Stage-Dependent Changes in Suspended Sediment, Nitrogen, and Phosphorus Transport Across the Lower Jinsha River Cascade, China

Sustainable management of cascade reservoirs requires joint consideration of sediment attenuation and downstream nutrient delivery, yet their relationships during cascade completion remain insufficiently understood. Using monitoring data from four reservoirs in the lower Jinsha River (2019–2024), we examined variations in suspended sediment concentration (SSC), total phosphorus (TP), and total nitrogen (TN) through stage-specific comparisons, correlation analyses, and flux decomposition. Relative to 2019, mean SSC decreased by 95.15% in Wudongde and 91.42% in Baihetan during 2022–2024, whereas nutrient responses varied among reservoirs. The pooled TP–SSC correlation weakened from 0.881 in 2019 to 0.354 during 2022–2024 and became nonsignificant after accounting for reservoir-specific mean differences. A two-month TP–SSC association persisted in Baihetan after seasonal and operational-stage adjustments, whereas corresponding associations were weak downstream. Despite persistently low SSC, TP fluxes increased by 25.11% in Xiluodu and 38.91% in Xiangjiaba from 2021 to 2022, with concentration changes contributing 68.3% and 88.4%, respectively. These findings demonstrate that sediment attenuation does not necessarily translate into reduced downstream phosphorus export, highlighting the importance of integrated sediment–nutrient monitoring for sustainable cascade-reservoir management.