Abstract: Cascade reservoir development has significantly altered water and sediment sources in the Upper Yangtze River, with tributary inputs in reservoir areas becoming the dominant control on sediment supply. This study analyzed observations from 16 tributary stations and combined statistical analysis, XGBoost modeling, and scenario simulations to quantify multi-factor controls on water and sediment fluxes. Runoff showed relatively minor changes, whereas sediment loads declined sharply. During 2007—2020, tributary sediment input to the Three Gorges Reservoir was only 34.2% of that in 1970—1987. In the Lower Jinsha River, tributary sediment loads decreased by 13.6%—90.6%. The monthly models for specific runoff and sediment yield performed well (R² > 0.74). Rainfall, month, and topography explained 80.2% of runoff variability, while runoff, rainfall, reservoir impact, land use, and month explained 81.0% of sediment variability. Future projections (2026—2056) suggest slightly lower mean annual rainfall than historical levels. The Beibei Station of the Jialing River is projected to deliver about 35.74 Mt/a, remaining the main sediment source to the Three Gorges Reservoir. High specific sediment yields are concentrated in several tributaries of the Lower Jinsha River, reaching up to ～829 t/(km²·a). These findings clarify the spatiotemporal evolution of tributary water and sediment inputs in the Upper Yangtze River.