River Sediment Load Research Articles

Wide bedrock valley development and sensitivity to environmental perturbations: Insights from flume experiments in erodible bedrock

AbstractWide bedrock valleys and their genetic descendants, strath terraces, can serve as morphological records of past climate that reflect river discharge and sediment load during periods of valley widening. Understanding how changes in sediment load and water discharge create such distinct morphological features is limited by a lack of robust understanding of the specific processes of bedrock valley widening. We present results from the first set of flume experiments specifically devoted to exploring the conditions necessary to create wide bedrock valleys and how bedrock valleys develop through time. We ran six experiments in a weak bedrock substrate representing valley widening in an easily erodible bedrock, with differing amounts of water discharge, sediment load and base level fall. We evaluated valley width, valley wall height, channel mobility, lateral and vertical bed incision and sediment cover on the bed to explore the conditions necessary for the development of wide bedrock valleys and better understand the processes that affect valley widening rates. The results of the experiments show that wide bedrock valleys developed slowly and only under long periods of high sediment conditions, while vertical incision occurred much faster and was easily induced under different forcing mechanisms. We found that high sediment flux, enough to cover the channel bed, was a necessary condition for substantial valley widening. However, sediment cover on the bed was not by itself a sufficient condition to create wide bedrock valleys in our experiments; other factors were also required, particularly mobile channels within the valleys and some channel curvature to induce lateral undercutting. The results from this set of experiments suggest that the creation of wide bedrock valleys has several necessary conditions that must be met, and the development of a wide bedrock valley can be disrupted by slight changes in one of these necessary conditions.

Evolution of the main channel of the Yangtze River

AbstractThe construction and operation of the Three Gorges Reservoir and the cascade reservoirs upstream have significantly altered the processes of flow and sediment in the main and tributary channels of the Yangtze River. This has led to substantial adjustments in the riverbed through erosion and deposition, thereby impacting flood protection, water resource utilization, navigation, and the aquatic environment in the Yangtze River basin. In this study, prototype measurements were used to analyze the variations in runoff and sediment load in the main channel of the Yangtze River, as well as the changes and evolution of the riverbed. Mathematical modeling was done to predict the trends in reservoir sedimentation and riverbed adjustments. The results indicate that, apart from the significantly increased runoff and sediment load in the river source region (Zhimenda station) over the past two decades, there is no clear unidirectional trend of increasing or decreasing in the main and tributary annual runoff of the Yangtze River. However, the release of reservoir outflows undergoes significant changes throughout the year due to reservoir regulation. Suspended sediment load in the upper Yangtze River has been decreasing since the 1990s, especially after the operation of the Three Gorges Reservoir and the four cascade reservoirs in the lower reach of the Jinsha River. The factors influencing flow and sediment variations include mainly climate change and human activities such as reservoir operation and soil and water conservation. The significant changes in flow and sediment conditions have disrupted the original relative equilibrium state of the main channel of the Yangtze River, leading to riverbed adjustments. The river sections in the upper reach, located within reservoir areas, have shifted from erosional state under natural conditions to accumulative state, while the middle and lower reaches have transitioned from a relatively equilibrium state to a process dominated by erosion and reconstruction. Overall, the river regime in the reservoir area and downstream of the Three Gorges Dam remains relatively stable, but there have been adjustments in some local river sections, for example, in the curved sections, particularly in the sharply curved sections downstream of the Three Gorges Dam, where gradual or abrupt chute cutoff has occurred. In the foreseeable future, the river channel downstream of the Three Gorges Dam will remain in an unsaturated state regarding sediment transport. The process of river channel erosion will persist for a long time and have far‐reaching consequences. Some long straight sections, multiple bifurcation sections, and sections with large curvature are expected to undergo certain adjustments in the river regime, necessitating continuous observation, long‐term monitoring, and timely river management and channel governance.

Scale‐Dependent Influence of Permafrost on Riverbank Erosion Rates

AbstractWhether permafrost systematically alters the rate of riverbank erosion is a fundamental geomorphic question with significant importance to infrastructure, water quality, and biogeochemistry of high‐latitude watersheds. For over four decades, this question has remained unanswered due to a lack of data. Using remotely sensed imagery, we addressed this knowledge gap by quantifying riverbank erosion rates across the Arctic and subarctic. To compare these rates to non‐permafrost rivers, we assembled a global data set of published riverbank erosion rates. We found that erosion rates in rivers influenced by permafrost are on average nine times lower than non‐permafrost systems; erosion rate differences increase up to 40 times for the largest rivers. To test alternative hypotheses for the observed erosion rate difference, we examined differences in total water yield and erosional efficiency between these rivers and non‐permafrost rivers. Neither of these factors nor differences in river sediment loads provided compelling alternative explanations, leading us to conclude that permafrost limits riverbank erosion rates. This conclusion was supported by field investigations of rates and patterns of erosion along three rivers flowing through discontinuous permafrost in Alaska. Our results show that permafrost limits maximum bank erosion rates on rivers with stream powers greater than 900 Wm−1. On smaller rivers, however, hydrology rather than thaw rate may be the dominant control on bank erosion. Our findings suggest that Arctic warming and hydrological changes should increase bank erosion rates on large rivers but may reduce rates on rivers with drainage areas less than a few thousand km2.

Multi-decadal erosion rates from glacierized watersheds on Mount Baker, Washington, USA, reveal topographic, climatic, and lithologic controls on sediment yields

Understanding land surface change in and sediment export out of proglacial landscapes is critical for understanding geohazard and flood risks over engineering timescales and characterizing landscape evolution over geomorphic timescales. We used automated Structure from Motion software to process historical aerial photographs and, with modern lidar data, generated a high-resolution DEM time series with coverage over 10 glacierized watersheds on Mount Baker, Washington, USA for the time period between 1947 and 2015. We measured basin-wide sediment yields and sediment redistribution on hillslopes and in stream channels. Slopes within most measured erosion sites are above theoretical and observed debris-flow thresholds. We observed significant erosion of hillslopes and limited deposition on hillslopes and in stream channels. Sediment delivery ratios during time periods with net erosion averaged 0.73. We determined, consistent with previous field observations, that debris flows originating from moraines are a primary erosion mechanism in proglacial zones on Mount Baker. Time series measurements indicate that temporal variability in erosion rates is associated with climate oscillations, with higher erosion rates during cooler-wetter periods. Basin-wide sediment yield is positively correlated with lithology (r2 = 0.54), hillslope angle (r2 = 0.52), drainage area (r2 = 0.82), and negatively correlated with stream channel slope (r2 = 0.67). Topographic differences between high and low yielding basins indicate that spatial variability in erosion on Mount Baker is sensitive to Pleistocene and Holocene glacial and volcanic activity. Specific sediment yields in six basins averaged 4600 ton/km2/yr, consistent with global measurements in glacierized catchments. Specific sediment yield decreased with increasing basin area, with total loads in the downstream main stem Nooksack River estimated between 480 and 820 ton/km2/yr. Proglacial sediment yields account for between 18 and 32 % of total sediment load in the main stem Nooksack River and exceed contributions by bluff and terrace erosion, which account for between 8 and 13 % of total load. Our findings indicate that erosion in glacierized basins is sensitive to decadal climate oscillations and that high proglacial sediment yields provide an important contribution to river systems downstream, particularly in catchments where upland topography and lithology is favorable.

Delta lobe development in response to changing fluvial sediment supply by the second largest river in Vietnam

While the importance of sediment supply to river delta evolution is well recognized, the extent to which sediment from upstream sources contributes to the subaerial and subaqueous components of its delta is not yet fully comprehended. To investigate this issue, the present study analyzed satellite images (Landsat) spanning from 1975 to present and nautical charts in different periods to examine the morphological evolution and its relationship with changes in river sediment loads of the Ba Lat delta lobe, located on the central coast of the second largest river system (Red) in Vietnam. We proposed a hybrid approach to estimate suspended sediment entering the sea via the Ba Lat mouth, based on a combination of one-line theory and long-term observation from a gauging station. The results show a significant reduction (∼91.5 %) in annual sediment load over the entire 64-year estimation period (1958–2021), which is associated with the operation of large dam-reservoirs upstream. The evolution of the Ba Lat delta lobe was found to be highly correlated with sediment load, with a huge sediment supply from the Red River causing the delta lobe to move seaward at a rate of more than 100 m/yr and the deltaic land area to accrete at a rate of 117 ha/yr between 1975 and 1990. However, since the early 1990s, the annual sediment flux has reduced of 57 %, resulting in more than 50 % of the delta lobe's shoreline experiencing severe erosion. The current amount of sediment load delivered to the sea (1.4 × 106 m3/yr) is insufficient to prograde the delta lobe, which could lead to continued land loss in the future. We attribute these erosions mainly to the reduced river sediment supply, generated by the large dam-reservoirs located upstream of the delta. The study suggests that these results and methods can inform coastal management and enhance understanding of delta lobe evolution by providing insights into changes in shoreline position, land area, and subaqueous component.

Subaqueous Topographic Deformation in Abandoned Delta Lobes—A Case Study in the Yellow River Delta, China

Reduction in river discharge and sediment load has left deltaic lobes in the world’s many river deltas starving, but knowledge of how the subaqueous topography of these abandoned subdeltas responds to environmental changes is limited. In this study, we aimed to determine the long-term dynamics of the subaqueous seabed of abandoned delta lobes to advance current knowledge. As a case study, we selected an abandoned subdelta on the Yellow River Delta of the Bohai Sea, China, for which three-decade long (1984–2017) bathymetric data and long-term river discharge and sediment load records are available. We analyzed the seafloor surface change and quantified the void space from the sea water surface to the seafloor. In addition, we surveyed the seafloor surface with an M80 unmanned surface vehicle carrying a multibeam echo sounder system (MBES) in 2019 to obtain high-resolution microtopography information. We found that a net volume of 5.3 × 108 m3 of sediment was eroded from the study seabed within an area of 3.6 × 108 m2 during 1984–2017. This volumetric quantity is equivalent to 6.89 billion metric tons of sediment, assuming a bulk density of 1.3 t/m3 for the seabed sediment. The seabed erosion from 0 to −5 m, from −5 to −10 m, and below −10 m has showed a similar increasing trend over the past 33 years. These findings suggest that seabed erosion in this abandoned subdelta will very likely continue, and that other abandoned delta lobes in the world may have been experiencing similar seabed erosion due to the interruption of the sediment supply and sea level rise. It is not clear if the seabed erosion of abandoned delta lobes would have any effect on the stability of the coastal shoreline and continental shelf.

Projection of Sediment Loading from Pearl River Basin, Mississippi into Gulf of Mexico under a Future Climate with Afforestation.

Sediment load in rivers is recognized as both a carrier and a potential source of contaminants. Sediment deposition significantly changes river flow and morphology, thereby affecting stream hydrology and aquatic life. We projected sediment load from the Pearl River basin (PRB), Mississippi into the northern Gulf of Mexico under a future climate with afforestation using the SWAT (Soil and Water Assessment Tool)-based HAWQS (Hydrologic and Water Quality System) model. Three simulation scenarios were developed in this study: (1) the past scenario for estimating the 40-year sediment load from 1981 to 2020; (2) the future scenario for projecting the 40-year sediment load from 2025 to 2064, and (3) the future afforestation scenario that was the same as the future scenario, except for converting the rangeland located in the middle section of the Pearl River watershed of the PRB into the mixed forest land cover. Simulations showed a 16% decrease in sediment load for the future scenario in comparison to the past scenario due to the decrease in future surface runoff. Over both the past and future 40 years, the monthly maximum and minimum sediment loads occurred, respectively, in April and August; whereas the seasonal sediment load followed the order: spring > winter > summer > fall. Among the four seasons, winter and spring accounted for about 86% of sediment load for both scenarios. Under the future 40-year climate conditions, a 10% reduction in annual average sediment load with afforestation was observed in comparison to without afforestation. This study provides new insights into how a future climate with afforestation would affect sediment load into the northern Gulf of Mexico.

Evolution of an Alpine proglacial river during 7 decades of deglaciation

Abstract. Alpine rivers have experienced considerable changes in channel morphology over the last century. Natural factors and human disturbance are the main drivers of changes in channel morphology that modify natural sediment and flow regimes at local, catchment, and regional scales. In glaciated catchments, river sediment loads are likely to increase due to increasing snowmelt and glacier melt runoff, facilitated by climate change. Additionally, channel erosion and depositional dynamics and patterns are influenced by sediment delivery from hillslopes and sediment in the forefields of retreating glaciers. In order to reliably assess the magnitudes of the channel-changing processes and their frequencies due to recent climate change, the investigation period needs to be extended to the last century, ideally back to the end of the Little Ice Age. Moreover, a high temporal resolution is required to account for the history of changes in channel morphology and for better detection and interpretation of related processes. The increasing availability of digitised historical aerial images and advancements in digital photogrammetry provide the basis for reconstructing and assessing the long-term evolution of the surface, in terms of both planimetric mapping and the generation of historical digital elevation models (DEMs). The main issue of current studies is the lack of information over a longer period. Therefore, this study contributes to research on fluvial sediment changes by estimating the sediment balance of a main Alpine river (Fagge) in a glaciated catchment (Kaunertal, Austria) over 19 survey periods from 1953 to 2019. Exploiting the potential of historical multi-temporal DEMs combined with recent topographic data, we quantify 66 years of geomorphic change within the active floodplain, including erosion, deposition, and the amounts of mobilised sediment. Our study focuses on a proglacial river that is undergoing a transition phase, resulting from an extensive glacier retreat of approximately 1.8 km. This has led to the formation of new channel networks and an overall negative cumulative sediment balance for the entire study area. We found that high-magnitude meteorological and hydrological events associated with local glacier retreats have a significant impact on the sediment balance. The gauge record indicates an increase in such events, as well as in runoff and probably in sediment transport capacity. Despite this, the sediment supply has declined in the last decade, which can be attributed to a lower contribution of the lateral moraines coupled to the channel network and less sediment sourced from the melting Gepatsch Glacier as evidenced by roches moutonnées exposed in the current/most recent forefield. Nonetheless, we observed significant erosion in the tributary, leading to the transport of sediment downstream. Overall, this study enhances our understanding of the complexity of sediment dynamics in proglacial rivers across various spatial and temporal scales and their relationship to climate change factors.

Variation characteristics of rainfall erosivity in tropical China and its impact on river sediment load

Rainfall erosivity is one of the important factors leading to soil and water erosion, affecting the intensity of soil erosion and the variation of river sediment load. This work aimed to explore the distribution and variation characteristics of rainfall erosivity in the Nandu River Basin in tropical China and to clarify the effect of rainfall erosivity on sediment load variation. The daily rainfall data of 13 meteorological stations in the Nandu River Basin during 1971–2020 were used to calculate the rainfall erosivity of the river basin by using the rainfall erosivity model based on the daily rainfall. The analyses were conducted using the Mann-Kendall non-parametric trend/abrupt test, wavelet analysis, inverse distance weighted interpolation and other research methods to determine the temporal and spatial fluctuation patterns of rainfall erosivity and sediment load. The annual mean rainfall erosivity in the Nandu River Basin during 1971–2020 was 16,497.67 MJ mm·ha−1·h−1, which showed an upward trend. The distribution of rainfall erosivity was concentrated from July to September. Except spring, the variation of rainfall erosivity in summer, autumn and winter showed an upward trend. Spatially, rainfall erosivity showed a decreasing trend from south to north. The variation coefficient of each meteorological station ranged from 0.24 to 0.43, showing high regional variability. The correlation between rainfall erosivity and erosive rainfall in the basin was the highest, followed by altitude and latitude. From 1980 to 2020, the sediment load in the Nandu River Basin showed an overall upward trend, with obvious periodic changes. The main change period was 25 years, and the sediment load had a large inter-annual variation. Taking the period before the inflection point of the double accumulation curve of sediment load and rainfall erosivity as the base period, since 2009, the river sediment load increased by 46.36% compared with the base period. The influence of the variation of rainfall erosivity on sediment load is −22.23%, and that of human activities on sediment load is 122.23%. These findings can provide a scientific reference for soil erosion model prediction, soil and water conservation and environmental management in tropical basins of China.

The sediment budget and migration of 137Cs in Chernobyl affected area: 30 years of investigations in the Plava River basin, Tula region

The Plava River basin (Tula region) is considered the most polluted with Chernobyl fallout region in Russian Federation. Detailed studies of sediment redistribution and migration of 137Cs produced by Chernobyl were conducted in the basin. The article provides an overview of results from various studies conducted during the last 30 years estimating the rate of erosion and sediment redistribution based on different methodology including 137Cs. Sediment budgets for different parts of the fluvial network were developed based on estimated rates of erosion and accumulation of sediments within the Plava River basin for the post-Chernobyl period. It has been established that eroded from agricultural fields sediments accumulate on the slopes of interfluves (38.4%) and in the bottoms of dry valleys (2738%). Part of the sediment delivered by slope runoff and temporary watercourses from the slopes of the interfluves to the bottoms of river valleys was redeposited on river floodplains (1011%), and the remaining part (13.124.7%) passed into the river sediment load. The value of basin contribution of sediments to the Plava River sediment runoff is somewhat overestimated due to the specifics of the erosion models used to calculate soil erosion from arable land. It has been established that for the period of more than 25 years since the Chernobyl accident, 5% of 137Cs deposits were lost due to soil erosion (less than 0.2% annually). According to evaluated sediment budget, only a quarter of mobilized material leaves basin as a sediment yield. Thus, only about 1% of the initial 137Cs fallout was removed from the Plava basin. The perspective directions for research on sediment redistribution in the plain river basins located in the temperate zone using 137Cs as a technogenic tracer are outlined.

River delta eco-morphodynamics under changing scenarios: The case of Lake Turkana, Kenya

In this work we explore the impact that changes in local climate and river hydrology have on the morphodynamics of a river delta, particularly focusing on the evolution of the delta generating at the lake inlet. We investigated the case of the delta in the lower reach of the Omo River in Ethiopia, which flows into lake Turkana, Kenya. The lake is the fourth largest lake and the largest desert lake in the world. This case study is of particular interest because within the last decades three dams have been built within the Omo basin. Among these, the Gibe III dam had a huge impact on the river hydrology and the sediment supply. To quantify changes in land use and river morphology and relate these to climate change and river hydrology we collated a historical dataset by combining information from different satellite sources. We observed that the amount of bare sediments progressively diminished and the biomass became denser compared to the existence of sparse biomass in the past. We argue that this is due to the changes in river hydrology and sediment load imposed by the dam, which increased sediment erosion by deepening the channel. The presence of the dam also increased the low flow level and reduced the peak flow during flood season, reducing the natural oscillations of the water table. In consequence, the possibility to plant for the local population was removed during floods. Our findings also indicate that the hydrology and reduced sediment discharge have changed the downstream (e.g. lake level), and upstream (e.g. mouth bar deposition) boundary conditions controlling the evolution of the delta structure, respectively. As a result, the delta morphology became less dynamic and less complex.

Changes in suspended‐sediment yields under divergent land‐cover disturbance histories: A comparison of two large watersheds, Olympic Mountains, USA

AbstractImprovements in timber harvest practices and reductions in harvest volumes over the past half century are commonly presumed to have reduced sediment loads in many western US rivers. However, direct assessments in larger watersheds are relatively sparse. Here, we compare 2019–21 sediment concentrations against those of the late 1970s in the Bogachiel and Calawah River watersheds, adjacent and similarly sized (~300 km2) basins in the western Olympic Mountains of Washington State. The Calawah River watershed has experienced significant land‐cover disturbance, including a large 1951 fire, extensive post‐fire salvage logging, and relatively high rates of timber harvest through the 1990s. In contrast, the Bogachiel River watershed did not burn, and experienced only modest timber harvest that largely post‐dated 1970s sediment monitoring. Channel‐width trends suggest the Calawah River was still recovering from 1950s disturbances in the late 1970s. We found that 2019–21 suspended‐sediment loads in the Calawah River were 2.3–2.6 times lower than would have been expected based on 1970s sediment rating curves, while recent loads in the Bogachiel River were a factor of 1.4 ± 1.0 lower. We consider the plausibility and possible explanations of declining concentrations in the less‐disturbed Bogachiel River. Suspended‐sediment yields in the Bogachiel River were two times higher than yields in the Calawah River, which is attributed to a combination of modestly higher precipitation, more efficient runoff generation, and more extensive and erodible Quaternary valley fills in the Bogachiel River. Regional shifts in flood hydrology have also influenced suspended‐sediment loads in both watersheds. Our results then document a significant decline in suspended‐sediment concentrations in the Calawah River over the past half century. Reduced land‐cover disturbance provides the simplest and most likely explanation for this decline, though the wide range of possible concentration changes in the Bogachiel River leaves open possibilities that other processes (human, natural, or methodologic) could be a factor.

Hybridizing Artificial Intelligence Algorithms for Forecasting of Sediment Load with Multi-Objective Optimization

Forecasting of sediment load (SL) is essential for reservoir operations, design of water resource structures, risk management, water resource planning and for preventing natural disasters in the river basin systems. Direct measurement of SL is difficult, labour intensive, and expensive. The development of an accurate and reliable model for forecasting the SL is required. Sediment transport is highly non-linear and is influenced by a variety of factors. Forecasting of the SL using various conventional methods is not highly accurate because of the association of various complex phenomena. In this study, major key factors such as rock type (RT), relief (R), rainfall (RF), water discharge (WD), temperature (T), catchment area (CA), and SL are recognized in developing the one-step-ahead SL forecasting model in the Mahanadi River (MR), which is among India’s largest rivers. Artificial neural networks (ANN) in conjunction with multi-objective genetic algorithm (ANN-MOGA)-based forecasting models were developed for forecasting the SL in the MR. The ANN-MOGA model was employed to optimize the two competing objective functions (bias and error variance) with simultaneous optimization of all associated ANN parameters. The performances of the proposed novel model were finally compared to other existing methods to verify the forecasting capability of the model. The ANN-MOGA model improved the performance by 12.81% and 10.19% compared to traditional AR and MAR regression models, respectively. The results suggested that hybrid ANN-MOGA models outperform traditional autoregressive and multivariate autoregressive forecasting models. Overall, hybrid ANN-MOGA intelligent techniques are recommended for the forecasting of SL in rivers because of their relatively better performance as compared to other existing models and simplicity of application.

Water Area Variation and River–Lake Interactions in the Poyang Lake from 1977–2021

Lakes are an important part of the Earth’s surface. Poyang Lake is China’s largest freshwater lake with complex hydrological relationships and unique rhythmic changes. There is no systematic understanding of the relationship between lake watershed area and river–lake connectivity. In this study, using remote sensing imagery, hydrological data, meteorological data, and land use surveys, we investigated changes in the Poyang Lake water area from 1977–2021. In addition, we examined the relationship between the lake water area and potential impact factors and analyzed the influence of river–lake interactions on the lake water area. The results showed that the water area of Poyang Lake decreased during the study period. River runoff and sediment load changes in the Poyang Lake basin are the main factors affecting the lake water area. Additionally, the Three Gorges Dam water impoundment situated in the upper reaches of the Yangtze River weakened and eliminated the block and reverse flow of the Yangtze River into the Poyang Lake, causing the water area to decrease toward the end of the flood season in 2005 and in the dry season in 2003. Changes in the lakebed elevation caused by sedimentation and anthropogenic activities have also accelerated the shrinkage of the water area. Overall, the results of this study indicate that variations in the river–lake interactions are the primary cause of the significant changes observed in the Poyang Lake water area in the last five decades.

Hydrodynamic variations and human activities have influenced sediment fluxes in the pearl river delta since the late holocene

Climate change and human activity can both exert a role in the river discharge and sediment load in river deltas when the sea level remains relatively stable. The Pearl River Delta constitutes the metropolitan region of the Guangdong–Hong Kong–Macau Greater Bay Area, and its evolution in the past and future impacts the sustainable development of this region. In this manuscript, a core situated in the northern South China Sea, adjacent to Qiao Island of Zhuhai city in the Pearl River Delta, was selected to investigate the hydrodynamic variation influenced by tropical processes and the sediment flux imposed by human activities. Using paleosecular variations (PSVs) in geomagnetic field correlations constrained by seven radiocarbon ages, an ∼2.5 kyr chronology was provided for core sediments ∼2.8 m in length. The magnetic fabric results suggest that deposition was influenced by the two types of hydrodynamic-weak energy modulated by El Niño–Southern Oscillation (ENSO) activity and higher energy regulated by high-frequency tropical storms. The cyclic variations in the degree of anisotropy of magnetic susceptibility (P) indicate two periodic changes at approximately 300 and 100 years, which were forced by ENSO activity, while the ratio of lineation with foliation (q value) implies a higher energy flow period of 1.3–1.9 kyr, arising from strong tropical storms. Moreover, the high saturation remanence (SIRM) introduces more sediment flux, which is impacted by growing human activity during the period of 1.0–1.6 kyr.

Sediment-Loading Processes in a Forested Catchment: Modeling and Observations

In order to investigate sediment-loading processes in a catchment, the daily time series of river discharge and sediment load were applied to a semi-distributed model, the Soil and Water Assessment Tool (SWAT). The time series of discharge and sediment load were obtained by monitoring the river stage and water turbidity of the Oikamanai River, Hokkaido, Japan, in the rainfall season (April-November) of 2011-2014. The catchment is forested (ca 90% area) but underlain by the Neogene sedimentary rocks with currently active faults and forest soils with tephra layers, which tend to frequently produce slope failure such as landslide and bank collapse by rainfall or snowmelt. The water turbidity, T, in ppm was converted into suspended sediment concentration, SSC, in g/L by applying the linear relationship between T and SSC. The acquisition of the time series of discharge, Q (m3/s) and sediment load, L (=Q·SSC in g/s) of the river allowed us to distinguish the fluvial sediment transport, accompanied by slope failure in the upstream, from that under no slope failure. The SWAT was used to simulate soil erosion and identify the region prone to the soil erosion in the Oikamanai River basin. The model’s results showed a satisfactory agreement between daily observed and simulated sediment load as indicated by the high Nash-Sutcliffe efficiency. This evidences that the upper mountainous region of the catchment provides a main sediment source, accompanied by slope failure.

Relationship between sediment load and climate extremes in the major Chinese rivers

Global warming has led to increase in climate extremes, which altered both hydrological cycles and soil erosion processes. However, the responses of river sediment load to climate change, especially climate extremes have not been well understood due to a lack of comprehensive assessment. This study investigated the temporal trends and spatial patterns of sediment load at 70 hydrological stations during the period of 1956–2019 in ten major river basins in China. Both the partial linear square regression model and the double mass curve method were used to quantify the contribution of key climatic indices and anthropogenic driving forces. Overall, the annual total sediment load exhibited a significant negative trend (P < 0.01), decreasing from 1842.18 Mt/yr in 1960 s to 322.72 Mt/yr in 2010 s (82.48 %). The significant decline of sediment load was particularly obvious in the Yangtze River basin and the Yellow River basin. The abrupt changes of sediment load mostly occurred in 1985–2000. Human activities were primarily response for sediment load changes in most basins. The sediment load in mainstream was mainly regulated by reservoirs, while the tributaries were largely affected by soil and water conservation measures. Intensive human activities have led to substantial changes in land surface and resulted in weakened correlation between sediment load and climate extremes. Generally, precipitation, erosive rainfall and precipitation amount in flood period were closely related to sediment load. In the Southwest River basin and Northwest River basin, the sediment load had a relatively good relationship with extreme temperature, which was related to alpine snowmelt and permafrost degradation due to climate warming. This study provides good insights for a better understanding of the interaction between river sediment load and climate extremes variables, which will be valuable for decision-making in river basin management.

Sediment dynamics in the Gulf of Lion (NW Mediterranean Sea) during two autumn–winter periods with contrasting meteorological conditions

A simulation based on a hydro-sedimentary model was conducted for the period between summer 2010 and spring 2012 in the Gulf of Lion (northwestern Mediterranean Sea) to understand the spatial and temporal variability of sediment transport, erosion and deposition on the continental shelf and slope. Datasets of both simulated and observed current, temperature and suspended matter from the shelf and the Cap de Creus Canyon which is the main export route towards the continental slope, were first compared to assess the reliability of the simulation. The simulation shows the massive sediment accumulation near the Rhone River mouth (∼56 % of the inputs), the accretion along the mid-shelf mud belt, and the impact of dense shelf water cascading on sediment resuspension and erosion inside the Cap de Creus Canyon. The two studied autumn–winter periods were strongly contrasted in terms of meteorological conditions and subsequent impacts on the sediment dynamics. During the first period (2010–2011) dominated by marine storms, the shelf sediment underwent strong changes, the Rhone River sediment load accumulated in a relatively small area, stock of littoral sands moved to the inner shelf (20–40 m) while inner shelf fine particles fed the mid-shelf mud belt and the upper Cap de Creus Canyon. During the second period (2011–2012) with very little marine wind and a particularly cold winter, sediment on the shelf underwent little change except for a southwestward growth of the Rhone River prodeltaic deposit. Sediment from the southwestern end of the shelf as well as from the upper Cap de Creus Canyon was flushed toward deeper reaches by dense shelf water cascading. Cascading also had a more moderate impact in the various canyons incising the continental shelf. Our work supports the view of an unbalanced sedimentary system, with a deficit mainly over the inner shelf, whose main driver is probably the strong and fast reduction of particulate matter inputs from the Rhone River (by a factor of 4 in less than one century).

Decadal Hydro-Morphodynamic Evolution of a Sandy Meander Bend Downstream of the Three Gorges Dam and Impact on Waterway Navigation

The Three Gorges Dam (TGD) in the Yangtze River profoundly regulates the downstream river discharge and sediment load, causing significant river morphodynamic changes since 2003. Understanding such changes is vital for managing the navigational waterway. Using the Wakouzi waterway 190 km downstream from TGD as an example, we examined its decadal hydro-morphodynamic evolution based on hydrological and topographical data collected from the 1990s onwards. The results showed that the suspended sediment load has reduced by 85.8% downstream of TGD, although the annual runoff has not changed. The suspended sediment has become coarser due to depletion of the fine sediment source. As a result, the river channel has been substantially eroded, particularly the deep-water parts of the channel. Erosion of the elongated sand bars has emerged as well, even in the presence of channel regulation works. Consequently, channel degradation has led to a lower river stage under the same river discharge compared with the pre-TGD condition, particularly in the dry season with a lower river discharge. These changes imply worsened navigability of the examined riverbend. The results of this study have implications for management of other river reaches in the middle Yangtze River and other rivers undergoing similar changes worldwide.

Human deforestation outweighed climate as factors affecting Yellow River floods and erosion on the Chinese Loess Plateau since the 10th century

Increasing proportions of the global population are exposed to floods, including many living in the Yellow River floodplain. Since records were first kept in 602 BCE, there have been ∼1500 floods on the Yellow River, resulting in the death of millions of people. Counteracting increased flood risk requires an understanding of the relationship between climate, forest cover, erosion, and river flow. However, to assess whether the Yellow River basin is currently experiencing a period of high flood risk requires a long-term perspective. Here we use a variety of paleoenvironmental proxies (pollen, magnetic susceptibility, cladoceran assemblages) preserved in high-resolution, well-dated sediment records retrieved from an alpine lake in the middle Yellow River basin (Chinese Loess Plateau) within the agro-pastoral ecozone to examine how climatic and land-use changes affected aquatic ecosystems over the past ∼2000 years. Further, to examine changes in erosion, runoff production and flood frequency, we synthesize previously published proxy reconstructions of vegetation density, soil erosion, and dry-wet changes, as well as flood records from historical documents. We demonstrate that, following the period around the 10th century, excessive cultural deforestation outweighed climate effects and became the dominant factor that led to an unprecedented flood-rich period when drought conditions were common, cultivation expanded to meet food shortages, heavy silting raised the riverbed, and runoff and flood risk increased. These watershed changes, including enhanced soil erosion, affected the biological communities of aquatic ecosystems that led to the disappearance of planktonic cladocerans that hitherto dominated the assemblages. Increasing temperatures and weakening monsoon precipitation during the past ∼50 years, together with decreased erosion indicated by unprecedented reductions in Yellow River runoff and sediment load resulting from reforestation and damming, led to the re-establishment of planktonic cladocerans. Despite these recent changes, vegetation cover has not recovered to pre-deforestation levels, suggesting that the Yellow River is currently experiencing a high flood risk period. Our results emphasize the importance of considering vegetation–flooding relationships to help improve risk assessments and management protocols for the Yellow River.