Flathead Lake Facts

How big is Flathead Lake? How deep is it? What fish live here, and does the lake ever freeze? Explore facts about Flathead Lake's size, water, wildlife, history and science on this page:

27.3 miles

Maximum length of Flathead Lake

15.5 miles

Maximum width of Flathead Lake

370.7 feet

Maximum depth of Flathead Lake

191.5 square miles

Area covered by the Flathead Lake's water

187.6 miles

Total mainland and island shoreline

2.2 years

Approximate flushing time for Flathead Lake

How Big Is Flathead Lake?

One of the West's Largest Natural Lakes

Flathead Lake is the largest natural freshwater lake by surface area in the western United States outside Alaska.

Lake Tahoe contains more water because it is much deeper, while the Great Salt Lake is larger in surface area but contains salt water rather than fresh water.

Flathead Lake is about 27 miles long and more than 15 miles across at its widest point. It covers 191.5 square miles and contains about 5.56 cubic miles of water.

How deep is Flathead Lake?

The deepest part of Flathead Lake is 370.7 feet (113 meters) deep. Its average depth is about 164.7 feet (50.2 meters).

How much shoreline does it have?

Flathead Lake has about 161.4 miles of mainland shoreline and another 26.2 miles around its islands, for a total of about 187.6 miles.

Water, Weather & Seasons

How warm does Flathead Lake get?

Average surface temperatures change dramatically through the year. They range from about 36°F (2.3°C) in mid-January to 56°F (13.5°C) in mid-June and about 68°F (20.3°C) in mid-August.

Does Flathead Lake freeze?

Usually, no. Flathead Lake contains an enormous amount of water, and wind can keep its surface moving. It takes an unusually long period of very cold and calm weather for the entire lake to freeze.

Bays and areas near the shoreline may freeze during winter. Historical FLBS observations show that the entire lake once froze about once each decade, but it has not completely frozen since the winters of 1988-89 and 1989-90.

What happens during spring runoff?

The Flathead River usually reaches its greatest flow during spring snowmelt, generally between May 15 and June 15. The river can carry enough sediment into Flathead Lake to create a visible plume that sometimes extends across the lake.

Why Does the Lake Level Change?

Flathead Lake's water level changes with the seasons. The lake and its outflow are regulated by Seli’š Ksanka Qlispe’ Dam on the Lower Flathead River near Polson.

Regulation of the dam causes the lake level to fluctuate by approximately 10 feet during the year, generally between elevations of 2,883 and 2,893 feet above sea level.

When mountain snowpack and flood conditions allow, the lake is typically raised during late spring and reaches full pool for summer recreation.

What Fish Live in Flathead Lake?

Flathead Lake is home to both native fish that evolved in this region and nonnative fish that were introduced by people.

Flathead Lake originally supported 11 native fish species. Some of the best-known native fish include bull trout, westslope cutthroat trout and mountain whitefish.

Bull Trout

Salvelinus confluentus — A native predator found in the Flathead Basin

Westslope Cutthroat Trout

Oncorhynchus clarkii lewisi — Montana's state fish and an important native species of the Flathead Basin.

Mountain Whitefish

Prosopium williamsoni — A native fish found in lakes, rivers and streams throughout the basin

Then and Now: A Changing Fish Community

The fish community in Flathead Lake looks very different today than it did more than a century ago.

Beginning in the late 1800s, fisheries managers introduced numerous nonnative fish species to increase fishing opportunities. The later arrival of nonnative Mysis, or opossum shrimp, in the mid-1980s caused additional major changes to the lake's food web.

Today, nonnative species such as lake trout, lake whitefish and yellow perch are prominent members of the lake's fish community.

Why does this matter?

Introducing a new species can affect many other organisms. Changes to Flathead Lake's fish and food web show how connected the different parts of an ecosystem are.

Complete Fish List

Native
Species, Common Name	Habitats¹	Current Status^2,3
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)
Species, Common Name	Habitats¹	Current Status^2,3
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,?

Habitats: FL, L, R, A, FLR
Distribution: A, C, R
Trend: S, D, E, ?
Listed: L

¹ Habitats: 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 Seli’š Ksanka Qlispe’ Dam.
² Distribution: Abundant throughout Basin (A), common in many areas (C) or restricted (R);
³ Trend: Population size stable (S), declining (D), expanding (E), or unknown (?). Listed (L) under the Federal Endangered Species Act.
⁴ Introduced 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.

How Was Flathead Lake Formed?

A Lake With an Ice Age Story

Flathead Lake is a remnant of ancient Glacial Lake Missoula, which covered much of western Montana until roughly 15,000 years ago.

During the Ice Age, an enormous glacier blocked the Clark Fork River and created a huge lake behind a natural ice dam. When that ice dam failed, tremendous amounts of water rushed west across what is now Idaho, Washington and Oregon.

These repeated floods moved enormous amounts of rock and sediment and helped create famous landscapes including the scablands of eastern Washington and the Columbia River Gorge.

Did you know?

Flathead Lake's Ice Age story is connected to landscapes hundreds of miles away in the Pacific Northwest.

Is Flathead Lake Clean?

Flathead Lake is known for its clear water, but keeping the lake healthy requires careful monitoring and protection.

Scientists spotted warning signs

Long-term monitoring by scientists at the Flathead Lake Biological Station detected signs of declining water quality, including increased algal growth and decreases in oxygen in deep water. Nutrients from human sources and shoreline erosion were among the concerns.

Science helped lead to action

Those observations helped encourage major water-quality protections, including restrictions on phosphate detergents and improvements to wastewater treatment.

Scientists are still watching

FLBS researchers continue to measure nutrients, water clarity, temperature and other characteristics of Flathead Lake. Long-term monitoring allows scientists to detect changes that might otherwise be difficult to see.

Why monitor a lake for decades?

A lake can look the same from year to year while important changes are happening underwater. Long-term measurements help scientists separate normal year-to-year variation from lasting changes.

Wild Horse Island

Wild Horse Island is the largest island in Flathead Lake. It covers approximately 2,100 acres and rises about 1,200 feet above the lake.

The island is known for wildlife including wild horses and Rocky Mountain bighorn sheep, as well as abundant native grasses and wildflowers.

Other public lands around Flathead Lake include state parks, fishing access sites and the National Wildlife Refuge on the North Shore.

Bighorn Sheep on Wild Horse Island - credit: Montana Fish & Wildlife Wild Horse Island

People & History Around Flathead Lake

Travel around the lake wasn't always easy

The first wagon trail from Polson to Somers followed the West Shore in the 1880s. The route was steep and hazardous. In some places, travelers had to lower wagons using ropes.

Before today's East Shore road

Work on an East Shore road began in 1911, but the road was not completed until 1946. Before its completion, students and researchers traveling to the Flathead Lake Biological Station sometimes arrived by horse or steamboat.

More than a century of lake science

The Flathead Lake Biological Station was established in 1899 by University of Montana biology professor Morton Elrod.

The Biological Station was originally located in Bigfork and later moved to its present location at Yellow Bay.

Flathead Lake by the Numbers

Maximum Length	27.3 miles (43.9 km)
Maximum Width	15.5 miles (24.9 km)
Maximum Depth	370.7 feet (113 m)
Mean Depth	164.7 feet (50.2 m)
Area Covered by Lake	191.5 sq miles (495.9 sq km)
Area Covered by Islands	5.5 sq miles (14.2 sq km)
Volume of Water	5.56 cu miles (23.2 cu km)
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 m)
Breadth	5.2 feet (1.58 m)
Flushing Time	2.2 years

Explore the Depths of Flathead Lake

A bathymetric map is like a topographic map of the land beneath the water. It uses contour lines to show the shape and depth of the lake bottom.

The printable FLBS Flathead Lake bathymetric map shows depth contours as well as public access locations around the lake.

View Printable Bathymetric Map

Looking for Today's Flathead Lake Level?

View current and historical Flathead Lake levels, weather and meteorological measurements from FLBS.

View Flathead Lake Weather & Level

How Do We Know These Facts?

The Flathead Lake Biological Station (FLBS) is a University of Montana research and education center located at Yellow Bay on Flathead Lake.

FLBS scientists have studied Flathead Lake and the Flathead watershed for more than a century. Long-term measurements of water quality, weather, aquatic life and other environmental conditions help scientists understand how the lake works and how it changes over time.

This long scientific record helps FLBS provide information to students, researchers, resource managers, decision-makers and communities throughout the region.