Trends in River Total Suspended Sediments Driven by Dams and Soil Erosion: A Comparison Between the Yangtze and Mekong Rivers.

Global river systems are experiencing rapid changes in sediment transport under growing anthropogenic and climatic stresses. However, the response of sediment discharge to the coupled influence of anthropogenic and natural factors and the associated impacts on the fluvial geomorphology in the Yangtze and Mekong rivers are not comprehensively assessed. Here, we recalibrated a seamless retrieval algorithm of the total suspended sediment (TSS) concentrations using in situ data and concurrent satellite data sets to analyze spatiotemporal patterns of the TSS concentrations in the lower Yangtze and Mekong rivers. Combined with soil erosion rates estimated by the Revised Universal Soil Loss Equation for the past 20 years, we examined the contributions of different factors to TSS trends. The results show that TSS concentrations in the Yangtze River decreased from 0.47 g L−1 in 2000 to 0.23 g L−1 in 2018 due to the construction of the Three Gorges Dam (TGD), especially in the Jingjiang reach, with a declining magnitude of 0.3 g L−1 (∼56%) since the TGD began operating. The Mekong River experienced increasing TSS concentration trends upstream and decreasing trends downstream from 2000 to 2018, possibly attributed to increased upstream soil erosion and decreased downstream water discharge. Declining TSS concentrations in both rivers have driven varying degrees of river channel erosion over the past two decades. This study investigated long‐term changes in the TSS concentrations and soil erosion in the Yangtze and Mekong rivers, and the results provide baseline information for the sustainable development of river sediment delivery. Plain Language Summary: Growing anthropogenic and climatic stresses have driven rapid changes in sediment transport for river systems globally. However, poorly available studies about the response of sediment discharge to coupled human and natural forces over the Yangtze and Mekong rivers limit the assessments of their impacts on river channel morphology. In this study, we estimated the spatiotemporal patterns of total suspended sediment (TSS) concentration in river channels of these two rivers and examined the contributions of driving factors to its temporal trends for the past 20 years. Three Gorges Dams caused TSS decrease of over 50% from 2000 to 2018 in the Yangtze River. In the Mekong River, soil erosion drove TSS increase in the upstream, while downstream TSS concentration decreased under declining water discharge during 2000–2018. Declining TSS concentrations enhanced river channel erosion in the Yangtze and Mekong rivers. We anticipate that the data sets here could serve as important information to assess the trade‐offs of future dam strategies and sustainable developments of river systems under human and climatic impacts. Key Points: Developed a seamless total suspended sediment (TSS) algorithm to determine the spatiotemporal patterns of the TSS concentration in channels of the Yangtze and Mekong riversExamined the contributions of dams and soil erosion to the TSS trendsExplored the impacts of declining TSS concentrations on river width dynamics [ABSTRACT FROM AUTHOR]