Dominant hydrological drivers of vegetation pattern changes in a river-connected lake after dam construction: outlet or inlet?

• The water–sediment environment of Dongting Lake was significantly altered by Three Gorges Dam. • Inundation patterns affected wetland vegetation distribution and succession. • Reduced water depth resulted in the space encroachment of Reed on Carex after Three Gorges Dam construction. • Water level change at the outlet was the major contributor to vegetation succession. Understanding the relationships between the spatiotemporal characteristics of vegetation and key hydrological factors is crucial for maintaining ecosystem stability in river-connected lakes. However, the complex hydrological interactions between the inlets and outlets of these lakes and rivers influence the direction and rate of vegetation succession, making wetland vegetation restoration in river-connected lakes more challenging. This study focused on Dongting Lake, an example of a river-connected lake in China, and examined the spatiotemporal variations in inundation and vegetation patterns following the construction of the Three Gorges Dam (TGD). Using the hydrodynamic model MIKE 21, we evaluated the contribution of hydrological conditions at the outlet and inlet to vegetation changes. The results revealed that after TGD operation, water discharge from the three channels and the water level at the outlet dramatically reduced, leading to a decrease in inundation duration and depth, particularly in East Dongting Lake. Concurrently, the vegetation area expanded and moved toward the lake’s center. The suitable inundation durations for Carex and Reed were 132–240 d and 110–186 d, respectively, suggesting that reduced water depths favored Reed growth and exacerbated the encroachment of Reed on Carex . Additionally, the decline in the water level at the outlet was identified as the key factor influencing vegetation succession in Dongting Lake after the TGD began operation. Therefore, water level regulation is key to mitigating the adverse impacts of the TGD on the lake’s ecosystem. This work provides significant insights for the restoration of wetland vegetation in river-connected lakes impacted by dams.