Lake Facts
Flathead Lake is the largest natural freshwater lake west of the Mississippi and the 79th largest natural lake in the world. It is over 27 miles long, 15 miles wide and has over 187 miles of shoreline with a maximum depth of 370 feet. Flathead Lake's waters are renowned for being one the cleanest as well. The lake water is completely flushed every 2.2 years. In comparison California/Nevada's similarly sized Lake Tahoe takes 650 years to completely flush.
The lake is bordered on the west by the Salish Mountains and the Mission Mountain Range on the east which is a part of the Rocky Mountains. The highest peak of the Mission Mountains is the glacier studded McDonald Peak at 9868’. Flathead Lake sits in the shadow of Glacier National Park just an hour drive from West Glacier to Bigfork. Polson is on the south side of the lake about an hour drive from Big Fork or Kalispell.
Average surface temperatures of the lake range from 36° in January to 56°in mid- June with the highs at 68° in mid-August. Due to its large volume and size Flathead Lake requires very cold and calm conditions to freeze entirely. Therefore, it does not freeze over in most winters, although some bays have ice cover. The lake has not completely frozen over since January 1990.
The southern half of the lake is on the Confederated Salish and Kootenai Tribes’ Flathead Indian Reservation. The lake level and its outflow are regulated by Salish Kootenai Dam (Se̓liš Ksanka Ql̓ispe̓ Dam) formerly known as the Kerr Dam which is located on the lower Flathead River near Polson and provides enough energy to power over 100,000 homes. Regulation by the dam has the lake level fluctuating seasonally 10 feet between 2,883 and 2,893 above sea level. The lake is typically completely full from mid-June until Labor Day.
Flathead Lake has many beautiful Islands. Wildhorse Island State Park is the largest at over 2100 acres and rising 1200 feet above the lake. It is noted for herds Rocky Mountain Bighorn Sheep, Mule Deer and a small band of wild horses. Osprey, Bald Eagles and many other birds populate the island as well.
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