Hydromorphological adjustment and river–lake disconnection in the Jingjiang Reach under long-term regulation of three Gorges Reservoir

Large dam operations can profoundly reshape downstream river morphology and disrupt river–lake interactions, leading to significant hydrological and ecological consequences. This study investigates long-term hydromorphological changes in the Jingjiang Reach of the Yangtze River following the operation of the Three Gorges Reservoir (TGR) since 2003. Based on daily discharge and water level data from three key hydrological stations (Zhicheng, Shashi, and Jianli), power-law stage–discharge relationships were developed to characterize channel evolution. Effective base level analysis indicated negligible change at Zhicheng but substantial riverbed incision at Shashi and Jianli, with mean lowering rates of 0.271 and 0.256 m/year, respectively. By 2024, cumulative channel storage had increased by approximately 2.45 billion m³, equivalent to 11 % of the average volume of Dongting Lake. Under the original hydrological regime, the Yangtze River flow was distributed into Dongting Lake through three main distributary inlets—Songzikou, Taipingkou, and Ouchikou. Results show a marked decline after TGR impoundment, particularly at Taipingkou and Ouchikou, where linear trends suggest potential inflow cessation by the mid-21st century. Extrapolated trends indicate that channel storage may reach 7.4 billion m³ by 2071, coinciding with a transition toward a “one-in, three-out” regime. This evolving hydrodynamic pattern is expected to weaken Dongting Lake's flood regulation capacity and exacerbate hydrological and ecological fragmentation. These findings highlight the necessity of integrated morphological monitoring and adaptive water management to mitigate the cumulative impacts of large-scale reservoir regulation on downstream river–lake systems.