Cross-Seasonal Heat Storage Driven by Autumn Impoundment: Thermal-Timing Evidence in Large Chinese Cascade Reservoirs

Quantifying the joint impacts of climate change and intensive cascade regulation on river thermal regimes is critical for managing ecological risks and optimizing hydropower production. However, most existing attribution studies primarily document broad, seasonally asymmetric warming and cooling patterns, offering limited mechanistic understanding of how specific reservoir operation strategies—particularly the widely implemented clear-water impoundment in China—regulate cross-seasonal heat storage and downstream winter warming. Here we developed an LSTM-based attribution framework to reconstruct counterfactual “no-dam” river temperatures and to quantify the relative contributions of anthropogenic regulation (ΔANT), long-term climatic warming (ΔTrend), and intra-annual climatic variability (ΔNCV) to downstream temperature changes in the lower Jinsha River, China. In addition, a suite of thermal-timing metrics is proposed to characterize seasonal heat states and to diagnose the cross-seasonal heat-storage processes responsible for the pronounced winter warming.Results indicate that anthropogenic regulation (ΔANT) is the dominant driver of observed downstream thermal changes, inducing substantial autumn–winter warming of up to ~2°C while dampening summer temperature extremes. Long-term climatic warming (ΔTrend) provides a persistent background increase, whereas intra-annual climatic variability (ΔNCV) imposes strong seasonal and interannual modulation. Notably, the magnitude of winter warming varies markedly among years and is strongly controlled by antecedent thermal-storage conditions, with thermal-timing metrics linking earlier autumn impoundment and greater cumulative heat storage to enhanced downstream winter temperatures (Pearson's r≈0.62). Overall, these findings elucidate the coupled roles of climate change and cascade reservoir regulation in shaping river thermal regimes and provide a mechanistic basis for optimizing multi-reservoir operations to balance hydropower generation with downstream thermal and ecological requirements.