Japan is characterized by steep mountain topography and weak geology as well as high precipitation and seasonal drastically changing temperature. As a result, sediment produced in mountain areas is frequently transported downstream and supplied to coastal areas through the river mouth which has formed many alluvial fans. In these upstream mountain rivers, many storage dams have been constructed for the purpose of flood control and water utilization. Nowadays, sediment accumulation in their active storage volume is a crucial issue both for reservoir and river basin sustainability.