Flathead Lake, 197 sq mi (510 sq km), 30 mi (48 km) long, NW Mont.; largest natural lake in Montana. Formed by the glacial damming of the Flathead River, which flows through it from north to south, Flathead Lake has an irregular shoreline and many small islands. Surrounded by mountains, the lake is a noted recreation area. Kerr Dam, at the southern end of the lake, provides hydroelectric power and water for irrigation.