The Kootenay River after crossing the border into Canada again, soon forms a large body of water known as Kootenay Lake. This lake is fed from the south by the Kootenay River, and from the north by the Lardeau and Duncan Rivers. The Duncan Dam, the first of the Columbia River Treaty dams to be built in 1967 holds back the Duncan River and is a storage dam with a 45-kilometre reservoir and does not generate any electricity. Its main purpose is to hold back water in relation to the U.S Army Corps of Engineers Dam at Libby, Montana to regulate what flows through the concentration of dams and powerhouses south of Nelson on the Kootenay River.
Its outlet flows west and creates the West Arm of Kootenay Lake. Near the Outlet, far above the level of the lake, the high power electric lines bridge the gap; one of many innovations created out of necessity because of the rugged terrain of the area.