How cascade dams reshape river thermal seasons: Satellite tracking of delayed spawning windows and range contractions in the Yangtze

The construction of cascade hydropower stations along the Yangtze River has fundamentally altered its natural thermal regime, with significant implications for aquatic ecosystems. This study integrates multi-source remote sensing data (Landsat 5/7/8 TIR), NCEP/NCAR reanalysis datasets, and long-term hydrological observations (1988–2021) to systematically evaluate the spatiotemporal impacts of major reservoirs on water temperature dynamics. Results reveal that cascade impoundment has amplified seasonal thermal fluctuations, with spring low-temperature discharges reducing downstream water temperatures by 0.4–1.8 °C and winter warm-water releases increasing temperatures by 1.2–2.2 °C. The timing of annual minimum water temperatures has been delayed by 35–42 days, disrupting critical life-cycle events for endemic fish species. Comparative analysis demonstrates that after impoundment, the temperature conditions often exceed the spawning threshold of protected species such as the Chinese sturgeon (16–20 °C). The successful spawning migration has decreased by 62%, and only 28% of the historical spawning grounds maintain appropriate temperature conditions. Thermophilic species have expanded their range upstream by 120–150 km, while cold - adapted species have suffered a range contraction of 45–60%. The study emphasizes the combined effects of multi - reservoir operations, indicating that later - built dams extend the thermal impact upstream. Moreover, in the warmer downstream river sections, the decomposition rate of organic matter has accelerated by 35–50%, significantly altering the nutrient cycling dynamics. These findings provide critical insights for optimizing ecological dispatching strategies and underscore the need for integrated management approaches that balance hydropower generation with ecosystem conservation in large river basins. The methodological framework combining remote sensing with ecological thresholds offers a transferable approach for assessing dam impacts in other regulated river systems worldwide.