Floods out of all other water problems cause very large damages in China. Previous flood management plans mainly focused on a single river by controlling its water level and conveyance capacity, while research on mitigation solutions to flooding issues in river-lake systems is scarce. This study considers the Huai River - Lake Hongze system, which is one of the largest river-lake systems in China. Very large damages associated with small floods are frequently observed in this river-lake system, although a series of flood management measures have been implemented in the Huai River Basin since the middle of the last century. An unstructured-grid finite-volume numerical model was applied to simulate hydrodynamic processes in this system, which is characterized by discrepant spatial scales between these two types of water bodies. It is found that the lake affects the upstream river flooding, but lowering the lake level would have limited effects that would be rapidly impaired by the sharp meander bend connecting the river and the lake. The artificial cutoff of this intensively-embanked bend has great potential in reducing the river stage and flood damages, as the construction of a diversion channel would shorten the flow path and increase the hydraulic gradient. This study extends the current knowledge about the hydrodynamics of river-lake systems and is beneficial to flood mitigation strategies for similar systems.