Evolution of Runoff–Sediment Regimes in the Upper Yangtze River Under Intensive Regulation and Implications for the Three Gorges Reservoir

Large regulated rivers are increasingly affected by combined main-stem dam regulation and tributary disturbances, which reshape sediment connectivity and alter downstream sediment delivery. Existing studies have established that large reservoirs reduce sediment load, but how intensive main-stem regulation and tributary-scale human activities jointly reshape the runoff–sediment regime entering the Three Gorges Reservoir (TGR) remains insufficiently clarified. Based on monthly runoff and sediment data from 1957 to 2024 at major stations in the upper Yangtze River, this study examines the stage-wise alteration, main-stem–tributary contrast, intra-annual redistribution, and statistical explanatory factors of TGR inflow sediment. Results show that annual runoff changed weakly, whereas sediment load decreased significantly at all stations, with major breakpoints in 1998 and 2013 for the main stem and TGR inflow. After the lower Jinsha River cascade entered operation, main-stem sediment supply was strongly disrupted, and TGR inflow sediment decreased by 84.1% compared with the pre-1998 period. In contrast, tributaries with limited sediment-retention capacity retained strong episodic sediment-supply potential during wet years. Reservoir regulation altered the intra-annual delivery pattern, reducing flood-season runoff proportions and markedly weakening flood-season sediment load from Xiangjiaba. The statistical explanatory analysis indicates that reservoir capacity and the normalized difference vegetation index (NDVI) are strongly associated with the long-term sediment decline, whereas precipitation accounts for a larger share of the interannual variability. These findings suggest that the present TGR inflow boundary is no longer a simple reduced-sediment condition, but a reorganized regime controlled by main-stem sediment supply disruption and tributary residual sediment pulses. This study provides a scientific basis for sediment management and future assessments of cascade-reservoir joint operation.