Hydrological regime changes in the river–lake system and their influences on the ecological environment downstream dams have attracted increasingly more attention all over the world. The Dongting lake downstream of the Three Gorges Dam (TGD) in the Yangtze River has been experiencing a series of hydraulic and hydrological changes over the last decade. The hydrological and ecological influences of the TGD flow regulation on the Dongting river–lake system and its functional mechanism during the impounding periods remain extremely unclear. This study examines the hydrological changes in the Dongting river–lake system based on a 1D/2D coupled hydrodynamic model. In particular, the inflow boundary of the model with and without the TGD was applied with the outflow and inflow of the TGD, respectively, during the same regulation periods. The results show that the diverted flow from the Yangtze River into the Dongting lake and outflow from the lake back to the river drastically decreased during the impounding periods, especially in October. The decreased water exchange between the Yangtze River and the Dongting lake impaired the water residence capacity to some extent in the lake. Stage decrease in the lake area resulted in a significant reduction in the water volume of the Dongting lake with the same time percentage. In addition, the obvious drainage effect in Dongting lake due to the increased stage difference and current speed after the TGD operation was the essential cause of hydrological changes in the lake area. These results provide an improvement in the understanding of impoundment influences on the large river–lake system and give some practical information for ecological environment management in similar river–lake systems.
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