How interannual hydrological variability governs thermal stratification and selective withdrawal efficacy: Insights from the lower Lancang River cascade reservoirs

Study region The cascade reservoirs in the lower Lancang River, southwestern China. Study focus Natural river ecosystems rely on specific thermal regimes, which are frequently disrupted by deep reservoirs releasing cold hypolimnetic water. This study investigated how interannual hydrological variability controls thermal stratification, and how selective withdrawal via stoplog gates influences downstream thermal habitats and hydropower generation. By coupling a three-dimensional hydrodynamic model with a streamwise temperature model, the thermal dynamics were simulated. A systematic assessment was then conducted to compare the thermal habitat improvement and the hydropower losses between bottom-outlet operation and selective withdrawal. New hydrological insights for the region Results indicated persistent vertical stratification in the reservoir forebay across typical hydrological years, with a stable hypolimnion temperature of 15 °C and a distinct stratified regime from May to August. Utilizing multi-layer stoplog gates shifted withdrawal flows to warmer surface layers, raising downstream temperatures by a maximum of 8.34 °C. Moreover, the thermal guarantee rate for the spawning of the indicator species ( Tor sinensis ) increased from 14.29% to 66.67%. This thermal habitat improvement incurred negligible hydropower losses, yielding a 0.33% reduction in total generation and a 0.058% decrease in guaranteed output. These results confirmed that coupling selective withdrawal with cascade optimization effectively balances thermal habitat suitability and hydropower generation, providing information to support reservoir management in highly regulated river basins.