Kootenai (kōˈtĭnā) [key], river, 407 mi (655 km) long, rising in the Rocky Mts., SE British Columbia, Canada. It flows S into NW Montana, NW through N Idaho, then N into Canada. There it flows through Kootenay Lake (64 mi/103 km long; 191 sq mi/495 sq km), an expansion of the river, before joining the Columbia River at Castlegar. The river is used to generate hydroelectricity. The Canadian name is spelled Kootenay.