (Yu and Shao, 2020).
2.1.6 Other River(s)
For other river basins, such as the Lixianjiang, Irrawaddy,
Nanpanjiang, Yuanjiang, and others on the Yunnan-Guizhou
Plateau, the overburden of the riverbeds are predominately
FIGURE 1 | Distribution of maximum thickness of riverbed overburden in the upper reaches of the Yangtze River.
Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 8957695
Yan et al. Riverbed Overburden in Southwest China

considered shallow (<40 m). More specifically the relative
overburden of the Lixianjiang and Irrawaddy are shallow,
while the Nanpanjiang and Yuanjiang are thicker in comparison.
2.2 Regional Thickness Variation
Characteristics
Summarizing the exploration data related to riverbed
overburden; the results show that the distribution of deep
riverbed overburden presents regional characteristics.
According to the characteristics of topography, terrain, and
the climate of the basin, they can be roughly divided into the
Tibetan plateau area, marginal alpine valley area, Piedmont or
marginal mountainous plain area, Yunnan-Guizhou Plateau area,
and the Sichuan Basin area.
The riverbed overburden is the thickest in the alpine valley
areas at the margin of the Tibetan Plateau, namely the Longmen
and Hengduan Mountain areas, and it is of a consistent thickness
along the following mountain ranges: Xuebaoding—Siguniang
Mountain—Gongga Mountain—Jinping Mountain—Yulong
Snow Mountain—Meili Snow Mountain. The contact zone
between the mountains and the Yunnan-Guizhou Plateau, and
the Piedmont area of the contact zone between the mountains
and the Sichuan Basin has an overburden that is thicker than the
mountain range in the previous example, however, the thickness
of the overburden is greater in the upper river valley than in the
lower river valley where it runs into the basin. The Jialing, the
Tuojiang, the Fujiang, the upper reaches of the Minjiang, and the
Qingyi Rivers all show the thickest overburden in the
Longmenshan section. The overburden becomes thinner after
FIGURE 2 | Spatial distribution of maximum thickness of riverbed overburdens of main rivers in southwest China.
Frontiers in Earth Science | www.frontiersin.org June 2022 | Volume 10 | Article 8957696
Yan et al. Riverbed Overburden in Southwest China

it enters the Sichuan Basin. The riverbed overburden in the
Yunnan-Guizhou Plateau and Sichuan basins is relatively
shallow. When the Jialing, Fujiang, Tuojiang, Minjiang and
other associated rivers enter the Sichuan Basin the overburden
becomes thinner. Additionally, the riverbed overburden of the Li,
Yuan, and Nanpan Rivers in the Yunnan-Guizhou Plateau is
generally less than 40 m. For example, the thickness of the
riverbed overburden of the Nanpan River is 15–25 m. After
the Nujiang, Irrawaddy, and Lancang Rivers flow into
Myanmar and Laos, the riverbed overburdens are generally
thinner. Finally, the riverbed overburden is not uniform across
the inner part of the Tibetan Plateau, but it is thinnest when it
reaches the periphery. For example, the eastern margin of the
Tibetan Plateau is the headwaters area of the Nujiang, Lancang,
Jinsha, and other major rivers and the thickness of the riverbed
overburden is predominantly less than 20 m. Additionally the
riverbed overburden in the basins above Lanzhou on the Yellow
River is shallower. However, the main stem of the Yarlung
Tsangpo River and its tributaries, the Lhasa, and Nianchu
Rivers all have a riverbed overburden that is more than 100 m
in most areas. Interestingly the overburden tends to decrease near
the Himalayan mountains.
In summary, the main rivers in Southwest China generally
have deep riverbed overburden, where the thickness is generally
between 10 and 100—m, local sections can reach into the
hundreds of meters. Generally, the thickness of riverbed
overburden is shallower in the upper reaches of the Tibetan
Plateau, thickest in the marginal mountain areas of the middle
reaches, and shallower in the contact areas of the lower reaches.
The Yunnan-Guizhou Plateau and Sichuan Basin show a shallow-
thick-shallow distribution pattern. Overall, it has the
characteristics of thickening gradually from the basin edge to
the plateau slope, and the river overburden also has the
characteristics of thickening from the downstream to the
upstream (Figure 2).
3 PRELIMINARY STUDY ON FORMATION
MECHANISMS OF RIVERBED
OVERBURDEN
The formation types of riverbed overburden are complex.
Different rivers, different sections of the same river, and even
different time periods of the same river section may have different
causes (Luo, 1995). Based on the engineering geological data, this
paper explores the distribution characteristics of the existing
riverbed overburden, and then analyzes the formation
mechanisms of the riverbed overburden in the primary rivers
of Southwest China.
3.1 Characteristics of Overburden Material
Composition
The main sources of riverbed overburden in Southwest China are
mainly alluvial-proluvial deposits, glacial (glaciofluvial, moraine)
deposits, dammed lake deposition, and landslide and collapse
deposits.
3.1.1 Alluvial-Proluvial Deposits
The alluvial-proluvial sediments in riverbeds in Southwest China
can be classified as coarse as boulders or fine to sand (and loess?).
The dominant species is gravel that is mainly composed of drift
pebbles intercalated with sand, others are gravelly sand and sandy
structure with good sorting and roundness. In addition, the sand
is usually lenticular in the horizontal or has an oblique laminar
distribution. The bed of the mountain river is narrow, the lateral
swing is limited, the floodplain is not developed, and downcutting
is obvious. The normal thickness of alluvial-proluvial deposits is
generally less than 30 m (Xu et al., 2008).
Alluvial-proluvial deposits are mainly distributed in the upper
and lower portions of the deep and huge thick overburden
reaches. The bottom layer is partially cemented and generally
unable to move. The deposits are widely distributed in riverbeds
and terraces.
3.1.2 Glacial Deposits
(1) Characteristics
Glacial deposits are one of the most important quaternary
deposits in northwest Yunnan, Western Sichuan, and Tibet.
Glacial moraines and glaciofluvial deposits are the major
contributors to the overburden layers. The glacial deposits in
China almost all belong to giant-grained or boulder soil and
crushed stone soil. The glacial deposits lack intermediate grain
size (or discontinuous deposits), and the lithology mainly
depends on the lithology of local parent rocks (Zhang et al., 2009).
(2) Distribution range
Glacial deposits mainly exist in riverbank slopes and terrace
zones. With the development of hydropower and accurate drilling
data of the riverbed, there is a range of glacial deposits differing in
thickness at the bottom of some riverbeds.
The glacial deposits found in the studied riverbeds are
distributed into separate layers. This distribution varies from
the central Longmen mountains to the Hengduan mountains.
Specifically, the glacial deposits were distributed in different
periods in the mountains between Minshan to Mangkang and
the Yunling river tributaries, and around the Yulong Mountains,
Haba Mountains, White snow Mountains, Gongga Mountains,
Siguniang mountains and other high mountains. The deposits are
oriented around the river tributaries of the riverbeds and valley
terraces. For example, there are several glacial deposits in the
Minjiang and Qingyi rivers around Siguniang Mountain. The
glacial deposits in the Minjiang River are mainly distributed in the
upper reaches of the main stem, the tributary Erhe River, and the
upper reaches of the Zagunao River. This is especially noted in the
northeast and east slopes of Siguniang Mountain. Glacial deposits
in the Qingyi River are distributed in the upper east river basin, in
the southwest slope of Siguniang Mountain and the southeast
slope of Jiajinshan Mountain.
Glacial deposits in the Dadu River around the Gongga
Mountains are distributed in the middle and upper reaches of
the river. The main stem is mainly distributed in the reaches
between the Qionglai Mountains and the Daxueshan Mountains.
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Yan et al. Riverbed Overburden in Southwest China

Glacial deposits in the tributaries are mainly distributed in the
upper reaches of the Nanya River, the upper reaches of the
Tianwan River, the lower reaches of the Yanzigou River, the
whole basin of the Wasihe River and the Geshizahe River, etc.
Glacial deposits in the Yalong River around Saneiri are
distributed in the Litang River section on the eastern slope of
the Shaluli Mountains. The glacial deposits of the Jinsha River
near the Yulong Mountains are distributed in the upper reaches
between the Shaluli Mountains and the Mangkang-Yunling
Mountains and the Qizong-Tacheng section of the Tiger
Leaping Gorge.
The sequence of distribution of glacial deposits is given in the
following examples. Along the margins of the Tibetan Plateau and
in mountain canyon valleys glacial deposits are primarily
distributed at the bottom and middle layers of the riverbed. At
the contact zone between the mountains and the Yunnan-
Guizhou Plateau and the Sichuan Basin, the deposits are
primarily distributed on the river terraces. In the Jinsha River,
for example, glacial deposits are located at the bottom of the
riverbed in the upper reaches. Near Shigu, they are in the middle
layer of the riverbed mixed with other deposits. In the lower
reaches they are above the river terraces. Essentially, the lower
altitude of the Sichuan Basin accounts for the higher distribution
of glacial deposits. For example, the shaft overburden on the left
and right sides of the Xiluodu dam site is mainly composed of
glacial and glaciofluvial deposits with a thickness of 42–59 m (He
and You, 2013). At the same time, the middle and lower layers of
the Yarlung Tsangpo riverbed are mostly glacial deposits
(Figure 3).
Analyzing longitudinal distributions between Hengduan
Mountain and Longmen Mountain, no glacial deposits were
found in the riverbed of the Jialing and Yellow Rivers in the
north, or from the Nujiang and Lancang Rivers in the south,
however glacial deposits were found on the terraces of the river
valleys. In the middle area of the previously mentioned river
sections, glacial deposits are found in the riverbed. In addition,
there are large accumulations in the valleys of the Hengduan
Mountains, many of which are of single or mixed glacial origins.
Tu et al. (2008) found that since the Quaternary, which was
influenced by glacier movement, a layer of glaciofluvial and
moraine deposits has been widely accumulated along the
middle reaches of the Dadu River at an altitude of
700–1500 m, with a maximum thickness of more than 100 m
and the thinnest point is just a few meters.
(3) Geogenic (Formation) Mechanism(s)
The eastern part of the Tibetan Plateau is divided into three
glacial periods: the last glacial period, the penultimate glacial
period, and the antepenultimate glacial period. There were no
quaternary ice sheets, but some intermittent distribution of ice
caps and mountain glacial centers formed by high mountains,
planation surfaces, and high-level basins (Li et al., 1991). The high
mountains in the Hengduan Mountain range have experienced
several glacial periods. As the glaciers melted, the water carried a
large amount of moraine debris along the way that finally
collected in the river valleys and basins. In the middle and
upper reaches of the Dadu River, Gongga Mountain is the
FIGURE 3 | Distribution of Main Causes of RiverBed Overburden in Southwest China (Distribution of ancien t dammed lakes, according to Wang Y. S. et al., 2007;Li
et al., 2010;Zhang et al., 2020;Wang et al., 2021.Distribution of Xigeda Formation, according to Xu and Liu, 2011).
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most concentrated and developed area of glaciers in the
Hengduan Mountains. The moraines of the last glacial period
(Dali Glaciation of late Pleistocene) are distributed below 2000 m,
and the ancient glacial erosion and accumulation remains are
distributed in bands from the center to the periphery downstream
of these mountains. The lower limit of the distribution ranges
from about 2000 m above sea level on the edge of the plateau to
more than 5,000 m above sea level in the interior of the plateau
(Shi and Zheng, 1995).
3.1.3 Dammed Lake Deposits
(1) Characteristics
Dammed lake deposition is primarily composed of
interbedded silty clay and silty fine sand. It is typically of
uniform grain size in thick layers ~ huge thick layers with
obvious horizontal bedding, and few other fossils except
spores and pollen (Zhang et al., 2020).
(2) Distribution range
The dammed lake depositions are primarily found as a mix
with other deposits or as a formation of separate layers generally
found in riverbed overburden (Figure 2). These deposits are
consistent with those of dammed paleolakes. For example, the
ancient Shigu Lake retains thick lacustrine sediments in the
middle of the riverbed of the Tiger Leaping Gorge section, the
Jinsha-Longjie section of the ancient Longjie Lake, as well as the
riverbeds of Zhaizicun, Xuelongbu, Temi, Qiaojia Lake, Xigda
Lake, Taoyuan Lake, Daju Lake, Shigu Lake, Jintang and other
dammed paleolakes have thick lacustrine sediments.
The dammed paleolakes existed in Diexi and downstream in
the Minjiang River Basin, as well as in the Zagunao River, a
tributary of the Minjiang River (Wang L. S. et al., 2007). These
have layers of dammed lake deposits in the Futang and Diexi
sections of the main stem of the Minjiang River, Shiziping in the
tributary of the Zagunao River, and the Yuzixi II in the tributary
of the Erhe River.
The Tongzilin dammed paleolake in the Yalong River is
mainly distributed in the Tongzilin-Ertan section. The
Lingguan River dammed paleolake (Li et al., 2010) in the
upper reaches of the Qingyi River has a thick layer of
dammed lake depositions in the riverbed segments such as
Xiaoguanzi, Minzhi, and Zuijugou.
Dammed lake depositions are widely distributed in the middle
and upper reaches of the main stem of the Dadu River, and
dammed paleolakes (deposits?) such as Kairaocun and Jiajun
have been discovered (Zhong and Ji, 2012).
Since the late Pleistocene, several river-blocking events have
occurred in the south-north river system in Southwest China,
forming several dammed lake deposits (Li et al., 2010). The
dammed lake depositions are roughly distributed along the
Gongga Mountain-Yulong Mountain line and extend along the
Jinsha and Dadu River valleys to the Tibetan plateau. The specific
distribution area is in the intersection of the Tibetan Plateau,
Yunnan Plateau, and Sichuan Basin where there is an abrupt
change in elevation.
(3) Formation Mechanism
Dammed lake deposits are from lakes formed by dammed or
regional subsidence (fault depression or depression) due to the
river flowing through the area. For example, the Xuelongnang
and Temi dammed paleolakes in the upper reaches of the Jinsha
River were probably formed by large landslides triggered by
seismic activity in the area (Chen and Cui, 2015;Chen
J. et al., 2021). The dammed paleolakes in the Benzilan-
Qiaojia section of the Jinsha River are mostly formed by
landslides or glacial deposits blocking the Jinsha River (Zhang
et al., 2020).
Due to the continuous uplifting process of the Tibetan plateau,
the increased erosion of the river has resulted in a large number of
dammed lake deposits being distributed downstream. There may
be multiple dammed river events in the same river section, many
of which are preserved in the formation of terraces. For example,
Zhang et al. (2007) studied the lacustrine lamellar clay layer and
its formation by landslide debris flow blocking the river in the
early to middle of the late Pleistocene as found under the fourth
terrace on both sides of the valley in the Benzilan area of the
Jinsha River. Some researchers have also found landslide dammed
paleolakes in the Lancang and Nujiang River basins, mainly
preserved on river terraces. Additionally, the Gushui paleo-
lake in the Lancang River in the Gushui area of Deqin
County, Yunnan Province, was formed in the late last
interglacial period of the early Pleistocene and the early
interglacial period of the last Pleistocene, it was found at the
base of the third and fourth terraces (Zhang and Zhao, 2008). A
further example is the sand-layer of a barrier lake formation that
is about 100 m thick in the ninth terrace of the Nujiang River
valley in the Daojie-Huitongqiao section which was formed
during the middle and late Pliocene between 4.2 and 2.6 Ma
(Zhao et al., 2012).
Based on the current dating of the riverbed overburden, it has
been concluded that the dammed lake depositions have formed
startingfromtheLatePleistoceneandHoloceneerasandcontinued
to the present. The upper age limit of dammed lake depositions in
the Benzilan-Qiaojia section of the Jinsha River is from the late
Pleistocene, and the lower age limit is from the late to the early
Pleistocene (Zhang et al., 2020). The dammed lake depositions in the
middle reaches of the Dadu River were formed from 18 Ka to 11 Ka
(Li C. Z. et al., 2015). The lacustrine deposit at the Tongzilin dam site
in the Yalong River started at 25 Ka and lasted to ~10 Ka (Ding et al.,
2021). The dammed lake depositions in Dixi, from Dixi to the lower
reaches of the Minjiang River and its tributary the Zagunao River,
weredatedtoabout20KaBP(Wang Y. S. et al., 2007). This period
from the late Pleistocene to Holocene, is also the period of
accelerated rise of the Tibetan plateau (Li et al., 1979).
There are also some lacustrine sediments widely distributed in
Southwest China, mainly preserved in terrace form, such as the
lacustrine sediments of the Xigda Formation, which are 4.2–3.3 or
2.6–1.8 Ma BP in the middle and late Pliocene or to the early
Pleistocene (Zhao et al., 2008). The Longjie silts along the valley of
the Jinsha River from Sanduizi of Panzhihua City to Baimakou of
Wuding County belong to the late Pleistocene (Li H. L. et al.,
2015).
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3.1.4 Geological Hazard Deposits
(1) Characteristics
Colluvial deposits are a primary source of riverbed overburden
in Southwest China. The colluvial deposits are a loose, non-
cohesive soil containing rock fragments. The particle size
composition of landslide deposits varies greatly, and the
particle size distribution range is very wide. Debris flow
deposits are mainly composed of gravel, sand, silt, and clay,
and the grain gradation varies significantly.
(2) Distribution range
The deposits formed by geological disasters, such as hillside
collapse, landslide and debris flow, are primarily distributed in
mountain rivers where the riverbed has a large gradient. These
deposits have a short transient time in the river and are usually
found in small, isolated pockets dotted in the longitudinal
direction of the river. Examples of this are in the Wudong
section of the Jinsha River and the Xiaoguanzi section or the
Qingyi River. The sediments are seldom distributed in bands,
instead they are mixed with other sediments to form layers during
river cutting. This can be seen in the Tiger Leaping Gorge section
of the Jinsha River and the Danba to Luding section of the Dadu
River. Large landslides and debris flows are typical mechanisms in
blocking a river. Once the dam breaks, the depositions will form a
long strip-shaped distribution pattern downstream. As the
Tibetan plateau continues to rise, the capacity for river erosion
is increased. Consequently, the remains of the disaster sediment
are eroded, leaving little residual sediment in the riverbed.
However, a large quantity of sediment is retained in the valley
terrace and bank slopes. For example, the Xuenongnang dammed
paleolake on the upper reaches of the Jinsha River was formed by
an earthquake-induced landslide. After the dam broke, the
depositions were distributed within a range of 3.5 km from the
downstream side (Chen and Cui, 2015). Additionally, disaster
deposits exist in the form of accumulation predominantly along
riverbanks as a result of the deep river cutting.
(3) Formation Mechanism(s)
The formation mechanisms for geological hazard deposits are
heavily influenced by external factors such as earthquakes and
rainstorms, this may also be coupled with rapid uplift and deep
river cutting. This is a result of the very active internal and
external geodynamic processes in the eastern margin of the
Qinghai-Tibet Plateau. Both large and giant landslides, hillside
collapses and debris flows occur frequently in mountain canyon
valleys, forming deep accumulations in local areas. The
concentrated occurrence time of paleo-landslides in mountain
canyon valleys of the Dadu River basin and the upper reaches of
the Minjiang River basin is 20–30 Ka, and the highest frequency
of the paleo-landslides in the Dadu River is 15–25 Ka, speculated
to have occurred in the late Pleistocene. The strong tectonic
activity of the Dadu River Basin resulted in frequent large-scale
landslides, consequently a large volume of paleo-landslide
deposits remains in the Dadu River basin today (Zhang et al.,
2021). The Xuelongnang and Temi dammed paleolakes in the
upper reaches of the Jinsha River were probably formed by large-
scale landslides triggered and blocked by paleo-seismic activity in
this area (Chen and Cui, 2015;Chen Y. et al., 2021). The age of the
existing geological disaster deposits is mainly in the 20–30 Ka
range, which is in the glacial peak of the last glacial period.
3.2 Geogenic Mechanisms
3.2.1 Overburden Formation Mechanisms in Primary
Rivers
The riverbed overburden thickness of the main rivers in
Southwest China presents a shallow layer with a single origin
that is primarily alluvium or alluvial-pluvial. The formation of
deep or huge thick riverbed overburden is the result of multiple
genesis. The controlling factors vary in different reaches of each
river. According to the material characteristics of current riverbed
overburden, the genesis of main river overburden is discussed.
(1) Minjiang River
The riverbed overburden in the Minjiang River basin was
formed by dammed lake deposition, glacial sediments and
alluvial-proluvial deposits. From the regional distribution data,
there are dammed lake deposits of differing thickness in the
riverbed of the main and tributary rivers in the upper reaches of
the Minjiang River, especially in the Diexi section. The bottom of
the overburden layer of the main stem and the left bank
tributaries of the Minjiang River in the east slope of
Siguiniang Mountain is mostly glaciofluvial sediments.
The deep riverbed overburden in the upper reaches of the
Minjiang River is mainly distributed in the middle portion of the
Longmen Mountains. During the uplifting process of the Tibetan
plateau, the main peaks of Qionglai and Sionglai Mountains,
formed from the surrounding highlands and above the snow line,
becoming the local center of quaternary glaciation. During the
glacial period, glaciers developed on Qionglai Mountain, During
the interglacial period, the glaciers melted anddeep river cutting
occurred carrying a large amount of the glacial moraine deposits
into the Minjiang River which was subsequently deposited in the
Minjiang River valley. During the uplift of Longmen Mountain,
seismic activity resulted in the river also uplifting causing the
deep valley to form, the result of which was increased erosion
deposits blocking the river. Wang L. S. et al. (2007) studied
landslide derived and other small-scale ancient barrier lakes in
Dixi, Maoxian, Wenzhen, Mianyang, Guogou, and Lixian in the
upper reaches of the Minjiang River. Thi indicated that
earthquakes and rainstorms lead to (hillside) collapse,
landslide and debris flows blocking the river, or the landslide
front is deposited directly on the early overburden, which resulted
in local accumulation of deep overburden.
(2) Qingyi River
The deep riverbed overburden is mainly deposited in the
upper reaches of the Qingyi River. The origin of the
overburden in the main source Baoxing River basin is
complex. The riverbed overburden in the East River showed
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that the bottom of the river is ice-water sediment, and the middle
layer is alluvial-pluvial, debris flow accumulation, colluvial and
barrier deposits, and the surface layer is the modern river
alluvium. The riverbed of the Xihe River has a deep
overburden. The bottom of the riverbed overburden is an
alluvial-proluvial layer, the middle is a dammed lake
deposition layer, and the surface is a modern river alluvial
layer. These are typical at the Chujugou Power Station sluice site.
The source of the Qingyi River is the Qionglai and Jiajinshan
Mountains, both of which are areas with intense crustal uplift
occurring from the late to recent periods. The local climate
characteristics, leading to frequent geological disasters such as
debris flow and river blocking events, and forming dammed lake
deposition in the riverbed are primary sources. The thickness of
the river deposits at the Xiaoguanzi dam site area reaches a
maximum depth of 87 m and has 10 distinct layers according to
its origin. The sixth and eighth layers are dammed lake deposits
and the fifth and seventh layers are debris flow deposits; it was
found that the left bank of the Baoxing River was blocked by two
extra-large debris flows in Guangou (Peng, 1996). In addition, the
Donghe River basin is located on the southeast slope of Siguniang
Mountain. The glaciation of Siguniang Mountain led to the
existence of glacial deposits at the bottom of the riverbed.
(3) Dadu River
The deep overburden in the Dadu River basin is distributed
continuously and stably in “bands”and has the characteristics of a
continuous regional development in spatial distribution.
According to the material origin of the overburden, the
Dajinchuan Section (above Danba County reach) is composed
of dammed lacustrine and alluvial-pluvial deposits. The middle
and upper reaches of the Dadu River (above Dagangshan) are
mainly composed of glaciofluvial, dammed lake, and alluvial-
diluvial deposits. The lower reaches of Dagang Mountain are
mainly composed of alluvial-pluvial deposits.
Tectonic movement controlled the development of the Dadu
River valley. Based on the north-south structure control the Dadu
River valley above Shimian county was an “L”shape. Below
Shimian county, due to the east-west uplift block of Shimian-
Ebian, the river turns sharply to the east. Under the action of neo-
tectonic movement, the broad valley basins of the Jinchuan
intermountain depression, the Jiajun tectonic adaptive basin,
the Hanyuan fault depression basin, and other wide valley
basins were formed.
During the late Pleistocene Dali glacial period, moraines piled
up in the middle and upper reaches of the Dadu River valley
forming weir dams. Meanwhile, the terminal moraine of the
tributary also blocked the Dadu River valley forming barrier lakes
allowing lacustrine sediments to be deposited in the riverbed. As
the climate warmed in the postglacial period, the melting of the
glaciers and the increased flowing water broke the dam. The
glacier descending height was higher in the Dajinchuan section
increasing the strength of the erosion. The glacier descending
height was lower in the upper part of Dagang Mountain, the weir
dam was taller, the lake deposits were thicker, the river erosion
had not cut through the moraine, and the glacier accretion
overburden was developed before the current riverbed. The
moraines in the lower reaches of Dagang Mountain are mainly
preserved on the river terraces (Shi, 1986).
The deep overburden of the Dadu River is the result of the
composite accumulation of tectonic subsidence and glaciation
(Shi, 1986). The Dajinchuan reach is a tectonic multilayer
accretion, and the middle and upper reaches of the Dadu
River are the products of the last glacial period (Dali
Glaciation of late Pleistocene) and tectonic-controlled adaptive
valley basins (such as Jiajun). Areas below Dagangshan are
mainly the result of tectonic control, such as faulted basins (e.
g. Hanyuan basins).
(4) Yalong River
The riverbed overburden in the upper reaches of the Yalong
River is relatively shallow, generally less than 40m, primarily
located in the eastern margin of the Tibetan plateau, and the river
erosion rate is relatively high. The deep overburden of the Yalong
River is mainly distributed in the middle and lower reaches. The
deposit layer can be divided into three parts along the depth, the
upper layer is modern river deposit; the middle layer is composed
of alluvial, pluvial, barrier lacustrine, and is comparatively thick;
the bottom layer is alluvial and colluvial deposits (Wang et al.,
2006;Wang Y. S. et al., 2007). The overburden of the lower
reaches of the Yalong River is composed of the middle layer
attributed to deposits originating from the Tongzilin dammed
lake. The dam site of the Tongzilin Hydropower Station has
developed a 51 m thick deposit profile, which is generally
composed of a lower alluvial layer (10 m thick), middle
lacustrine layer (26 m thick) and upper alluvial layer (15 m
thick) (Ding et al., 2021).
The origin of overburden of the Upper-Yalong River is
dominated by deposits that are caused by blocking, while the
middle and upper Yalong River sections are dominated by
deposits typically associated with a composite origin. The deep
overburden is caused by the second layer accretion sequence, and
the dating of the accretion layer are 10–20 Ka bp (Wang Y. S.
et al., 2007). This period was during the last glacial maximum
when glaciers were widely developed. The decreased river flow
coupled with the low potential energy and increased glacial debris
resulted in the river filling and damming, forming a thick multi-
geogenic accretion overburden. During the post-glacial period,
the rivers incised rapidly, but did not cut through the accretion
layer. Therefore, the deep overburden in the Yalong River is a
climatic geogenic model formed during the last glacial maximum
(Wang L. S. et al., 2007).
(5) Jinsha River
The riverbed overburden in the Jinsha River can be roughly
divided into four sections according to the thickness: the source
area (above the Lawa dam site) (<40 m), the lawa dam site to Daju
section (>40 m), the Daju to Panzhihua city (<40 m), and the
section below Panzhihua city (>40 m). Among them, the riverbed
overburden in the source area and Daju to Panzhihua city is
thinner, with a predominant alluvial layer, the cause of which
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may be mainly tectonic. The source area is in the Tibetan Plateau,
which has been in the process of continuous uplift, and therefore
the river erosion capacity is greater than the accumulation rate.
The main reason for the thin riverbed overburden from Daju to
Panzhihua city may be that it is in the same tectonic region, the
Yangtze platform, so the uplift and subsidence of the crust and
river erosion are at roughly the same level.
The separate layers of overburden from the Lawa dam site to
the Daju section are composed of dammed lake deposits, glacial
deposits, and deposits of geological hazards. Wang (2009)
proposed that the thick riverbed overburden in this reach was
formed by the ancient Gulongpan fault basin under the influence
of neo-tectonic movement.
The Jinsha River overburden from Panzhihua city to Yibin city
are primarily composed of alluvial-proluvial sediments in the top
and bottom layers, and dammed lake deposits or mixed
sediments in the middle layers. Ge et al. (2006) studied the
Jinsha River from Tuoding to Yibin city and proposed that
climate change directly affected the precipitation conditions of
the Jinsha River basin. During the rainy period, the river channels
were strongly incised, developing deep grooves, and included
mass wasting of the bank slopes. During dry periods, the Jinsha
River cuts down slowly, and the river channels form large
deposits.
(6) Yarlung Tsangpo River
According to the available geological data, the riverbed
overburden of the main Yarlung Tsangpo River and its
tributaries, such as the Lhasa and Nianchu Rivers, has a large
burial depth, mainly affected by glaciation and tectonic factors.
The stratigraphic structure of the longitudinal riverbed
overburden in the Yarlung Tsangpo River is basically a binary
structure, with the upper layer being a modern alluvial-pluvial
layer and the middle and lower layers primarily consisting of
glacial moraine deposits.
The main stem of Yarlung Tsangpo River is characterized by a
narrow and wide valley and thick river and lake deposits. Wang
et al. (2014) studied the riverbed thickness from Xetongmen to
the Yalu Tsangpo Canyon and found that, due to the variation in
uplift rates of different river sections, the uplift rates of the canyon
section were high, while the uplift rates of the wide valley section
were low, the bedrock surface of the canyon river channel were
several hundred meters higher than the sediment-bedrock
interface of the broad valley river section in its upper reaches.
The result was the formation of a huge “sediment reservoir.”Over
the past million years, the pebble sediment had been filling this
sediment reservoir forming a thick layer. At the same time, it also
caused the valley floor to rise and widen, turning the v-shaped
valley into a u-shaped valley (Figure 4).
Lacustrine sediments of dammed paleolakes are widely
distributed along the valley of the Yarlung Tsangpo River
basin across the southeastern Tibetan Plateau. There are
several ancient dammed lakes in the middle and lower reaches
of the Yarlung Tsangpo River, such as Dazhuka, Jiedexiu, Gega,
Yigong, Zhongyu, Talu, Guxiang, Songzong, Yupu, Dongjiu,
Lulang and others. The spatial distribution of these paleolakes
may be controlled by the north-south normal fault system or
follow active fault lines such as the Jiali fault. The paleolakeshad
developed since the last glacial period, mainly during the last
glacial maximum and the early Holocene, lasting for thousands to
tens of thousands of years (Wang et al., 2021).
(7) Other river(s)
The deep riverbed overburden in the Jialing and Tuojiang
River basins are mainly distributed in the Longmen Mountain
area. Among them are the Jialing River tributaries; the Bailong
River, the Baishui River, and the Fujiang branch of the Liuhuo
River have developed deep overburden with zonal distribution.
These deep overburden layers are mainly composed of alluvial or
alluvial-proluvial deposits. Additionally, there are collapse
FIGURE 4 | Longitudinal profiles of interface between bed rock and sediment deposits along the Yalu Tsangpo River Valley (Wang et al., 2014, modification).
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deposits on the side of the river and dammed lake deposits in
some sections of the river.
The deep riverbed overburden of the Nujiang, Yellow, and
Lancang Rivers are mainly distributed and are similar in having a
point-type distribution, where its origin is mainly formed by
collapse, landslides, and debris flow.
3.2.2 Discussion on the Geogenic Mechanisms of
Thick Riverbed Overburden in Southwest China
According to existing and ongoing research achievements the
formation of deep overburden is very complicated with a deposit
composition of fluvial and non-fluvial origins.
The change of dynamic conditions in the river system can only
control the self-organization process of low-level streams (Yi et
a1., 2021). It can cause the river to accumulate and cut down, but
the influence is generally limited to a certain reach of a river, the
time scale of the influence is short (10–1000 a), and the
overburden is shallow (<40 m) (Maddy et a1., 2001).
There are many non-fluvial factors (contributing to the
geogenic mechanisms), including erosion base level change,
tectonic movement, climate change and so on (Bridgland,
2000;Alexey et al., 2019;Delaney and Adhikari, 2020); Xu
et al. (2008) proposed that the existence of deep riverbed
overburden in Southwest China was directly related to the
increase and decrease in sea level since the Quaternary.
However, some studies found that the influence of sea level
rise and fall on river geomorphology development was only
limited to the reach near the estuary (mainly downstream). It
has little impact on the middle and upper reaches of inland rivers
(Merritts et a1., 1994).
Tectonic movement and climate change are two important
factors controlling the riverbed overburden thickness. However,
the coupling of tectonic movement and climate change in the
evolution of riverbed overburden is of primary concern.
Tectonic movement is a major contributor to the formation
and evolution of rivers. Macroscopically, the Tibetan Plateau
controls the sedimentary and biogeochemical exchange between
mountains and oceans, and there are dynamic linkages and
coevolution between tectonic plates and river systems, the
macro-structure of the river system in the eastern Tibetan
Plateau is controlled by tectonic movement (Yi et a1., 2021) A
large number of low-temperature thermochronology studies have
revealed the rapid exhumation of the southeastern margin of the
plateau in Cenozoic stages at about 60–40 Ma BP, 30–20 Ma BP
and 20–0MaBP(Clift and Sun, 2006;Liu et al., 2018). The South
Slope of the Western Kunlun Mountains Plateau is the highest,
and the southeast part of the Qinghai-Xizang Plateau, which
flows east to the north of Tibet and is the source of the Yangtze
and Yellow rivers, is the lowest, and descends by a steep slope
(fault) to the mountains on the border of western Sichuan and the
Yunnan-Guizhou Plateau. This general tilt from northwest to
southeast is undoubtedly an important factor in controlling the
flow of rivers like the Jinsha River, the Salween River, and the
Lancang River (Li et al., 1979). On a smaller scale, the integral
block uplift of the Tibetan plateau also has local differences. From
the point of view of the specific river reach, the river spans
different tectonic units, and the ascending and the up-and-down
movement between tectonic units further contribute to river
erosion and accumulation change, forming the riverbed
overburden aggradation layer (Xu et al., 2008). For example,
the longitudinal profile of the Dadu riverbed from the upper to
the lower reaches shows a gentle-steep-gentle fold shape. The
thickness of the valley deposits on the steep slope is different, the
thin part is about 20 m, and the steep slope is in the Sichuan-
Yunnan south-north tectonic belt, which is obviously related to
the continuous uplift of neotectonics (Luo, 1995).
The variation in topography caused by tectonic movement not
only affects slope stability, but also affects climate. The uplift on
the eastern margin of the Tibetan Plateau, increases the potential
energy of flowing water to carry out strong erosion and river
cutting. There are various types of gravity based physical and
geological processes active on these slopes. At the same time, in
the middle-lower reaches of many rivers in Southwest China, the
longitudinal gradient of the riverbed becomes steeper as it moves
down the valley. As the steepness increases river erosion and
cutting are also increased, and the physical and geological effects
such as valley slope collapse, landslide and debris flow also
increases, and these in turn form dammed lakes and
colluvium deposits. The periodic uplift of the Tibetan Plateau
and the resulting water and heat conditions, combined with
global climate changes, have caused four alternating glacial
and interglacial periods to occur on the plateau (Li et al.,
1979). Some mountains in the eastern margin of the Tibetan
Plateau formed glaciers when the mountains uplifted above the
height of the snow line during the last glacial period. Some
mountains at higher altitudes developed quaternary glacial
periods, such as Mt. Gongga and Mt. Yulong, etc., but at least
12 relatively low-lying mountains between 4,200 m and 4,500 m
only developed during the last glacial period. These mountains
were strongly uplifted by about 1000 m due to the Gonghe
Movement in the late Pleistocene causing them to move above
the snow line and thus were only affected by the last glacial period
(Cui et al., 2011). During the late Pleistocene, the glaciers
advanced along these river valleys (Li et al., 1979). As a result
of intense erosion of the upper source of the glacier, the
longitudinal slope of the bedrock valley floor is shallower than
that of the modern riverbed, and the overburden of the valley
floor gradually thickens from the marginal mountain mouth of
the basin to the upper reaches, forming deep deposits (e.g.,
Minjiang River valley) (Luo, 1995). At the same time, the
strong rise of the Tibetan Plateau changed the atmospheric
circulation, the monsoon had become dominant, and impacted
the occurrence and development of quaternary sediments of
different geogenic types in the valley (An et al., 2006).
As the climate warms (the glacial-interglacial transition), glacier
melting accelerates in the alpine valley region, and river flow velocity
and flux increase. According to model simulations, river sediment
discharge increases 3–19 times after 100 years of accelerated glacial
retreat (Delaney and Adhikari, 2020). Such high-speed sediment
discharge leads to a reduction in the sedimentation of the riverbed,
and only thinner sediments can be formed on the original
overburden of the river valley. During the transition from the
interglacial period to the glacial period, the climate gradually
becomes colder and more unstable, which may lead to higher
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Yan et al. Riverbed Overburden in Southwest China

flood frequency and intensity, causing rivers to undercut. As the
climate begins to cool, vegetation coverage decreases or even
disappears, and as the average temperature continues to decrease,
the vegetation coverage also decreases or even disappears. This
cooling combined with the increased freeze-thaw action, more
sediments enter the river, and the combination of higher
sediment flux and lower stream flow cause the increased
deposition of river overburden (Bridgland, 2000;Maddy et al.,
2001;Liu et al., 2006) studied the four sets of lake terraces and
lacustrine sediments of the Yarlung Tsangpo Grand Canyon and its
tributary the Niyang River, using 14C dating found that during
glacial advances the growth of the Zelunglung glacier on Mt.
Namche Barwa formed ice dams and barrier lakes in the Yarlung
Zangbo River, and developed corresponding lacustrine deposits.
During the interglacial period the ice dam broke, the lake water
was released and the lacustrine sediments decreased. In addition,
according to the study, the rapid melting of plateau glaciers during
the interglacial period was the trigger for earthquakes. During the
last deglacial period, the rapid melting and erosion of mountain
glaciers (or ice caps) unloading in the Tibetan Plateau might have
induced frequent seismic activity in this area (Zhong et al., 2021).
The thickness of riverbed overburden is a geological
phenomenon of the coupling effect of tectonic, climatic, and
fluvial sedimentary processes during the Quaternary period. Take
Qiaojia County section of the main stem of the Jinsha River as an
example, the thickness of the overburden layer is nearly 900 m as
revealed by wide-area electromagnetic resistivity sounding
profile. Additionally, its lowest point is 100 m below sea level.
According to the latest drilling data, the platform overburden
thickness of Qiaojia County is more than 733 m (Shan et al., 2021;
Cheng et al., 2022).
The formation of riverbed overburden in the Qiaojia section of
the Jinsha River is mostly controlled by tectonic activity and
fluvial sedimentation. Initially, the deposition process is
controlled by these two processes (Figures 5,6). The Qiaojia
County section is mainly controlled by Xiaojiang fault and
Zemuhe fault, forming a nearly elliptical pull-apart Basin that
formed 1.1 ma ago and ended at about 34 ka (Li et al., 2016;Shan
et al., 2021). According to the 14C dating data, the relative
subsidence time of the basin stopped at about 34 ka, and its
average subsidence rate was about 1–2 mm/year, slightly higher
than the erosion rate of 0.4 mm/year in the Qiaojia section of the
Jinsha River (Huang et al., 2010;Shan et al., 2021), indicating that
it was the river accumulation stage.
At the same time, the tectonic and fluvial processes affected
the evolution of drainage catchment and backflow, and then
affected the sediment. Before the Jinsha River was connected,
there was a south-flow paleo-drainage system extending along
the Xiaojiang fault zone in the Qiaojia section. The upper
reaches of the Xixi River flowed into the ancient Honghe River
system after passing the Paleo-Xiaojiang River. According to
the heavy mineral analysis of paleo-fluvial sediments in the
overburden of this section, the provenance area is close to
Qiaojia basin, which is inferred to be the product of the Paleo-
Jinsha River flowing south. There are three main layers of
paleo-fluvial alluvial facies, accounting for about 50% of the
total thickness (Table 2).ThelateQuaternaryupliftofthe
Yunnan Plateau blocked the Xixihe-Baihetan-Qiaojia paleo-
drainage from flowing south, capturing the upstream erosion
process of the ancient Duiping River (one of the branches of
the Niulanjiang paleo-drainage). This resulted in the reversed
flow of the Qiaojia River segment to the north, through the
Jinsha River (Li et al., 2009;Cheng et al., 2022).
The effects of climate change should now also be considered as
an important factor in the formation of overburden. Luoji
Mountain in the upper reaches of Zemu River on the north
side of Qiaojia Basin still retains well-preserved Quaternary
moraines of Penultimate Glaciation (MIS 6) early stages of the
last Glaciation(MIS 4) and late stage of the Last Glaciation (MIS
2) (Tang et al., 2021) Since the last glacial period, the sedimentary
climate in this region was dry and cold, with less precipitation,
and the sediments were mainly from physical weathering.
According to stratigraphic sporopollen analysis, the
overburden in this section transitioned through a dry-cold
grassland environment, mild-humid environment, warm-
humid environment, and a modern dry-hot climate. The
sporopollen assemblage reveals that the environmental
evolution cycle was consistent with the sedimentary cycle.
Each sedimentary layer of the overburden in this section is
composed of multiple cyclic deposits with a coarse-fine grain
size change (Shan et al., 2021).
FIGURE 5 | Tectonic setting of the Qiaojia Basin (Shan et al., 2021,
modification).
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Yan et al. Riverbed Overburden in Southwest China

Finally, the coupling effect of the three factors is conclusive.
Both tectonic activity and climate change lead to frequent
landslides, floods and debris flows in this section, which then
affect the river geological processes. The overburden in this
section can be traced to lacustrine sediments, debris flow
alluvium and slope-alluvium, accounting for about 50% of the
total thickness (Cheng et al., 2022). Among them, the evidence
suggests that the lacustrine sediments are from dammed lake
deposits. There are many ancient landslide bodies near Qiaojia
basin, and there is a huge ancient landslide residue about 3 km
away from the lower boundary of the Qiaojia Basin, which
certainly blocked the Jinsha River and formed a large barrier
lake, providing conditions for the static water deposition (Li et al.,
2016).
4 HYDROENGINEERING GEOLOGICAL
PROBLEMS AND TREATMENT OF DEEP
OVERBURDEN
The spatial distribution of hydropower resources in China are
uneven as they are affected by topography and climate conditions.
The hydropower resources in major rivers such as the Jinsha
River in Southwest China are very rich, thus, it has become the
center of hydroengineering construction in China. Flood control
and hydroelectric are two of the main engineering activities in
Southwest China. The construction of dams often encounters
thick overburden. The diverse genesis of deep overburden
restricts the site and dam type for hydroelectric power
stations, These are often based on differences in lithology,
sediment grading, structure, particle shape, density, and degree
of cementation. Altogether it increases the complexity of the dam
junction layout and adds further complexity to the foundation
design.
Faced with complex and thick overburden and the general risk
of dam construction, there are two main treatment methods for
overburden: excavate all the overburden layers, or alternately,
partial excavation of the overburden layers.
4.1 Excavation of all the Overburden
When the overburden layer is relatively thin and its removal
will not add further technical problems and will not increase
the amount of engineering and investment too great, the
overburden layer can be removed completely and treated
according to the conventional damming technology on
bedrock.
FIGURE 6 | Geological model of the Qiaojia Basin (Shan et al., 2021, modification).
TABLE 2 | Sedimentary sequence revealed by boreholesin Qiaojia Basin(Cheng et al., 2022, modification).
Stratigraphic position Sedimentary facies type Top boundary altitude/m Core depth/m Single layer thickness/m
Quaternary Strata ⑪Slope-pluvial facies 834.13 0.00 160.80
673.33 160.80
⑩alluvial- Pluvial facies 665.04 169.09 8.29
⑨Lacustrine facies 613.73 220.40 51.31
⑧alluvial- Pluvial facies 560.13 274.00 53.6
⑦Fluvial facies 523.83 310.30 36.3
⑥alluvial- Pluvial facies 500.04 334.09 23.79
⑤Fluvial facies 472.79 361.34 27.25
④Lacustrine facies 445.23 388.90 27.56
③alluvial- Pluvial facies 436.59 397.54 8.64
②Fluvial facies 128.75 705.38 307.84
①alluvial- Pluvial facies 105.93 728.20 22.82
Bedrock section
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Yan et al. Riverbed Overburden in Southwest China

For example, the thickness of the riverbed overburden at the
Wudongde Hydropower Station on the Jinsha River is 52–73 m.
After all the overburden was excavated, a hyperbolic thin arch
dam was built on the limestone bedrock. The dam has a
maximum height of 265 m. For the Shuangjiangkou
Hydropower Station on the Dadu River, the riverbed
overburden depth is 48–57 m (partial 67.8 m). The decision
was made to excavate all the overburden layer at the bottom
of the core wall, build the core wall directly on granite, and
construct a 314 m high rockfill dam with a gravel-soil vertical core
(Liu, 2015). In both cases the removal of the relatively thin
overburden did not add unnecessary complexity and
additional financial burden onto the project allowing the dam
to be built on bedrock.
4.2 Partial Excavation of Overburden
When the thickness of the overburden is very large, the cost of
excavation may be too high and overall construction becomes
more difficult. It may be decided to retain part of the overburden
and build the dam directly on the overburden layer, but the
engineering characteristics of the thick overburden often result in
three major problems: dam foundation deformation, dam
foundation seepage, and/or sand liquefaction. These three
problems are directly related to the safety and financial
investment of the project.
(1) Dam foundation deformation
The riverbed overburden layer of the dam foundation may
have different geogenic types, a complex structure, differing
thicknesses of each layer, and different physical and
mechanical properties, each of which adversely affect the stress
distribution and overall deformation of the dam body, core, and
anti-seepage walls. Therefore, based on the specific situation of
the project it is necessary to reinforce the overburden layer within
a certain range of the dam foundation, this increases the strength
and stability of the foundation and reduces the uneven
deformation of the dam body. the reinforcement measures
such as the excavation and replacement, consolidation
grouting, vibro-replacement stone columns and Jet Grouting
are often adopted.
The Pubugou Hydropower station dam in the main stem of
the Dadu River is located on a deep overburden with a depth of
78 m. The uneven deformation of the dam’s foundation is the key
to dam foundation treatment. All soils above 670 m of the
originally designed foundation surface should have been
excavated. During construction, the soft soil and soil layers
which are susceptible to liquefaction were found in the dam
foundation. The result was the final foundation surface was
excavated to a depth of 667 m, and a backfill measure was
adopted to 670 m of the dam foundation design height. At the
same time, 10 m deep consolidation grouting was carried out
under the dam foundation. The results show that the overburden
after grouting meets the design requirements of foundation
bearing capacity and deformation modulus, and the continued
operation of the reservoir indicates that these measures were
successful (Xue et al., 2012).
The original design of the composite geomembrane inclined-
wall rockfill dam of the Renzonghai Hydropower Station was to
excavate all the silt loam layers within 20 m of the dam
foundation. Two additional problems presented themselves in
this plan, first because it is difficult to excavate silt loam and
second, the resulting pile up after excavation which may cause soil
erosion and pollution of the river channel, it was decided to
reinforce the foundation by vibro-replacement stone columns in
the project implementation stage (Fan and Wang, 2012).
(2) Dam Foundation Seepage
Due to the nature of riverbed overburden and its variable
sedimentary makeup there is a strong tendency for dam
foundation seepage. The specific reasons for foundation
seepage are that the overburden has coarse grain, boulders,
low permeability resistance, strong permeability, and the more
common local overhead. More specifically seepage deformation
failure forms, such as contact scouring and soil piping resulting
from glacial deposits, alluvial-proluvial deposits, and colluvial-
sliding deposits add to foundation seepage. Engineering measures
must necessarily be taken to prevent seepage and premature dam
failure. Curtain grouting of the dam foundation or the
construction of a concrete anti-seepage wall to cut off the
seepage path and reduce hydraulic gradient are common
methods (Hedayati Talouki et a1., 2015; Luo and Huang,
2020). The seepage control measures mainly include the
vertical cutoff technique, the horizontal anti-seepage
technique, and joint seepage control. In China, the concrete
cut-off wall or the combination of cut-off wall and curtain
grouting are the most common for the construction of high
earth-rock dams on a deep overburden layer. This is mainly due
to their mature technology and their highly effective ability to
protect against permeability. Horizontal anti-seepage techniques
are more suitable for dam foundations with weak permeability.
When the overburden is thick and the vertical anti-seepage
conditions are unfavorable or are too cost intensive, the
horizontal anti-seepage scheme should be studied and
adopted, but the effect of the Horizontal anti-seepage
technique is limited. For tall and medium height dams,
complex strata and projects with large anti-seepage
requirements, the final technique should be carefully selected.
A typical example is the Tarbela Dam in Pakistan, this high
dam of 147 m was built on a 230 m thick overburden layer. Using
horizontal seepage prevention measures, the dam used a length of
1,432 m of clay paving seepage prevention. The result was that
after initial water filling the dam foundation permeability was
unacceptably high. During water storage in 1974, there were more
than 100 collapse pits, in 1978 after a dumping soil treatment, the
overall seepage and the seepage of the dam foundation has tended
to be stable (Dang and Fang, 2011).
(3) Sand Liquefaction
Loose sand layers are typical in thick riverbed overburden and
are susceptible to liquefaction. In particular, the sand lens
interlayers are mostly distributed in the nearshore along the
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Yan et al. Riverbed Overburden in Southwest China

river. The thickness is generally less than 2 m, and the maximum
thickness is about 13 m. The liquefaction of sand layers often
occurs during earthquakes. Sand liquefaction has several harmful
effects, such as sand-boils and waterspouts, ground subsidence,
inducing high-speed landslides, creating foundation instability,
etc. Sand liquefaction results when the pore water pressure
increases, the effective stress of the soil skeleton decreases, and
the soil is transformed from solid to liquid.
Chen et al. (2013) has reviewed the site liquefaction
phenomenon during earthquakes over the past 20 years and
concluded that the depth of soil liquefaction can reach 20 m,
however, below 20 m sand liquefaction becomes more difficult.
For a sand layer that is in the location of the dam foundation
where it is judged to be possible for liquefaction, the measures of
excavating or changing the soil should be taken. When excavation
is difficult or uneconomical, artificial infill measures may be
taken, or concrete diaphragm wall or other methods can be
used to enclose the liquefiable layer under the foundation. The
possible liquefied sand layer of the dam foundation of the
Huangjinping Hydropower Station on the Dadu River was
treated using basic excavation and setting the pressure on the
slope downstream of the dam. The foundation of Jinping
Hydropower station in the Yalong River prevents the
liquefaction of the Sand Lens body through the technique of
consolidation irrigation.
5 CONCLUSION
Based on the analysis of the exploration data of riverbed
overburden in the primary river basins of Southwest China,
the results show that:
(1) The riverbed overburden in Southwest China shows a certain
regularity. The riverbed overburden in the Tibetan Plateau is
shallower in the headstream area, it is thickest in the middle
reaches along the mountain canyon valleys, and entering the
plain area, and into the piedmont slope areas and the Yunnan
Plateau, the overburden is shallower.
(2) The deep overburden in most river reaches can be vertically
divided into three layers. The middle layer is a sequence of
aggradation of multi-genesis deposits, mainly consisting of
glacial deposits, damming lake deposits, and landslide and
debris flow caused by geological disasters. The Glacial
deposits are mainly distributed in the valley belt around
the mountain which is above the snow line. Dammed lake
deposits and geological disaster deposits are widely
distributed, mainly in mountain canyon valleys where the
riverbed gradient is large. Typically, the upper layers are
modern river deposits, while the bottom layers are made up
of paleo-riverbed deposits.
(3) The combination of regional tectonic movement, climate
change, and river system development are major
contributing factors to the origin of the aggradation
resulting in the deep overburden of major rivers in
Southwest China, this includes the Jinsha, Dadu,
Yalong, Qingyi, Min, and Yarlung Tsangpo Rivers.
However, the overburden of each river is the result of a
complex action of multiple auxiliary sources guided by the
effects of these main controlling factors. Due to the
specific topography and geomorphology, geological
environment, drainage evolution, and the local
differential ascending and descending movement, there
are clearly differences in the genesis of each river basin.
However, no matter what factor or multiple factors are at
work, the coupling of tectonic-climatic-fluvial
sedimentation processes always affects the thickness
variation and spatial distribution of riverbed
overburden in Southwest China.
(4) In hydroengineering projects over thick overburden of
riverbeds, two primary treatment methods are adopted: to
remove the overburden completely or to remove it partially.
When the dam foundation is placed on the overburden layer,
there are three primary engineering problems that need to be
addressed: dam foundation deformation, dam foundation
seepage, and sand liquefaction.
DATA AVAILABILITY STATEMENT
The raw data supporting the conclusion of this article will be
made available by the authors, without undue reservation.
AUTHOR CONTRIBUTIONS
ZY contributed to the data acquisition, investigation and wrote
the original draft. MX and XK contributed to the data acquisition,
the conception and design of the study. LG and SZ contributed to
the data acquisition and graphic editing. The authors would like
to thank Joshua Schmidt for his help on the English editing of the
manuscript. All authors have read and agreed to the published
version of the manuscript.
FUNDING
This research is supported by the State Key Laboratory of
Geohazard Prevention and Geoenvironment Protection at
Chengdu University of Technology (SKLGP2018Z018);Power
China Chengdu Engineering Corporation Limited (P238-
2014);The College Science Foundation of Sichuan Water
Conservancy Vocational College (KY2021-07).
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at:
https://www.frontiersin.org/articles/10.3389/feart.2022.895769/
full#supplementary-material
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Yan et al. Riverbed Overburden in Southwest China

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Conflict of Interest: SZ was employed by Power China Chengdu Engineering
Corporation Ltd.
The remaining authors declare that the research was conducted in the absence of
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