Makio dam in the Kiso River is a multipurpose dam which generates hydroelectric power and supplies water to the Aichi Irrigation Project. Reservoir sedimentation increased in this Dam because of Nagano West Earthquake in 1984. It cost about 30 billion yen to recover active storage capacity in 1996 by excavating deposited sediments. In the future, flow regime is going to change due to global warming which will have another impact on water resources management. In this study, we assessed multiple effects of the several reservoir sedimentation and future in flow change by GCM model and distributed hydrological model (Hydro-BEAM). We evaluated economic effect in future (2093-2110) in terms of hydropower generation and water supply to the Aichi Irrigation Project. Regarding hydroelectric power generation, annual total generation can be almost maintained by modifying seasonal dam operation rule. Regardless of the sedimentation scenarios and flow-regime changes, annual total hydroelectric power generation can be maintained. However, water supply to the Aichi Irrigation Project will be seriously damaged due to a loss of active storage volume by reservoir sedimentation and changes in seasonal availability of discharge.