The 2005 typhoon disasters taught us that the Mimikawa River system in Miyazaki Prefecture had various problems with sediments in the river basin (the riverbed in dam reservoirs rise so much that flooding risk increases, sediments captured at dams granulate the downstream river channel and destabilize the bridge piers in the downstream river channel). In one of the approaches to solve these problems simultaneously, since 2017, sediment sluicing at a series of dams has been operated cooperatively. Using this approach, we can expect that the river will recover its continuity of sediment movement, returning its physical environment and animal habitats to their original states. Based on environmental surveys and numerical analyses of the current conditions, we have predicted the environmental impacts due to sediment sluicing at dams and defined the method to assess them. This paper presents: 1) an overview of sediment sluicing at dams in the Mimikawa River system, 2) the analysis of the river environment prior to operating sediment sluicing at dams, 3) the prediction of the impacts of sediment sluicing at dams in the river environment, 4) the environmental impact assessment after operating sediment sluicing at dams.

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