Hydrological Regime Shifts in River-Connected Lakes under Upstream Dam Regulation: Insights from the Three Gorges Project and Poyang Lake

Abstract. Revealing the impacts of mainstream dams on the hydrological regime of connected river–lake systems is crucial for elucidating river–lake interaction mechanisms and providing a scientific basis for basin-scale water resources regulation and ecological protection. Taking the Three Gorges Project–Poyang Lake system as a representative case, this study integrates the Light Gradient Boosting Machine (LightGBM) model with the SWAT model to analyze the lake’s hydrological responses to dam regulation. Based on long-term runoff, water level, and meteorological series, simulated and observed hydrological events were compared to quantify the influence of the Three Gorges Project on Poyang Lake. Results indicate that backflow events declined significantly in frequency (−11.58 %), duration (−22.6 days), water level (−7.79 %), and discharge (−35.60 %), with the dam contributing 72.19 % of the variation in backflow events discharge. The backwater effect at Hukou weakened markedly (fitted backflow −150.58×10³ m³/s), triggering cascading effects across the lake. Normalflow events were notably prolonged (+25.9 days), whereas flood events decreased in both frequency (−5.18 %) and discharge (−11.03 %), demonstrating a significant flood peak attenuation effect (fitted backflow −10512.89×10³ m³/s). In contrast, a decline in water levels (−7.47 %) and discharge (−12.90 %) during normalflow events, and a 15.18 % increase in drought event frequency, to which the dam contributed 30.03 % of the runoff variation. During drought events, discharge deviated substantially from model predictions (+391.34×10³ m³/s), indicating enhanced hydraulic resistance at Hukou. Moreover, under drought events, the hydrological relationship at Hukou shifted from mainstream water-level dominance to lake-outflow dominance. Overall, the construction of the Three Gorges Dam has made hydrological variations in the Poyang Lake region more stable and secure, but has also increased drought risks, marking a gradual transition from a naturally regulated lake system to a semi-natural, dam-regulated system.  Future management should aim to optimize dam operation schedules and enhance river–lake connectivity to promote the coordinated and sustainable development of water resources and ecosystems.