A unified ecological flow indicator reveals long-term regime shifts and connectivity changes in a regulated river–lake system

Large reservoir operation has substantially altered flow regimes and ecological connectivity in river–lake systems, yet long-term, spatially explicit ecological responses remain insufficiently quantified. An indicator-based ecological flow assessment was applied to evaluate hydro-ecological regime changes in the Jingjiang reach of the Yangtze River and its three major outlets to Dongting Lake following the operation of the Three Gorges Reservoir (TGR). Daily discharge records from 2003 to 2024 were analyzed at three mainstem stations (Zhicheng, Shashi, Jianli) and three outlets (Songzikou, Taipingkou, Ouchikou) using a unified ecological flow scoring framework to ensure consistency across sites and seasons. Results reveal a pronounced improvement in non-flood-season ecological flows along the Jingjiang mainstem, with increasing adjustment rates downstream, while flood-season scores remain comparatively stable under flood-control constraints. In contrast, the outlets exhibit coordinated yet divergent responses: Songzikou closely mirrors mainstem trends, whereas Taipingkou and Ouchikou are increasingly modulated by lake storage capacity and backwater effects, showing declining ecological scores, particularly during the flood season. These spatial patterns reflect the combined influence of reservoir-driven flow redistribution, channel adjustment, and river–lake connectivity alteration. By integrating long-term observations with a unified indicator framework across multiple nodes, the analysis provides robust evidence of differential ecological flow responses within a regulated river–lake system. The findings offer a transferable methodological approach for diagnosing ecological connectivity changes and support adaptive management of large river–lake ecosystems under sustained human regulation. • Spatiotemporal ecological flow in Jingjiang–Dongting Lake system (2003–2024). • Non-flood season ecological flow improves greatly; flood season show little change. • Three river–lake outlets respond differently to reservoir and lake effects. • Long-term analysis diagnoses hydro-ecological regime shifts in river–lake system.