A hydrothermal connectivity framework for identifying ecological operation windows in a tributary bay during early impoundment

Early impoundment creates highly non-stationary hydrothermal conditions in tributary bays, making broad seasonal ecological operations too coarse to effectively support fish migration. However, indicator-based approaches that translate hydrothermal dynamics into ecologically interpretable migration accessibility and operation timing remain limited. Here, we developed a hydrothermal migration connectivity framework to identify migration windows and ecological operation windows in Heishui Bay, a tributary bay of Baihetan Reservoir. Multivariate change point detection was used to divide the early impoundment period into distinct hydrothermal regimes, and a CE-QUAL-W2 model calibrated and evaluated against observed water levels and vertical temperature profiles was used to reconstruct backwater intrusion, circulation, and thermal stratification. Species-specific thermal and hydraulic requirements were translated into corresponding migration-supporting corridor states, including thermal corridors, velocity corridors, and composite corridors. To characterize corridor performance, we quantified occurrence timing, persistence, continuity, and fragmentation, and further evaluated corridor quality using composite connectivity and corridor fragmentation indicators. The results showed that the early impoundment period was divided into five hydrothermally distinct regimes. Backwater intrusion length varied from approximately 22 to 30 km and was jointly controlled by water level fluctuation, the mainstream to tributary discharge ratio, and thermally induced density structure. Composite corridor connectivity was highly transient: the first through-going composite corridor appeared on 2 March but lasted only 1 day, while a second brief through-going event occurred on 16 May. In addition, indicator-based corridor quality analysis further identified 2–9 March as a low-fragmentation interval forming the core of the primary ecological operation window, with a weaker secondary window in mid-May. These findings show that ecological operations in tributary bays should be refined beyond broad breeding-season schedules to reflect species-specific and process-dependent connectivity dynamics. The proposed framework provides a basis for translating hydrothermal variability into operation relevant timing in newly impounded tributary bay systems.