Kootenay River, a tributary in the Columbia River system in western North America. From its source in the Rocky Mountains of southeastern British Columbia, the Kootenay flows south into the United States (where its name is spelled Kootenai), turns northwest, and flows back into Canada. It passes through Kootenay Lake and empties into the Columbia River at Castlegar, British Columbia, after a course of 485 miles (780 km). Libby Dam, a large flood-control and hydroelectric project, is on the river near Libby, Montana. The river is named for the Kutenai Indians.