It marks the southern limit of the Rocky Mountain Trench, a structural depression extending northward to the Liard Plain of British Columbia, Canada.
Bordered on the eastern shore by the Mission Range and on the west by the forested foothills of the Salish Mountains, it is a remnant of the ancient, massive glacial dammed lake, Lake Missoula.
The lake is 48 km (30 miles) long, 26 km (16 miles) wide, covering 510 square kilometers (197 square miles).
It is larger in surface area than Lake Tahoe, but it is much smaller in volume due to Tahoe’s depth.
Flathead Lake has a maximum depth of 113 m (370.7 ft), and an average depth of 50.2 m (164.7 ft).
The lake has an irregularly shaped shoreline and a dozen small islands, the largest of which is Wild Horse Island at 876 hectares (2,164 acres).
Flathead Lake is in a scenic part of Montana, 48 km (30 miles) southwest of Glacier National Park and is flanked by two scenic highways, which wind along its curving shoreline. On the west side is U.S. Route 93, and on the east, is Route 35.
The Flathead River and the Swan River (known also as the Bigfork River where it enters the lake) are the lake’s major tributaries.
The lake is inhabited by the native bull trout and cutthroat trout, as well as the non-native lake trout, yellow perch, and lake whitefish.
Flathead Lake is one of the cleanest lakes in the populated world for its size and type.
The mild climate allows for cherry orchards on the east shore and vineyards for wine production on the west shore. There are also apple, pear and plum orchards around the lake as well as vegetables, hay, honey, nursery tree, Christmas tree, sod/turf, and wheat production bordering or near the lake.
Once known as “Salish Lake”, this body of water was named for the Salish Indians. Early European Americans called them the Flathead Indians because of a misinterpretation of early Native American sign language. A common misconception is that the name is derived from a practice of head flattening more common among tribes such as the Chinook. There is no evidence to show that the Salish ever had this custom. Since the late 19th century, the Salish were mostly removed to the Flathead Indian Reservation, located at the southern end of the lake.
In 1812, Canadian explorer David Thompson rode to a hill near Polson and described Flathead Lake in his journal.
In 1855, treaty established the Flathead Indian Reservation, which includes the south half of Flathead Lake.
Fur traders employed by the North West Company and Hudson’s Bay Company entered the Flathead Valley in the early 19th century. Trading posts were established north of Flathead Lake. The first settlers began arriving in the 1860s.
Commercial boating developed on Flathead Lake beginning in the 1880s to transport passengers, merchandise, lumber, agricultural products, and livestock.
In 1899, University of Montana Flathead Lake Biological Station established. It is one of the oldest active biological field research stations in the United States.
The lake was dammed in 1930 by Kerr Dam at its outlet on Polson Bay, and the lake level was raised by 3 meters (10 feet). The dam was designed to generate hydroelectricity but also serves recreational and irrigation uses.
In 1958, Flathead Lakers was founded, one of the oldest and largest lake protection organizations in North America.
The largest island in the lake Wild Horse Island is a state park since 1977. This island has been a landmark since the Salish-Kootenai Indians were reported to have used it to pasture horses to keep them from being stolen by other tribes. The park is noted for its wildlife.
Kalispell is a city with population of about 20,000. It is located near the lake. The name Kalispell is a Salish word meaning “flat land above the lake”.