High aggradation and degradation in a river induced by the last flood event will raise and decrease the risk of an upcoming flood event, respectively. To quantitatively investigate this effect, a two-dimensional (2D) hydrodynamic model coupled with a sediment transport the river-bed evolution models is developed. After being validated against two experimental benchmark tests, the model is applied to simulate the effects of the morphological change on flood patterns. It is found in two experimental scale tests that bed evolution may have substantial influence on water level, which is directly related to flood risk extent. In the application of the model computing a realistic flood event in Bayangaole Reach of Yellow River, the results show that the water surface level becomes lower for an upcoming flood event with the same hydrograph, indicating the flood risk considerably decreases in the next flood event. The effect of a perturbed bed therefore could be of significance for fluvial flood over a movable bed and therefore to flood risk management.