Evolution of the Water–Sediment Regime in the Yangtze River Basin Under the Synergistic Effects of Extreme Precipitation and Water Conservancy Development

Under the synergistic effects of extreme precipitation and water conservancy development, the water–sediment regime in the Yangtze River Basin has undergone significant changes. This study integrates daily precipitation records (1981–2023), runoff and sediment observations (2002–2023), and water conservancy statistics (2009–2023), applying the Mann–Kendall test, principal component analysis (PCA), and a coupling coordination degree model (CCDM) to diagnose spatiotemporal patterns and mechanisms. The CCDM quantifies the coordination between extreme precipitation and water conservancy subsystems using a composite index derived from PCA-selected key indicators, with coordination degree (D) ranging from 0 (Dysregulation) to 1 (Quality Coordination), classified into Mild Dysfunction, Primary Coordination, Intermediate Coordination and Better Coordination. The results reveal increased runoff variability but a marked decline in sediment load (MK Z = − 4.82, p < 0.001), with the steepest reduction upstream tapering downstream, primarily driven by reservoir trapping. The Three Gorges Dam accounted for ~ 76.3% of sediment reduction at Yichang, and cascade operations post-2012 further decreased sediment to ultra-low levels upstream. Extreme precipitation intensified across most of the basin, with PRCPTOT and R95p identified as principal natural drivers, while reservoir operations considerably weakened the runoff–sediment linkage. Coupling coordination between extreme precipitation and water conservancy subsystems improved from mild dysfunction (D = 0.42, 2009–2012) to intermediate coordination (D = 0.71, 2019–2023), showing a clear spatial decrease downstream. These findings elucidate the joint natural–engineering control mechanisms governing water–sediment dynamics and provide critical insights for adaptive basin management aimed at balancing flood safety, sediment continuity, and ecosystem stability.