Floodplain systems are affected by seasonal shifts between dry and wet cycles, and therefore cause marked changes in surface water and groundwater flow regimes. Surface water-groundwater interactions are rarely investigated in large floodplain systems, where few surface and groundwater data are obtained or monitored. This study used field observations, statistical analysis, and Darcy's law approach to explore surface water-groundwater dynamics, interactions, and the associated fluxes in a geographically complex river-floodplain wetland-seasonal lake system (Lake Poyang, China). Results indicate that the wetland groundwater is more sensitive to variations in the river levels than the seasonal lakes. Groundwater levels are generally lower than those of the seasonal lakes, but slightly higher than the surrounding river levels, due to the influence of the complex topography. Statistical analysis further reveals that the river hydrology tends to play a more significant role in controlling groundwater dynamics, relative to the seasonal lakes. Generally, the river shows gaining conditions (i.e., groundwater recharges river) and occasionally losing conditions (i.e., river recharges groundwater) with variable Darcy fluxes by up to 0.4 and 0.2 m/d, respectively, while the seasonal lakes are more likely to show slightly losing conditions (<0.1 m/d). Additionally, the annual accumulated flux rates range from 7.5 m/a to 48.2 m/a for surface water-groundwater interactions in the floodplain system. The accumulated flux rate for river-groundwater interactions is around four to seven times higher than that of the seasonal lake-groundwater interactions. Also, the accumulated flux in autumn and winter is higher than that of the spring and summer on a seasonal scale. The findings from this study have important implications for improving the understanding of the water resources joint management, water quality, and eco-environmental protection for both the river and the lake.