Pengfei Zhan, Liguang Jiang, Kai Liu, Tan Chen, Chenyu Fan, Fanxuan Zeng, Chunqiao Song
Abstract The bidirectional exchange between rivers and floodplain lakes is critical for regional hydrology, nutrient cycling, and ecosystem management. However, observing the two‐dimensional water surface elevation (WSE) fields that govern these interactions remains a long‐standing challenge, limiting understanding of hydraulic regime shifts. The Surface Water and Ocean Topography (SWOT) mission provides unprecedented capability to resolve these dynamics with high‐resolution, spatially continuous WSE data. Taking the Poyang Lake‐Yangtze River system as a case study, we reveal a divergent seasonal response in water surface slope. The river mainstream slope steepens during the dry season (2.61 cm/km), while the river‐lake confluence paradoxically exhibits steeper or reversed slopes during wet season. This mechanism leads to a seasonal backwater effect, more extensive in the dry season (17.2 km) than wet season (14.8 km). Our findings offer a globally applicable framework for understanding river‐lake hydrodynamics, showcasing SWOT's potential for advancing hydrodynamic modeling and water management.