With the flood disaster experienced in the Mimikawa River system due to Typhoon Nabi in 2005, various problems caused by sediment in the river basin became clear, including increased risk of flooding with rising riverbed levels in dam-regulating reservoirs, grain coarsening in the downstream river channel with the entrapment of sediment in dams, and the destabilization of bridge piers in the downstream river channel. As part of an effort to resolve all these problems simultaneously, from 2017 sediment sluicing at a series of dams was implemented. Sluicing is an operation carried out when there is river flooding due to typhoons. Dam-regulating reservoir drawdown is carried out, and sediment flowing into these reservoirs from the upstream is allowed to flow downstream of dams. This paper principally covers 1) an overview of sediment sluicing at dams in the Mimikawa River system, 2) understanding and analysis of the river environment prior to sluicing, 3) predictions of the effect on the river environment due to sluicing, and 4) testing of these predictions.