River Sediment Load Research Articles

Kernel-Based Versus Tree-Based Data-Driven Models: On Applying Suspended Sediment Load Estimation

River sediment load estimation poses a critical challenge for water engineers due to its complex and nonlinear hydrological processes. This study assessed the amount of suspended sediment at the Bagh-e-Kalayeh hydrometric station on the Alamut River in the Qazvin province of Iran using two hydrological and meteorological variables, including discharge and rainfall, by considering three scenarios (discharge, discharge + monthly rainfall, and discharge + monthly rainfall + daily rainfall). For modeling, kernel-based data-driven methods, including Gaussian process regression (GPR) and support vector regression (SVR), and tree models, including the M5 tree, random forest (RF), random tree (RT), extra trees, reduced error pruning tree (REPT), and multi-search methods, were used. The results showed that the best performance was achieved by the SVR, with r = 0.948, Wilmot index = 0.965, and RMSE = 0.011 in the first scenario (only discharge). Discharge had the most significant impact on sediment estimation compared to rainfall. It was determined that the suspended sediment load in the Alamut River can be successfully estimated by the SVR method, where only the discharge was used as the input parameter. Additionally, the results indicated that given its characteristics and inherent features, the multi-search method can be used as a complementary approach in sediment modeling, especially in situations where the data volume is not extensive.

Economic impacts of large dams on downstream brickmaking in developing countries

Large dams have positive and negative impacts, including disrupting brickmaking on the floodplains downstream due to flow regulation and sediment reduction, affecting the supply of essential construction material, notably in developing countries. In this study, we introduce an analytical framework to assess the economywide effects of large dams on downstream brickmaking, focusing on Traditional Fired Clay Brick (TFCB). The framework includes three steps: characterizing the impacts on river flow and sediment load using river system modeling and secondary data, understanding the role of TFCB production in the economy based on survey and economic data, and quantifying the economywide impacts of changes in TFCB production using dynamic computable general equilibrium modeling. We demonstrate the functionality of the approach by conducting a case study of the impacts of the Grand Ethiopian Renaissance Dam (GERD) on the Sudanese economy due to changes in TFCB production by comparing two scenarios: “with GERD” and “no GERD.” Results show that Sudan’s accumulated (2023–2050) discounted (at 0.5% annually) Gross Domestic Product (GDP) at factor cost would decline by US$ 6 billion (−0.38%) due to a reduction in TFCB production. Consumer flexibility regarding brick types and the ability of alternative brick sources to fill the demand gap are key determinants of the impacts.

Statistical studies of the characteristics and key influencing factors of sediment changes in the upper Yangtze River over the past 60 years

Sediments in the upper Yangtze River have significantly changed over the past 60 years. The characteristics and causes of these sediment changes must be determined in the upcoming phases of the comprehensive management of the Yangtze River Basin. Based on sediment observation data from 1960 to 2020, this study examined the trends and sudden changes in the sediments in the mainstream and its significant tributaries in the upper Yangtze River. The relative contributions of precipitation change and various human activities to sediment load changes are quantified. The relationships between sediment variations and the construction and large dam projects, soil and water conservation measures, and earthquake disasters are established. The results indicate that there was a significant decline in sediment load with main changes occurring around the 1985s and 2010s. Multiple double mass curves are used to quantify the relative contributions of precipitation change and human activities to sediment load changes. The relative contribution of dams to the decrease in sediment load ranges from 53 % to 74 %. Precipitation change serves a significant role in the observed changes, accounting for a range of 10 % to 27 %. Soil conservation measures take some time to show their impact on sediment reduction. These measures work by stabilizing the soil and improving hydrological conditions. On average, they contribute between 7 % and 20 % towards reducing sediment. In recent times, there has been a noticeable rise in sediment load in rivers like the Jialing and Min Rivers. This unexpected increase could potentially be linked to a combination of factors including earthquakes, heavy precipitation, and localized sediment scouring. The results of this study offer valuable insights into the variations in river sediment load, aiding in the improved management of water resources and sediment in large dams. These findings provide helpful insights for dam management.

One third of African rivers fail to meet the ’good ambient water quality’ nutrient targets

The ambition of Sustainable Development Goal (SDG) target 6.3 is to improve global water quality by 2030. SDG indicator 6.3.2 monitors progress towards this target by assessing water bodies against ‘good’ ambient water quality criteria, with nutrients (nitrogen and phosphorus) as part of the key metrics. However, large data gaps present a fundamental challenge, especially for Africa on how to assess the progress being made with respect to both the current and desired future situations. Here, a continental water quality model for Africa is presented to simulate river sediment load, Total Nitrogen (TN) and Total Phosphorus (TP) loads and concentrations. Furthermore, critical areas and hotspots of TN and TP pollution are mapped for the period 2017 – 2019, in relation to the United Nations Environment Programme (UNEP) target thresholds used for the assessment of SDG indicator 6.3.2. Utilizing the UNEP’s criteria, which designates a water body as having “good ambient water quality” if 80% or more of its monitored values meet their targets, it is estimated that 44 % and 15 % of African rivers fail to meet the set water quality thresholds for simulated TP and TN, respectively. When synthesizing data for both TP and TN, 34 % of the rivers do not qualify as having “good ambient water quality”. Geographically, the most pronounced nutrient pollution hotspots were in North Africa, Niger River Delta, Nile River basin, Congo River basin and specific zones in Southern Africa. These areas correlate with regions characterized by high inputs of fertilizers, manure and wastewater discharge.

Scale-specific controls of monthly suspended sediment load in a typical inland river

Sediment transport is a multi-scale and non-linear process controlled by multiple variables. Given the intricate nature of sediment transport mechanisms and the overlap of different factors’ effects on it at various time scales, unraveling the relationship between sediment load and its influencing factors is challenging. This study aimed to elucidate the multi-timescale effects of factors on monthly suspended sediment load in a seasonal inland river. In this study, monthly sediment loads measured by the depth-integrating method were obtained from the Tabu River Basin in northern China from 2007 to 2021, alongside data on four controlling factors: runoff, precipitation, water surface evaporation, and the normalized difference vegetation index (NDVI). The results showed that sediment load, runoff, and precipitation showed strong variability during 2007–2021, while water surface evaporation and NDVI exhibited moderate variability. Multivariate empirical mode decomposition (MEMD) decomposed the temporal patterns of sediment load and associated variables into five intrinsic mode functions (IMF) and a residue. IMF1 (4.7 months), IMF2 (9.0 months) and IMF3 (14.2 months) collectively explained 115.88 %, 91.73 %, 91.28 %, 107.69 %, and 106.09 % of the total variability in sediment load, runoff, precipitation, water surface evaporation, and NDVI, respectively. Subsequently, the time-dependent intrinsic correlation (TDIC) was utilized to illustrate the inherent relationships between sediment load and its controlling factors across various time scales. The associations between sediment load and the controlling factors changed significantly with the scale. Runoff was the dominant factor controlling sediment load at IMF1 (4.7 months), IMF2 (9.0 months), IMF3 (14.2 months), and IMF4 (25.1 months). Water surface evaporation controlled the sediment transport process at IMF5 (63.6 months). The prediction of sediment load on the measurement scale, using scale-specific controls analyzed with MEMD (R2 = 0.993), demonstrated superior performance compared to traditional multiple linear regression prediction (R2 = 0.748). This study will provide a theoretical basis for effective prevention and control of soil erosion and watershed management in inland river basins.

Runoff and sediment effect of the soil-water conservation measures in a typical river basin of the Loess Plateau

Soil erosion and sediment transport processes, which are intimately connected with hydrological processes, strongly affect land surface morphology and earth system dynamics. Over the past 60 years, intensive Soil-Water Conservation (SWC) measures implemented on the Loess Plateau have significantly reduced sediment load and concurrently exacerbated runoff declines, while the quantitative evaluation of the effects of SWC measures remains challenging. This study develops a sediment module which simulates the hillslope and in-stream sediment processes, and then incorporates the module into a Geomorphology-Based Ecohydrological Model (GBEHM). Besides, the model also couples with an explicit representation of different types of SWC measures, including both Hillslope SWC (HSWC) and River-network SWC (RSWC) measures. The newly developed hydro-sediment model is then applied to the Kuye River basin, a typical river basin on the Loess Plateau, and used for quantifying the effects of SWC measures on river discharge and sediment load during 1982–2019. Compared with Period 1 (1982–2000), the river discharge decreased by 48.7% in Period 2 (2001–2019), while the sediment load experienced a more substantial decrease of 95.3% between the two periods. Based on the numerical experiment results, HSWC and RSWC contributed to 25.4% (58 million m3/yr) and 28.9% (66 million m3/yr) of the annual runoff decline, as well as 58.1% (36 million t/yr) and 33.9% (21 million t/yr) of the annual sediment load reduction between the two periods, respectively. Besides, HSWC plays an increasingly important role in sediment reduction in recent years due to check dam silting. The model developed in this research is useful to evaluate the multi-faceted effects of existing SWC measures and to help with future SWC planning and policymaking.

Changes in sediment load in the Lower Yellow River and its driving factors from 1919 to 2021

Rivers are not only an essential component of the development of civilization and the carbon cycle worldwide, but also a main contributor to natural disasters, especially the Lower Yellow River (LYR). With the functional degradation of the water–sediment regulation scheme (WSRS), LYR has reached a new stage. Thus, the changes in the sediment load in the Suspended River and its driving factors have significant practical applications. In this study, the sediment load in the LYR was analyzed from 1919 to 2021 based on improved sediment identity factor decomposition, wavelet analysis, and a double cumulative curve. The results showed that the changes in discharge and sediment exhibited poor synchronicity at different timescales. The sediment load decreased significantly, with evident periodicity of 9–10 years (years denoted as ‘a’) since 1950, and 69-a, 32-a, and 9-a since 1919. The changes in the sediment load can be divided into four phases: 1919–1959, 1960–1979, 1980–1998, and 1999–2021. Artificial levees can effectively constrain water flow and enhance sediment transport when the levee spacing is less than 6 km. To restrain deposition of the LYR, large dams control the incoming sediment coefficient so as to not exceed 0.009 kg∙s m−6. However, the WSRS reached its limit in 2010, and the wandering reach showed a deteriorating trend. Human activities control the changes in the sediment load. The reduction in the sediment load was mainly attributed to decreases in effective water yield capacity (53 %–75 %) before 1999 and sediment concentrations (46 %–65 %) after 1999. These results can provide a reference for further management of the suspended river.

Quantification of Bed Load Transport in the Northern Part of the Central Plateau of Morocco: Case of Wadi Skhirate

Rivers play a vital role in our ecosystems, providing fresh water, supporting rich biodiversity, and contributing to human well-being. However, in the face of climate change and intensive human activities, the sediment load in rivers can reach critical levels, presenting a complex set of challenges that require immediate action. The increased sediment load can alter aquatic habitats, clog channels, reduce reservoir storage capacity, and increase the risk of flooding. These direct threats entail high costs in terms of material and ecological damage, loss of life, and expenditure on rebuilding damaged infrastructure. The quantification of bedload in watercourses is therefore crucial for maintaining water and soil resources, safeguarding riparian communities, and preserving ecological balance. The study reports the findings of a three-year monitoring of the bed load of Skhirate Wadi, a river that drains a part of the western Moroccan Meseta. The study used the colorimetric monitoring method, which quantifies the volumes of coarse sediment that were transported by monitoring topographic variations in the riverbed and measuring the distances covered by the sediment. The study showed the sediment was found to move around seven times annually on average. However, the frequency and magnitude of floods and the size of particles affect the variation in this displacement. It also showed sediments travel an average distance ranging from 649 to 883 meters per year, and that the average specific bedload at the watershed scale is 30 m3/ Km2/ year. Relationships between flood peaks mobilized sediment volumes, and average particle distances are established and discussed. These results are fundamental to understanding of coarse sediment transfer processes in the small rivers of the central plateau. They are also essential for assessing the impact on the aquatic ecosystem, on downstream dams, and on the various existing road and hydro-agricultural infrastructures. This assessment will enable the implementation of appropriate management strategies to anticipate changes and plan the planning of the river and its watershed.

Vectorized dataset of silted land formed by check dams on the Chinese Loess Plateau

Check dams on the Chinese Loess Plateau (CLP) have captured billions of tons of eroded sediment, substantially reducing sediment load in the Yellow River. However, uncertainties persist regarding the precise sediment capture and the role of these dams in Yellow River flow and sediment dynamics due to the lack of available spatial distribution datasets. We produced the first vectorized dataset of silted land formed by check dams on the CLP, combining high-resolution and easily accessible Google Earth images with object-based classification methods. The accuracy of the dataset was verified by 1947 collected test samples, and the producer’s accuracy and user’s accuracy of the dam lands were 88.9% and 99.5%, respectively. Our dataset not only provides fundamental information for accurately assessing the ecosystem service functions of check dams, but also helps to interpret current changes in sediment delivery of the Yellow River and plan future soil and water conservation projects.

Can Turbidity Data from Remote Sensing Explain Modelled Spatial and Temporal Sediment Loading Patterns? An Application in the Lake Tana Basin

Understanding the spatial and temporal patterns of sediment loading in water bodies is crucial for effective water quality management. Remote sensing (RS) has emerged as a valuable and reliable tool for monitoring turbidity, which can provide insights into sediment dynamics in water bodies. In this study, we investigate the potential of turbidity data derived from RS to explain simulated spatial and temporal sediment loading patterns in the Lake Tana basin, Ethiopia. Utilizing existing RS lake turbidity data from Copernicus Global Land Service (CGLS) and simulated seasonal and multiyear trends of river sediment loadings into Lake Tana from the Soil and Water Assessment Tool (SWAT + model), we estimate correlations at different river inlets into Lake Tana. The results reveal a strong positive correlation (R2 > 0.66) between the multiyear monthly average sediment load from inflow rivers and RS lake turbidity at most river inlets. This indicates that the simulated river sediment loads and lake turbidity at river inlets exhibit similar seasonal patterns. Notably, higher turbidity levels are observed at the river inlet with the highest sediment load export. These findings highlight the potential of RS turbidity products in characterizing temporal and spatial patterns of sediment loadings, particularly in data-scarce regions, contributing to a better understanding of water quality dynamics in such areas.

Inter‐ and intra‐annual fluctuations and attribution analysis of runoff and sediment transport in the middle and lower Yangtze River

AbstractAssessing the changing regimes of runoff and sediment load in rivers, as well as analysing their underlying causes, is essential for maintaining the ecological integrity of rivers and promoting the sustainable development of water resources. Current methods of attribution analysis focus on accounting for the effects of climate change and human activities on inter‐annual variability in streamflow and sediment. However, the contribution of individual drivers to intra‐annual hydrologic variability is often overlooked. The study analysed the inter‐ and intra‐annual fluctuations in runoff and sediment load in the middle and lower Yangtze River using a water‐sediment variation attribution model, the Extreme Gradient Boosting algorithm, and the Multiple‐Factor Attribution method. The results showed that human activities were responsible for over 70% of the inter‐ and intra‐annual fluctuation in runoff and sediment load in the Yichang, Hankou, and Datong sections. Additionally, it was found that human activities were the main contributor to the reduction in both runoff and sediment load in all sections. Closer to the Yangtze estuary, human influence on intra‐annual fluctuation of runoff decreases while climate change influence increases, and human influence on both inter‐ and intra‐annual fluctuation of sediment load becomes less significant. The sensitivity of annual runoff to precipitation is greater than that of air temperature and evaporation in the Yichang, Hankou, and Datong sections. An increase of 10% in precipitation from baseline values leads to a corresponding increase of 3.13%, 2.24%, and 3.91% in runoff for the Yichang, Hankou, and Datong sections. The annual sediment load is more sensitive to changes in runoff than to precipitation, air temperature, and evaporation in each section. A 10% increase in runoff from the baseline value leads to an annual sediment load increase of 20.36%, 10.11%, and 8.78% at Yichang, Hankou, and Datong, respectively. This study provides a theoretical basis and guidance for decision‐making to protect and sustainably develop the water and sediment resources in the Yangtze River Basin.

Historical climate impact attribution of changes in river flow and sediment loads at selected gauging stations in the Nile basin

The Nile basin is the second largest basin in Africa and one of the regions experiencing high climatic diversity with variability of precipitation and deteriorating water resources. As climate change is affecting most of the hydroclimatic variables across the world, this study assesses whether historical changes in river flow and sediment loads at selected gauges in the Nile basin can be attributed to climate change. An impact attribution approach is employed by constraining a process-based model with a set of factual and counterfactual climate forcing data for 69 years (1951–2019), from the impact attribution setup of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3a). To quantify the role of climate change, we use the non-parametric Mann-Kendall test to identify trends and calculate the differences in long-term mean annual river flow and sediment load simulations between a model setup using factual and counterfactual climate forcing data. Results for selected river stations in the Lake Victoria basin show reasonable evidence of a long-term historical increase in river flows (two stations) and sediment load (one station), largely attributed to changes in climate. In contrast, within the Blue Nile and Main Nile basins, there is a slight decrease of river flows at four selected stations under factual climate, which can be attributed to climate change, but no significant changes in sediment load (one station). These findings show spatial differences in the impacts of climate change on river flows and sediment load in the study area for the historical period.

A Half-Century of Human Impact on Nan River Runoff and Sediment Load Supplied to the Chao Phraya River

The construction of large dams in the upper tributary basin of the Chao Phraya River (CPR) has been linked to a significant decrease in sediment load in the CPR system, estimated between 75–85%. This study, utilizing historical and recent river flow and sediment data from 1922 to 2019, examines the impact of three major dams constructed in the Nan River basin (the Sirikit, Naresuan, and Khwae Noi dams) on river runoff and sediment loads in the CPR. The investigation employed the Mann–Kendall (MK) test and the double mass curve (DMC) for analysis. Findings indicate that the Nan River is a major contributor to the CPR, accounting for around 40% of the runoff and 57% of the total sediment load (TSL). The Naresuan diversion dam’s water regulation was found to significantly reduce annual runoff and TSL downstream of the dam. Despite an initial increase in sediment load at the CPR headwater (C.2) post the construction of the Sirikit dam, attributed to expanded irrigation downstream and channel improvements in the lower Nan River, the operation of the three dams eventually led to a 31% reduction in sediment load at C.2 compared to pre-construction levels.

Exploring transformative processes at the intersections of land, water and livelihoods: a case study from the Tsitsa Project, South Africa

ABSTRACT The Tsitsa River catchment is a complex social-ecological system (cSES) in a rural area of the Eastern Cape Province of South Africa and the site of the Tsitsa Project (TP); a multi-stakeholder, transdisciplinary landscape restoration project aiming to improve sustainable livelihoods and ecological infrastructure. We investigated transformation mechanisms in a framing of multidimensional linkages, including the recognition of differentiated scales and levels. Linkages were analysed through the development of two vignettes: 1) a citizen technician employed to monitor sediment loads in rivers to inform landscape restoration activities (local scale); and 2) a senior government official responsible for (regional scale) operational and on-the-ground restoration initiatives. Vignette data were generated during a workshop, from TP researcher reflexivity, and interviews with the TP Catchment Coordinator and vignette subjects. Data were analysed and presented: i) as a heuristic diagram, ii) through a narrative, and iii) as a matrix table. Each analysis incorporated a different conceptualisation of scale in relation to four social processes related to transformative change: learning, agency, power and structure. Transformation is demonstrated and leverage points and areas of intractability for promoting and constraining future transformation towards social-ecological sustainability, were identified respectively. Further, we suggest that an understanding of transformative processes was enriched and nuanced by combining a triad of complementary analytical exercises. These allowed a focus on unique stories and contexts, but also the identification of generalisable patterns and mechanisms.

Update of Empirical Models for Predicting Specific Degradation in South Korea and Future Sediment Management Considering Climate Change

In this study, the existing empirical models for sediment management for ungauged watersheds in South Korea developed in 2019 were updated. The existing models were developed based on the Universal Soil Loss Equation structure with domestic river sediment data obtained from 2005 to 2014 and watershed characteristic values. To update the models, the specific degradation (SD) value was calculated first in the same way as the existing method by adding sediment measurement and daily discharge data obtained from 2015 to 2020. The calculated SD values with recent data decreased compared with the past and differed from the results predicted using the existing models and the watershed characteristics investigated in this study. For this reason, the existing models were updated using the latest watershed characteristics and SD values. The root-mean-square error values of the existing models, which ranged from 131 to 90 tons/km2·year, decreased to 102 to 62 tons/km2·year in the updated models with the same structures. Multicollinearity was tested to evaluate the reliability of the updated models, and a new 95% prediction interval for usability was estimated. In addition, quantitative changes in the future sediment load of domestic rivers were analyzed using the future rainfall data obtained from climatic change scenarios and the watershed area. The results of this study reconfirmed that periodic updates are required for the existing empirical models. It is expected that the empirical models will be used in various ways, such as for the prediction of future sediment loads for river sediment management.

Variation of runoff-sediment relationship at flood event scale in three typical watersheds of the Loess Plateau

River runoff and sediment load are characterized by topography, climate patterns, and vegetation in the watershed. The relationship runoff and sediment load has been greatly altered in many rivers throughout the world, as well as rivers in the Chinese Loess Plateau (LP) due to large-scale ecological restoration and soil conservation measures implementation. This study attempts to identify the changes of runoff-sediment relationship in three watersheds: the Gushanchuan River watershed, Jialu River watershed and Xichuan River watershed by using the flood event data during the periods of 1974–1989 and 2007–2019 in the LP. The results indicated that both runoff depth and sediment yield of flood events during 2007–2019 reduced significantly compared with those during 1974–1989 in all the watersheds. The highest decrease was found in the Gushanchuan River watershed, exhibiting reduction of 56.48% and 95.03% in runoff and sediment yield, respectively. The ratios between sediment yield and runoff depth in all three watersheds showed significant decrease during 2007–2019 when comparing to those in 1974–1989. The correlations between runoff and sediment variables became worse in 2007–2019, compared with those during 1974–1989. The implementation of soil and water conservation (SWC) measures (vegetation restoration, terraces and check dams) accounted for the changes in runoff-sediment relationship in all three watersheds. The result of this study could provide valuable information for the influences of SWC implementation and guide soil erosion control in the LP.

New data-driven equation for estimating total sediment loads in rivers: application of the MHBMO algorithm

New data-driven equation for estimating total sediment loads in rivers: application of the MHBMO algorithm

Review on erosion phenomenon, maintenance, and financial calculation of lifetime as an asset for Pelton turbines

Prevention of greenhouse emissions is the top priority for all countries, which urges them to switch to renewable energy as much as possible. Hydropower is one of the renewables that have high flexibility and at the same time compatibility to be used with any other renewable sources. Moreover, hydropower plants operating in the Himalayas, Andes, and Alps are facing operational challenges due to the high concentration of sediment loads in rivers. Although the arrangement of traditional sediment control mechanisms like dams and sand traps, the erosion tendency of hydro-turbine components operating in this sediment-laden water increases with the increased concentration of sediments. Much past research has been directed towards understanding sediment behaviors, investigation of flow, and effect of concentration, shape, and size, especially with Francis turbines. However, there are very fewer studies regarding sediment erosion and flow behavior in the case of the Pelton turbine. Hence, delving deeper into the flow characteristics, sediment behavior, and performance of the Pelton turbine is important to better understand the flow and sediment pattern of these types of turbines. The paper consists of the evaluation of studies conducted on the flow pattern in the Pelton turbine buckets and its validation with the numerical analysis models using image processing. It is being used in the Waterpower Laboratory at the Norwegian University of Science and Technology, NTNU. This paper also evaluates the scope of investigations about erosion by sediments in Pelton buckets using image analysis and state-of-the-art technology in the hydropower sector. In addition, a review is done about the predictability of erosion based on the measurements of the quantity of sediments that passes through the turbine. This research paper can build a background for quantifying sediment erosion in Pelton turbines with a certain degree of error, which can be utilized as a reference in future studies. The life cycle estimation of a turbine is also analyzed with the consideration of its location and financial return requirements together with the type of maintenance that it may have and the repair that is foreseen, in the case of a non-coated surface.

A global rise in alluvial mining increases sediment load in tropical rivers.

Increasing gold and mineral mining activity in rivers across the global tropics has degraded ecosystems and threatened human health1,2. Such river mineral mining involves intensive excavation and sediment processing in river corridors, altering river form and releasing excess sediment downstream2. Increased suspended sediment loads can reduce water clarity and cause siltation to levels that may result in disease and mortality in fish3,4, poor water quality5 and damage to human infrastructure6. Although river mining has been investigated at local scales, no global synthesis of its physical footprint and impacts on hydrologic systems exists, leaving its full environmental consequences unknown. We assemble and analyse a 37-year satellite database showing pervasive, increasing river mineral mining worldwide. We identify 396 mining districts in 49 countries, concentrated in tropical waterways that are almost universally altered by mining-derived sediment. Of 173 mining-affected rivers, 80% have suspended sediment concentrations (SSCs) more than double pre-mining levels. In 30 countries in which mining affects large (>50 m wide) rivers, 23 ± 19% of large river length is altered by mining-derived sediment, a globe-spanning effect representing 35,000 river kilometres, 6% (±1% s.e.) of all large tropical river reaches. Our findings highlight the ubiquity and intensity of mining-associated degradation in tropical river systems.

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.