The forestation in rivers causes several problems, because it reduces river flow capacity, and the forestation by invasive spices sometimes affects endemic species and the biodiversity in a river habitat. The wash-out of vegetation during a flood is known to be a reasonable measure for preventing the forestation. However, the relationship among the wash-out condition, a river discharge and a bed deformation has not been clarified yet. This paper investigates the vegetation growth rate and the wash-out condition of vegetation during the flood at Ohara area in the Asahi River, based on the field observations and the numerical simulations. The observation results showed that the large discharge during the flood induced the bed deformation and the wash-out of vegetation, and the simulated results indicated that the non-dimensional bed shear stress of can evaluate the washed-out condition more suitably.