Flathead Lake is 27.3 miles (43.9 km) long and up to 15.5 miles (24.9 km) wide with a maximum depth of 370.7 ft (113.0 meters), making it deeper than the average depths of the Yellow Sea or the Persian Gulf and about half the area of San Francisco Bay (main bay). The lake was raised 10 feet (3.0 meters) by Kerr Dam and is one of the cleanest in the populated world for its size and type.
Located in the northwest corner of the state of Montana, 7 miles (11 km) south of Kalispell, it is approximately 30 miles (50 km) long and 16 miles (25 km) wide, covering 191.5 square miles (495.9 km²). On the west side, the lake is flanked by U.S. Route 93, and on the east, by Route 35. The lake is bordered on its eastern shore by the Mission Mountains and on the west by the Salish Mountains.
 Open Water Swimming
- Flathead lake is the site of the LAKE Monsters and Flathead Lake Open Water Swimmers (FLOWSWIMMERS), coached by Mark Johnston as well as the site of the 1.2-mile Water Daze in Polson Bay and the Lakeside Lighthouse Loop event on the western shore of Flathead Lake, both sanctioned by the World Open Water Swimming Association.
- Flathead Lake is the site of Enduring Waves for Isabella, a 24-hour non-stop charity swim, scheduled to begin on 5 September 2014, by Emily von Jentzen, an open water swimmer who raises money for children with special medical conditions. The swim will be a perimeter swim about Flathead Lake.