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Floodplain systems are most often hydrologically complex settings characterized by highly variable surface water-groundwater interactions that are subjected to wide-ranging wetting and drying over seasonal timeframes. This study used field methods, statistical analysis, and the Darcy's law approach to explore surface water-groundwater dynamics, interactions, and fluxes in a geographically complex river-floodplain wetland-isolated lake system (Poyang Lake, China). The floodplain system of Poyang Lake is affected by strongly seasonal shifts between dry and wet processes that cause marked changes in surface water and groundwater flow regimes. Results indicate that wetland groundwater is more sensitive to variations in river levels than the seasonal isolated lakes. In general, groundwater levels are lower than those of the isolated lakes but slightly higher than river levels. Statistical analysis indicates that the river hydrology plays a more significant role than the isolated lakes in controlling floodplain groundwater dynamics. Overall, the river shows gaining conditions and occasionally losing conditions with highly variable Darcy fluxes of up to +0.4 and -0.2 m/day, respectively, whereas the isolated lakes are more likely to show slightly losing conditions (less than -0.1 m/day). Although seasonal flux rates range from 7.5 to 48.2 m/day for surface water-groundwater interactions in the floodplain, the flux rates for river-groundwater interactions were around four to seven times higher than that of the isolated lake-groundwater interactions. The outcomes of this study have important implications for improving the understanding of the water resources, water quality, and ecosystem functioning for both the river and the lake.