Backflow is an important mechanism controlling flow and transport in many connected river-lake systems.This study used statistical methods and hydrodynamic-particle tracking models to examine primary causal factors of backflow,indication of backflow occurrence and influences of backflow on the hydrodynamics in a large floodplain lake (Poyang Lake,China).Statistical analysis indicates that although both catchment inflows and the Yangtze River discharge are all the influencing factors of backflow,the Yangtze River discharge is the main controlling factor of backflow during July and October,rather than catchment inflows to the lake.The ratio of Poyang Basin inflows to Yangtze River discharge can be used as an indication of the daily occurrence of backflow,which is most likely to occur during periods when this ratio is lower than 5%.If this ratio is higher than 10%,the probability of backflow occurrence is lower than 2%.Hydrodynamic modeling reveals that,in general,backflow causes an increasing in lake wa-ter levels ranging from 0.2 to 1.5 m,and tends to increase the magnitude of the water velocities (by up to 0.3 m/s) in the lake's main flow channels.Backflow disturbs the normal northward water flow direction in range of 90°-180° across Poyang Lake,and transports mass varied from a few kilometers to approximately 20 km southward into the lake.Backflow tends to increase the magni-tudes of the water velocities and water levels in the lake's main flow channels more than in the shallow floodplain areas.The influ-ence of backflow on the hydrological regimes in Poyang Lake attenuates with distance from the Yangtze River,as expected,but nonetheless propagates to virtually its upstream extremity.