Flathead Lake

Flathead Lake is a large natural lake in northwest Montana, 7 miles (11 km) south of Kalispell, spanning Lake and Flathead counties. It covers 197 sq mi (510 km2), reaches a maximum length of 27.3 mi (43.9 km) and width of 15.5 mi (24.9 km), with a maximum depth of 370.7 ft (113.0 m) and an average depth of 164.7 ft (50.2 m). By surface area it is the largest natural freshwater lake west of the Mississippi within the contiguous United States. The lake is a remnant of Glacial Lake Missoula, the massive proglacial lake that occupied much of western Montana during the last ice age. Flathead Lake sits behind the Polson Moraine — a terminal moraine marking the southernmost extent of glacial advance during the Pleistocene — with a bedrock hill beneath the moraine preventing complete drainage as the ice retreated. The lake's primary inflows are the Flathead River and the Swan River, and its watershed encompasses 8,587 sq mi (22,240 km2). Flathead Lake is co-managed by Montana Fish, Wildlife and Parks and the Confederated Salish and Kootenai Tribes, with outflows regulated at the Seĺiš Ksanka Qlispé Dam (formerly Kerr Dam) near Polson. It is widely described as one of the cleanest lakes in the populated world for its size and type.