River bank stability is very sensitive to the variation of the water level (WL) of the river. This is the case along the Lower Mekong River which WL change between dry and wet seasons is about 10m. This paper presents a study of river bank stability subjected to variations of the water level. Dupuit assumption is supposed to find the variation of WL in the soil using a numerical solution by an explicit finite difference method. The mass sliding Safety Factor (SF) predicted by the simplified slices equilibrium method varies significantly as a function of WL variation and the risk of land slide is high when the SFs are minimums. By using different hydrographs we can demonstrate that the maximum risk of mass slide appears to occur after a rapid drop of the WL and is worse if the permeability of soil is small. Furthermore, by imposing different distances between the slip surfaces and the top of the bank, a safety zone is determined, that results useful for local risk management.