Reservoir sedimentation is one of the most important problems for securing long term achievement of hydropower plant operation in the future. We have already classified the sedimentation problem regarding the reservoir types and proposed effective sediment control measures for regulating reservoirs. On the other hand, sedimentation problem in storage reservoirs needs to be more studied. In this paper, we have evaluated the influence of sedimentation progress in storage reservoirs on power generation by analyzing long-term operation record, and predicted the future influence of reservoir sedimentation from the view points of active storage capacity and the maximum hydropower discharge.