At the small town of Canal Flats the Kootenay River passes within of Columbia Lake, the headwaters of the Columbia River, as it merges into the Rocky Mountain Trench along the eastern foothills of the Selkirk Mountains. It receives the Lussier River near Skookumchuck Station of the Canadian Pacific Railway and the St. Mary and Wild Horse rivers at the historic mining town of Fort Steele, then receives the Bull River at the settlement of the same name. At Wardner, British Columbia, the Kootenay widens into the Lake Koocanusa reservoir, formed by Libby Dam over downstream at Jennings, Montana. The Elk River, the Kootenay's longest tributary, enters Lake Koocanusa south of Elko, British Columbia.