About Flathead Lake

Flathead Lake Facts

Flathead Lake is one of the 300 largest natural lakes in the world and is the largest natural freshwater lake in the western United States. Of those large lakes, Flathead is one of the cleanest. Studies at the Flathead Lake Biological Station show that water quality in Flathead Lake is among the best in the world.
The Lake's major tributaries are the Flathead and Swan Rivers.
Numerous small streams flow directly into the Lake at its shoreline, particularly on the wetter East Shore.
Kerr Dam is located at the outlet of Flathead Lake in Polson, at the southern end. Regulation of outflow by the dam maintains the Lake's level between 2,883 and 2,893 feet above sea level. If runoff conditions in the mountains don't warrant flood threats, the Lake level is brought to 2,890 feet by the end of May and to full pool by June 15.
Maximum flow in the Flathead River generally occurs during spring freshet between May 15 and June 10, creating a plume of sediment that covers the Lake surface.
Due to its massive volume and normally active winds over the surface, Flathead Lake does not freeze over most winters, although the bays often have winter ice cover. The Lake did freeze over in the winters of 1978-79 (all winter), 1987-88 (all winter), 1988-89 (March only) and 1989-90 (January only).
Open space on the shoreline includes the National Wildlife Refuge on the North Shore and State land managed by the Flathead Lake Biological Station as a refuge on the South Shore (Polson Bay).
Wildhorse Island, near Big Arm Bay, is the largest island in the lake, at 2,100 acres. It is managed by the Montana Department of Fish, Wildlife and Parks as a wildlife refuge. It is noted for its herd of Rocky Mountain Bighorn sheep and several wild horses. Native grasses and flowers are abundant.
Bird Island near The Narrows are State owned and are managed jointly by Montana Fish, Wildlife and Parks and the Flathead Lake Biological Station as a bird refuge. Geese, osprey, herons and eagles frequent the Bird Islands.
The name Yellow Bay was derived from the yellow rock outcrop on the point which is Precambrian without fossils. The outcrop belongs to the Algonkian substrata, which also outcrops in Glacier National Park.
Flathead Lake Biological Station was established in Bigfork in 1899 by Dr. Morton J. Elrod. the Flathead Lake Biological Station was moved to its present location in 1908 and is currently operated as a year-round academic and research facility mainly dealing with aquatic studies.
The first wagon trail (1880's) from Polson to the north end of the Lake followed the west side of the lake and was steep and hazardous. At some places, wagons had to be lowered by ropes.
In 1911, work started from the south end of the Lake to build an east shore road with the work primarily done by convict labor. It was not completed until September, 1946.
Flathead Lake is currently described as oligomesotrophic (oligotrophic means being deficient in plant nutrients while mesotrophic means having a moderate amount of dissolved nutrients).
In spite of its renowned purity, studies at the Bio Station show the water quality of Flathead Lake has declined over the last decade due to the combined effects of increased pollution from human sources, erosion of the shoreline caused by dam operations and introduction of nonnative biota (flora and fauna).
Average surface temperatures of the lake range from 2.3° C (36°F) in mid-January, to 13.5°C (56°F) in mid-June, to 20.3°C (68°F) in mid-August.
Of the 25 fish species most commonly found in the Flathead River-Lake ecosystem, 10 species are native and 15 have been introduced. The native species include redside shiner, peamouth minnow, squawfish minnow, largescale sucker, longnose sucker, sculpin, bull trout, cutthroat trout, pygmy whitefish and mountain whitefish. Lake trout, lake whitefish and yellow perch are the most common nonnatives and have increased in abundance since 1970, whereas native species have declined.

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
Mainland	161.4 miles	259.7 km
Island	26.2 miles	42.2 km
Total	187.6 miles	301.9 km
Curvature of Lake Surface
Length	13.2 feet	4.02 meters
Breadth	5.2 feet	1.58 meters
Flushing Time		3.4 years

Fish of the Flathead Basin

Common Name	Species	Habitats¹	Current Status^2,3
Native
Bull trout	Salvelinus confluentus	All	C,D, L
Westslope cutthroat trout	Oncorhynchus clarkii lewisi	All	C,D
Mountain whitefish	Prosopium williamsoni	All	C,?
Pygmy whitefish	Prosopium coulterii	FL,L,A	R,?
Largescale sucker	Catostomus macrocheilus	FL,L,R	C,S
Longnose sucker	Catostomus catostomus	FL,L,R	C,S
Northern pikeminnow	Ptychocheilus oregonensis	FL,L,R	C,S
Peamouth chub	Mylocheilus caurinus	FL,L,R	C,S
Redside shiner	Richardsonius balteatus	FL,L,R	C,S
Slimy sculpin	Cottus cognatus	FL,L,R	C,S
Longnose dace	Rhinichthys cataractae	FLR	C,S

Nonnative (year introduced)
Lake whitefish (1890)	Coregonus clupeaformis	All	A,E
Lake trout (1905)	Salvelinus namaycush	All	A,E
Rainbow trout (1914)	Oncorhynchus mykiss	FL,L,R	A,E
Brook trout (1913)	Salvelinus fontinalis	FL,L,R	A,E
Yellowstone cutthroat trout (1910s)	Oncorhynchus clarkii bouvieri	L,R,A	R4,S
Golden trout (1960s)	Oncorhynchus mykiss aquabonita	L	R4,S
Brown trout (1910s)	Salmo trutta	R,FLR	R,?
Kokanee salmon (1916)	Oncorhynchus nerka	A	R,D
Arctic grayling (1960s)	Thymallus arcticus	L,R,A	R4,S
Yellow perch (1910)	Perca flavescens	FL,L	A,S
Northern pike (1960s)	Esox lucius	FL,L,R	C,E
Largemouth bass (1898)	Micropterus salmoides	FL,L	R,E
Smallmouth bass (1960s)	Micropterus dolomieu	FL,L,FLR	C,E
Pumpkinseed sunfish (1910)	Lepomis gibbosus	FL,L	C,S
Black bullhead (1910)	Ameiurus melas	FL,L	C,S
Central mudminnow (1990s)	Umbra limi	L	R,?
Brook stickleback (??)	Culaea inconstans	FL,L,R	R,?
Walleye (??)	Sander vitreus	L	R4,?
White sucker (??)	Catostomus commersonii	L	R4,?
Black crappie (??)	Pomoxis nigromaculatus	L	R4,?

¹Habitats: Populations found in: Flathead Lake (FL), other Basin lakes (L), rivers and streams (R), with adfluvial (A) life cycle (adults in lakes, spawn in tributary streams); the Lower Flathead River (FLR) downstream of Kerr Dam.
²Distribution: Abundant throughout Basin (A), common in many areas (C) or restricted (R);
³Trend: Population size stable (S), declining (D), expanding (E), or unknown (?). Listed (L) under the Federal Endangered Species Act.
⁴Introduced to a few small lakes in the Basin.

Compiled by FLBS and adapted from: B.K. Ellis, J.A. Stanford et al. 2011; Stanford, J.A. and B.K. Ellis 2002; Holton and Johnson 1996; Montana Fisheries Information System; unpublished records; and professional opinion.