Lake Cushman is a 4,014.6-acre (16.247 km2) lake and reservoir on the north fork of the Skokomish River in Mason County, Washington. The lake originally was a long narrow broadening of the Skokomish River formed in a glacial trough and dammed by a terminal moraine from the Vashon Glaciation during the most recent ice age. The lake was expanded after construction of the Cushman Dam No. 1. The lake is maintained by this dam and provides electrical power to the Tacoma Power system.