Flathead Lake is the largest natural lake in Montana and the largest freshwater lake in the western United States. It is the one of the cleanest lakes of its size in the world. Located in the northwest corner of the state, 7 miles south of Kalispell, it is nearly 30 miles (44 km) long, 16 miles (25 km) wide, and covers 197 square miles (510 square km). Formed by the glacial damming of the Flathead River, the lake is bordered on its eastern shore by the Mission Mountains and on the west by the Salish Mountains . Once known as Selish Lake, the lake takes its name from the Salish (Flathead) Indians who live on the Flathead Indian Reservation located at the southern end of the lake. Kerr Dam, near Polson, regulates the lake's water level and provides hydroelectric power and water for irrigation. The lake has an irregular shoreline and several small islands, the largest of which is Wild Horse Island, a state park. Besides the Flathead River, the Swan River is the lake's other major tributary. The lake is inhabited by native bull trout and cutthroat trout, as well lake trout, yellow perch and lake whitefish .

Contents

1 Geology 2 Statistics 3 References 4 External Links

Geology

Flathead Lakes lies at the end of a geological feature called the Rocky Mountain Trench . The trench, which formed with the Rocky Mountains roughly 60 million years ago, extends north into the southern Yukon as a straight, steep valley. During the last ice age this trench was filled by an enormous glacier. As the glacier moved southward, it carved out the trench. Present day Polson, Montana marks the southernmost extent of the glacier during the last ice age. Polson is the site of the glacier's end moraine .

The large size of the Polson Moraine indicates that the glacier stalled here for several years before retreating. As the climate warmed, a portion of the glacier in the Mission Valley receded more slowly than the main body, which kept the lake basin from being filled with sediment. Eventually this ice also melted, forming a lake behind the moraine. Once the water reached the top of this moraine dam, it began to cut a channel through it. Most moraine dammed lakes drain quickly because water cuts entirely through the moraine. However, Flathead Lake remains because a bedrock hill buried underneath the Polson Moraine prevented the moraine from being completely cut through so the meltwater never completely drained.

Statistics

Maximum Length	27.3 miles	43.9 km
Maximum Width	15.5 miles	24.9 km
Maximum Depth	370.7 feet	113.0 meters
Mean Depth	164.7 feet	50.2 meters
Area Covered by Lake	191.5 sq miles	495.9 sq kms
Area Covered by Islands	5.5 sq miles	14.2 sq kms
Volume of Water	5.56 cu miles	23.2 cu kms
Length of Shoreline	187.6 miles	301.9 km

References

Alt, David. "The Making of Flathead Lake" in Profiles of Montana Geology: A layman's guide to the Treasure State. Butte, MT: Montana Bureau of Mines and Geology, 1984

Flathead Lake Biological Station. "About Flathead Lake." Available at http://www.umt.edu/biology/flbs/aboutflbs/flatheadlake.htm

External Links

Flathead Lake Biological Station
Flathead Lakers
The Flathead from the Space Shuttle by James Conner