Backwater and stormwater has significnat impacts on simulating water quantities. In the downstream area of a river-bay system, the water level fluctuations will attenuate or dampen the river discharge, resulting in extended inundation areas.On the other hand, in urbanized area, the stormwater drainage network might transport water in a direction opposite to the surface gradient. If the flow exceeds the maximum capacity of the drainage system, urban flash flooding will occur. The impacts of backwater and stormwater should be considered to appropriately simulate water quantities at the downstream East River.