River deltas rank among the most economically and ecologically valuable environments on Earth. Even in the absence of sea-level rise, deltas are increasingly vulnerable to coastal hazards as ...declining sediment supply and climate change alter their sediment budget, affecting delta morphology and possibly leading to erosion
. However, the relationship between deltaic sediment budgets, oceanographic forces of waves and tides, and delta morphology has remained poorly quantified. Here we show how the morphology of about 11,000 coastal deltas worldwide, ranging from small bayhead deltas to mega-deltas, has been affected by river damming and deforestation. We introduce a model that shows that present-day delta morphology varies across a continuum between wave (about 80 per cent), tide (around 10 per cent) and river (about 10 per cent) dominance, but that most large deltas are tide- and river-dominated. Over the past 30 years, despite sea-level rise, deltas globally have experienced a net land gain of 54 ± 12 square kilometres per year (2 standard deviations), with the largest 1 per cent of deltas being responsible for 30 per cent of all net land area gains. Humans are a considerable driver of these net land gains-25 per cent of delta growth can be attributed to deforestation-induced increases in fluvial sediment supply. Yet for nearly 1,000 deltas, river damming
has resulted in a severe (more than 50 per cent) reduction in anthropogenic sediment flux, forcing a collective loss of 12 ± 3.5 square kilometres per year (2 standard deviations) of deltaic land. Not all deltas lose land in response to river damming: deltas transitioning towards tide dominance are currently gaining land, probably through channel infilling. With expected accelerated sea-level rise
, however, recent land gains are unlikely to be sustained throughout the twenty-first century. Understanding the redistribution of sediments by waves and tides will be critical for successfully predicting human-driven change to deltas, both locally and globally.
Concentrations of arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), and zinc (Zn), grain sizes, and concentrations of organic carbon (Corg) were measured in 323 river ...sediment samples from the Pearl River Delta (PRD). Results showed that the heavy metal concentrations in the sediments ranged from 1.6–93 mg/kg for As, 0.04–9.3 mg/kg for Cd, 2–315 mg/kg for Cr, 1.1–352 mg/kg for Cu, 0.01–0.67 mg/kg for Hg, 11–221 mg/kg for Pb, and 11–1234 mg/kg for Zn. The highest values of As, Cr, Cu, Hg, Pb, and Zn appeared in the Beijiang River, whereas Cd was high in the Xijiang River. The overall sediment quality in the area with respect to metal concentrations generally met the primary standard criteria of China (Marine Sediment Quality), except for Cd and Cu. The spatial distributions of the heavy metals were influenced by both grain sizes and Corg concentrations. The Igeo geo-accumulation index indicated that there was no significant Cr, Cu, Hg, or Zn pollution, slight to moderate pollution by As and Pb, and moderate Cd pollution in the study area. Spatial distributions of an eco-toxicological index based on probable effect levels indicated that there was a 21% probability that the combination of the seven metals was exerting a toxic stress in the PRD river sediments.
•Heavy metal pollution was assessed in river sediments of the Pearl River Delta•Beijiang River had the highest values of As, Cr, Cu, Hg, Pb, and Zn•Slight to moderate pollution by As and Pb, and moderate Cd pollution appeared in the region.•A 21% probability of being toxic in the river sediments of PRD
Sediment connectivity plays a fundamental role in sustaining ecosystem goods and services in fluvial systems, including hydropower production. Dams alter the natural processes of sediment transport ...by trapping sediment and reshaping downstream hydrology and geomorphology. Due to these processes' interconnected nature, dams' impacts extend in time and space beyond the dam site to the entire river system. System‐scale approaches to reduce dam impacts commonly only consider dam siting, overlooking the potential of sediment management strategies integrated into the dam operations to offer more flexible solutions for mitigation. Herein, we contribute a sediment routing model (D‐CASCADE) to assess the impacts of reservoirs and their management strategies on river sediment connectivity. D‐CASCADE is applied to the 3S river system, a tributary of the Mekong River, a hotspot of potential dams in the Lower Mekong. We analyze three dam development portfolios. The effect of reservoir management is examined by assessing daily sediment delivery with specific dam release strategies. Model results predict sediment yield to the Mekong to reduce by 31%–60%. Finally, we explore trade‐offs between hydropower generation and sediment connectivity across cascades of multiple reservoirs. Results show that repeated flushing operations during the early wet season could significantly increase sediment delivery with minimal (max 6%) hydropower losses. While poor trade‐offs between sediment and hydropower have been locked‐in in the Mekong, our results highlight the potential of including sediment connectivity models in multi‐objective decision‐making frameworks to devise integrated water and sediment management strategies that mitigate connectivity disruptions while minimizing losses in other sectors.
Key Points
The D‐CASCADE model can reproduce robust, basin‐wide patterns of river sediment connectivity in the Se Kong‐Se San‐Sre Pok (3S) river basin
We quantify the impact on sediment transport for different grain sizes of three dam development scenarios on the lower 3S system
We optimize the design of drawdown sediment flushing operations considering both sediment connectivity and hydropower generation
Water composition in floodplains plays a key role in ecological processes and is affected by both local water sources and flooding from the main river. Despite local studies, still lacks a complete ...understanding on the relationship between hydrological processes and sediment distribution in the river-floodplain system of the Amazon basin. This paper presents the first mapping of the dynamics of suspended sediments in rivers and lakes (>0.25 km2) of central Amazon, considering different water types. Previous studies have considered only white-water rivers with high sediment concentration. This study also describes some river-lake systems in terms of their spatial-temporal pattern of sediments and water flows. Time series between 2003 and 2017 of red and infrared reflectance of Moderate Resolution Imaging Spectroradiometer (MODIS) images were temporally filtered to retrieve incomplete and low-quality data. These images were assessed as a proxy of the surface suspended sediments concentration; and maps of the spatial-temporal variation of sediments were created, such as the class frequency map. This map represents a 15-year frequency at which each pixel remains in one of the surface suspended sediments concentration classes: high, moderate, and low, with an overall accuracy of 71%. Our findings allowed to observe the variation of sediment concentration along the Solimões-Amazonas River, such as, for instance, its increase from the confluence with the Tapajós River to the mouth. Some hydrological processes were also observed in lakes of the middle reach, such as overbank flow and resuspension of sediments in depression lakes. In some ria lakes, the main water source comes from local basin with the backwater promoting sediment input in these lakes during the low-water period. The maps produced by this study are available and useful for supporting biogeochemical studies.
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•Sediment dynamic maps from black, clear and white water in Amazon are presented.•Low-quality MODIS data (2003–2017) were retrieved with a time series smoothing filter.•Sediments' spatial-temporal pattern in river-lake systems is assessed in case studies.•Implications for hydrology, geomorphology and biogeochemical studies
Abandoned river channels on alluvial floodplains represent areas where sediments, organic matter, and pollutants preferentially accumulate during overbank flooding. Theoretical models describing ...sedimentation in floodplain lakes recognize the different stages in their evolution, where the threshold for hydrological connectivity increases in older lakes as a plug‐bar develops. Sedimentary archives collected from floodplain lakes are widely used to reconstruct ecological and hydrological dynamics in riverine settings, but how floodplain lake evolution influences flow velocities and sedimentation patterns on an event scale remains poorly understood. Here we combine sediment samples collected in and around a floodplain lake with hydraulic modelling simulations to examine inundation, flow velocity, and sedimentation patterns in a floodplain lake along the Trinity River at Liberty, Texas. We focus our analyses on an extreme flood event associated with the landfall of Hurricane Harvey in August 2017 and develop a series of alternative lake bathymetries to examine the influence of floodplain lake evolution on flow velocity patterns during the flood. We find that sediments deposited in the lake after the Hurricane Harvey flood become thinner and finer with distance from the tie‐channel in accordance with simulated flow velocities that drop with distance from the tie‐channel. Flow velocity simulations from model runs with alternative plug‐bar geometries and lake depths imply that sedimentation patterns will shift as the lake evolves and infills. The integration of sediment sampling and hydraulic model simulations provides a method to understand the processes that govern sedimentation in floodplain lakes during flood events that will improve interpretations of individual events in sedimentary archives from these contexts.
We combine sediment samples from a floodplain lake with hydraulic modelling simulations to examine inundation, flow velocity and sedimentation patterns in a floodplain lake along the Trinity River at Liberty, Texas. We find that sediments deposited in the lake after the Hurricane Harvey flood become thinner and finer with distance from the tie‐channel which is in accordance with simulated flow velocities. The integration of sediment sampling and hydraulic model simulations can improve interpretations of sedimentary archives from these contexts.
While mountainous rivers in subtropical East Asia deliver a disproportionately large amount of sediments to the global ocean, the controlling mechanisms for sediment supply and transport remain ...understudied. Here we target a mesoscale tectonically stable mountainous river, the Minjiang River, in southeast China. We present petrography, heavy minerals, and Sr‐Nd isotopic data from suspended particulate matter (SPM) and riverbed sediments to characterize sediment‐climate feedback processes. Results show Sr‐Nd isotopic compositions of the SPM vary seasonally, corresponding well with the spatiotemporal variations of precipitation. River sands display low compositional and textural maturity and represent first cycle‐dominated and fast‐transported detritus. Provenance analysis suggests prominent contributions of upstream Proterozoic‐Paleozoic rocks to downstream SPM and riverbed sediments. We propose that climate‐induced hydrological variations exert a major control on sediment supply, transport, and compositions. Our study highlights the crucial role of climate in driving physical erosion of mountains in tectonically stable, subtropical East Asia.
Plain Language Summary
The subtropical‐tropical rivers in East and South Asia, particularly those draining mountains, account for a significant proportion of global sediment and nutrient exports. Based on tectonic settings, river basin morphology, climatic conditions, and extents of anthropogenic impacts, two distinct types of sediment supply and transport regimes in the low‐middle latitudes of East Asia have been identified. Type 1 rivers are large rivers on the tectonically stable continent with slow, multicycled sediment transport, modulated by widespread flood plains, lakes, and human activities. Type 2 rivers are small mountainous rivers on tectonically active plate boundaries with rapid sediment transport, which are commonly triggered by extreme tectonic and climatic events. However, sediment transport mechanisms of the mountainous rivers in the tectonically stable, subtropical regions remain highly enigmatic. We combined petrology and geochemistry methods to track sediment sources of the Minjiang River (southeast China) and to characterize river sediment transfer processes. We find that hydroclimate changes, rather than anthropogenic forcings, exhibit the major control on discharged sediment compositions. We suggest that the mesoscale, tectonically stable, mountainous rivers have similar rapid sediment transport behavior to those small mountainous river systems, unlike other large river systems in subtropical East Asia.
Key Points
Petrography, heavy mineral, and Sr‐Nd isotopic data were used to trace sediment provenance for a tectonically stable mountainous river
Provenance data indicates rapid sediment delivery in response to hydrological changes over seasonal timescales
Climate variability exerts a major control on sediment compositions, revealing a third type of sediment transport regime in East Asia
Hydromorphodynamic models are powerful tools for predicting the potential mobilization and transport of sediment in river ecosystems. Recent studies have shown that they are able to predict suspended ...sediment matter concentration in small river systems satisfactorily. However, hydro-sedimentary modelling exercises often neglect suspended sediment properties (e.g. sediment densities and grain-size distribution), which are known to directly control sediment dynamics in the water column during flood events. The main objective of this study is to assess whether a better representation of such properties leads to an improved performance in the model. The modelling approach utilizes a fully coupled hydromorphodynamic model based on TELEMAC-3D (v7p1) and an enhanced version of the sediment transport module SISYPHE (based on v7p1), which allows for a refined sediment representation (i.e. 10-class sediment mixtures instead of 2-class mixtures and distributed sediment density instead of uniform). The proposed developments of the SISYPHE model enable us to evaluate and discuss the added value of sediment representation refinement for improving sediment transport and riverbed evolution predictions. To this end, we used several model set-ups to evaluate the influence of sediment grain-size distribution, sediment density, and suspended sediment concentration at the upstream boundary on model predictions. As a test case, we simulated a flood event in a small-scale river, the Orne river in north-eastern France. Depending on the model set-up, the results show substantial discrepancies in terms of simulated bathymetry evolutions. Moreover, the model based on an enhanced configuration of the sediment grain-size distribution (10 classes of particle sizes) and with distinct densities per class outperforms the standard SISYPHE configuration, with only two sediment grain-size classes, in terms of simulated suspended sediment concentration.
Tracing sediments back to their catchment sources using biogeochemical and physical fingerprints involves multiple assumptions. One of the most fundamental assumptions is that these fingerprints are ...consistent during sediment generation, transportation, and deposition processes. Accordingly, the biogeochemical fingerprints used to trace sediment must remain constant, during detachment and redistribution, or they must vary in a predictable and measurable way. One key challenge to this assumption is the sorting effect of particles by size during detachment, mobilization, transportation and deposition processes. Owing to the notable effect of particle size on sediment fingerprints, we believe it is important to review the main approaches used to address the effects of changes in particle size composition on sediment fingerprints. The two main approaches to addressing particle size impacts on fingerprint properties are: fractionation of source and sediment material to a narrow particle size range (e.g. isolation of <10μm or <63μm fractions), and concentration corrections (e.g. normalizing concentrations by parameters such as specific surface area). These approaches are often used in combination. The utility of fractionation and corrections to address particle size effects has received increasing attention and the relative merits of these procedures have been subject to debate. Accordingly, alternative techniques to address particle size effects in sediment fingerprinting studies are being adopted. For example, a tributary tracing technique or edge-of-field samplers may minimize particle size effects on sediment source fingerprints. The interrelationships between particle size and biogeochemical tracer properties suggest that particle size may also contribute to the formation of contrasts in sediment fingerprints between sources. Indeed, there may be a significant opportunity to derive further sediment source information through comprehensively investigating and unravelling the complexity of particle size–biogeochemical interactions.
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