Sediment replenishment is one of the effective methods to prevent a sediment deficiency and detaching overgrowing algae in downstream reaches of dams. In this study, we examined the effect on a bed material size and algal biomass based on data measured in Futase dam. We assumed that algae detaches from the bed materials when the bed materials are moved by flow. So, threshold bed material size in motion Dcri for a given τ is calculated. The result showed that in winter, algal biomass was very high if flow was constantly low (No.1). However, other result showed that the algal biomass was not so high if Dcri was temporally high (No.2). This is probably because the bed materials of No.2 were finer and easier to move by flow than those of No.1. Thus, sediment replenishment promotes detachment of algae not only by enhancing sediment transport during major floods but also by covering riverbed with fine materials and reducing bed stability in winter.