- 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
- 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
- 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
- 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,
- Flathead Lake is currently described
as oligomesotrophic (oligotrophic means being deficient in plant
nutrients while mesotrophic means having a moderate amount of
- 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
- 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
| Maximum Length
| Maximum Width
| Maximum Depth
| Mean Depth
| 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
| Curvature of Lake Surface
| Flushing Time
Flathead Lake Bathymetric
Flathead Lake level information is now available on the FLBS weather and meteorological data page.
Flathead Lake Annual
Link to the Flathead
Lakers' Flathead Lake Historical Annual Hydrographs
Satellite Views of