Reshaping spatial–temporal pattern of suspended sediment concentration in the Yangtze River mainstem by damming reservoirs based on nearly 40 years Landsat observations

The proliferation of dams has significantly disrupted sediment flux from land to ocean, reshaping estuarine geomorphology and altering sediment–associated biogeochemical cycles. However, understanding of how dams influence the spatiotemporal distribution of sediment in the Yangtze River remains limited due to sparse observational records. To addresses this gap, this study introduces a novel satellite–derived framework that leverages 36 years of Landsat imagery to quantify patterns of suspended sediment concentration (SSC) along the river. Given the inherent limitations of traditional SSC monitoring, particularly spectral saturation and scalability constraints when quantifying high concentration over large spatial domains, this study calibrated machine learning models using a combination of a public dataset (N=2410) and a field cruise sampling dataset (N=214). The XGBoost–based model achieved robust predictive performance (Public: R²=0.80, MAE=17.19 mg/L, and RMSE=54.13 mg/L; Cruise: R²=0.88, MAE=9.63 mg/L, and RMSE=13.86 mg/L), enabling detailed mapping of SSC spatiotemporal dynamics along the Yangtze River mainstem. Over the study period, SSC exhibited a pronounced decline, decreasing from 767.23±6.51 mg/L in 1986 to 48.14±0.70 mg/L in 2022. The construction of upstream cascading dams shifted high–SSC zones from the upper reach to the middle and lower reaches, while reversing the upstream sediment regime from erosion to deposition, with an average reservoir accumulation of 161.77 Mt yr⁻¹. Among these dams, the Liyuan (LY), Ludila (LDL), Ahai (AH), Jinanqiao (JAQ), and Three Gorges Dam (TGD) exerted the most pronounced influence, with their commissioning closely aligning with marked SSC reductions and abrupt regime shifts. Cascading dams were identified as the dominant drivers of the reshaped sediment distribution, responsible for 32.30 % of the change, exceeding the contributions of upstream soil and water conservation measures (land use transition: 26.12 %, vegetation restoration: 9.13 %) and climate factors (30.44 %). This study quantifies sediment redistribution in the Yangtze River mainstem, elucidates multidecadal SSC responses to dam construction, and provides a transferable framework for sediment–related environmental assessments in ungauged regions.