Flathead Lake is the largest natural freshwater lake west of the Mississippi in the lower 48 states, with over 200 square miles of water and 185 miles of shoreline. The southern half of Flathead Lake is within the boundary of the Confederated Salish and Kootenai Tribes Flathead Reservation. Recreationists must purchase a tribal recreation permit.
Flathead Lake is located in the northwest region of Montana. The east shore is bordered by Montana Highway 35 and the west side of the lake is accessible from US Highway 93 between Polson and Kalispell.
Montana Fish Wildlife and Parks maintains thirteen public access sites around the lake. These sites include: Sportsmans Bridge, Somers, Big Fork, and Juniper Beach fishing access sites; Wayfarers, Woods Bay, Yellow Bay, Finely Point, Walstad Memorial, Big Arm, and Elmo state recreation areas, which have toilets, boat launch, camping, swimming, and picnic facilities; and West Shore State Park, located twenty miles south of Kalispell on Highway 93. Also located along the west shore near the Big Arm is Wildhorse Island, a 2,165-acre state park. The park is accessible only by boat and is a public day-use and picnic area only, no overnight camping.
Recreation on and around the lake includes: sailing, power boating, waterskiing, swimming, fishing, picnicking, and camping. In the summer, roadside stands along the east shore offer a variety of locally grown cherries, apples, plums, and other fruits. There are numerous motels and rental cabins in addition to public campgrounds scattered all around the shoreline. The major population centers around the lake are Kalispell, Bigfork, and Polson, all of which offer a complete variety of groceries, supplies, and information.
Why is Flathead Lake so clear?
The primary reason is that it is relatively low in nutrients. Phosphorus and nitrogen are two elements that promote the growth of algae, and since Flathead Lake has very low levels of these nutrients, it remains clear
Another key factor is its location. Flathead Lake is situated in a glacial valley, which was carved out by ancient glaciers. As a result, the lake is very deep — up to 370 feet in some places. The deep water helps to prevent sediments from being stirred up, resulting in clearer water.
Additionally, the lack of inflowing rivers means that there is less chance for pollutants to enter the lake. The combination of these factors makes Flathead Lake one of the clearest lakes in the world.
Can you swim in Flathead Lake?
As one of the cleanest in the world, Flathead Lake is perfect for OPEN WATER SWIMMING! No sharks, no jellyfish, and no lane lanes…just clear, clean, and crisp water!
Salish Point is located on the lake’s southern shore in Polson, MT. The park offers a very large, roped-off swimming area between two fishing docks (about 250m apart) and stretching 30m out from the beach. In addition to a nice area for open water swimming, kids can be found during the summer months jumping and playing on the intermediate docks located within the swimming area.
Unless you like your water COLD, the effective swimming season is June through September. Average water temps in June are in the 60s, 70s in July and August, and then back down again in September.
Summer air temperatures average in the mid-70s to mid-80s.
Bathrooms are available and parking is easy. There are no available concessions, so bring your own food and beverages.
Always obey signs at the beach or advisories from official government agencies. Stay alert and check for other swimming hazards such as dangerous currents and tides. Please report your pollution concerns so Affiliates can help keep other beach-goers safe.
Beach conditions change constantly, and it is never wise to swim in open water up to 48 hours after a heavy rain. Please use caution!
Flathead Lake Facts
- The Flathead Lake Biological Station (FLBS) is a year-round University of Montana Center of Excellence that conducts ecological research with an emphasis on fresh water, particularly Flathead Lake and the Flathead watershed. FLBS also provides field ecology courses for college students, natural resource professionals and educators from around the nation; trains graduate students for professional and teaching careers; and provides scientific data, interpretation and outreach to help resolve environmental problems and inform public policy.
- FLBS is one of the oldest active biological stations in the US. It was established in 1899 by Dr. Morton Elrod, the first Biology Professor at the University of Montana. Dr. Elrod was instrumental in the creation of the National Bison Range and Glacier National Park for which he was the first naturalist and wrote its first guide book. FLBS was originally located in Bigfork, but was moved to Yellow Bay in the 1908.
- Flathead Lake is the 79th largest of the natural freshwater lakes in the world, and it is one of the cleanest.
- Flathead Lake’s high water quality results from its watershed being mainly National Park, Wilderness and managed forest lands (>60%); having a relatively low human population (~95,000); being dominated by very old, low nutrient geology; receiving high amounts of precipitation (mostly as mountain snow); and rapid flushing of the Lake (about 2.2 years for all the water to be replaced). In contrast, Lake Tahoe’s flushing time is about 650 years.
- FLBS serves as the “Sentinel of the Lake”, having collected samples and data on Flathead Lake ecology and water quality for over 100 years, and provides insights into ecological conditions and changes over time. Since 1977, FLBS researchers have conducted a rigorous scientific monitoring program, which has shown declining water quality (e.g., increases in algal growth and algal blooms, declines in oxygen in bottom waters). These changes appear to be due to increases in nutrient pollution from human sources, shoreline erosion, changing climate and introduced species (particularly Mysis shrimp); and would have gone unnoticed if FLBS researchers were not conducting long-term monitoring. Currently, FLBS researchers are developing biological and physical models to better understand the influence of increasing nutrients and temperatures plus highly complex community interactions on Flathead Lake’s water quality.
- Decreases in water quality have led Federal and State agencies to classify Flathead Lake as “Impaired” due to human caused increases in nutrient and sediments, and to work on creating a long-term plan for water quality protection.
- Flathead Lake is currently described as oligotrophic which means lacking in plant nutrients, but FLBS monitoring indicates that nutrient inputs are increasing.
- Flathead Lake is the largest natural freshwater lake in the western US (by surface area) outside of Alaska. Lake Tahoe has more water than Flathead because it is significantly deeper (nearly 1650 ft vs. 380 ft). The Great Salt Lake in Utah is significantly larger than Flathead but is salt, not fresh water. And there are numerous larger man-made reservoirs.
- 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.
- Flathead Lake’s biological community is much different today than when FLBS was founded. The Lake originally had 11 native fish species, notably westslope cutthroat trout (Montana State fish) and bull trout (top predator). However, since the late 1800s, fisheries managers have introduced 19 nonnative fishes to “enhance” the Lake and its angling opportunities. These fish introductions, along with the arrival of the nonnative Mysis (opossum) shrimp in the mid-1980s, changed the biological community dramatically. Today, the fish community is more similar to the Great Lakes than Rocky Mountain lakes, as it is dominated by nonnatives, particularly lake trout, lake whitefish and yellow perch.
- Flathead Lake is a remnant of Glacial Lake Missoula, which covered much of Western Montana until roughly 15,000 years ago. Periodic rupturing of the ice dam that created the lake resulted in cataclysmic floods that swept across Washington and Oregon, removing and transporting huge amounts of sediments, creating the scablands of Eastern Washington, and carving out the Columbia River Gorge.
- The Lake’s major tributaries are the Flathead and Swan Rivers. There are numerous small streams that flow into the Lake, particularly on the wetter East Shore.
- Maximum river flow in the Flathead generally occurs between May 15 and June 15 during peak snowmelt, creating a sediment plume that can cover the entire lake surface.
- The Lake level and its outflow are regulated by Kerr Dam, which is located on the Lower Flathead River near Polson. Kerr Dam was completed in 1938 by the Montana Power Company, raised the Lake level 10 feet above its natural level, and generates 194 megawatts of electricity. It is cooperatively operated by PPL Montana and the Confederated Salish and Kootenai Tribes. Regulation by the dam results in the Lake level fluctuating seasonally 10 feet between 2,883 and 2,893 feet above sea level. If snowpack conditions in the mountains do not threaten flooding, lake level is brought to 2,890 feet by the end of May and to full pool by June 15 for summer recreation.
- Due to its large volume and fetch (distance of water across which wind blows), Flathead Lake requires very cold and calm conditions to freeze entirely. Therefore, most winters it does not freeze over, although some bays and margins have ice cover. FLBS historic observations show that the Lake froze over about once each decade, however the Lake has not entirely frozen since 1988-89 (March only) and 1989-90 (January only), perhaps reflecting warmer climatic conditions.
- Public lands around the Lake include a National Wildlife Refuge on the North Shore, six State Parks (including Yellow Bay which is on FLBS property) managed by Montana Fish Wildlife and Parks (FWP), and nine Fishing Accesses managed by FWP or the Confederated Salish and Kootenai Tribes.
- Wildhorse Island, one of the State Parks, is the largest island in the lake at 2,100 acres, and rises 1,200 feet above the Lake. It is noted for herds of wild horses and Rocky Mountain Bighorn sheep. Native grasses and flowers are abundant.
- The first wagon trail (1880s) from Polson to Somers followed the West Shore of the Lake and was steep and hazardous. In places, wagons were lowered by ropes.
- In 1911, work started from the south end of the Lake to build an East Shore road. The road, which was primarily built by convict labor, was not completed until 1946. Until then, FLBS students and researchers arrived by horse or steamboat.
- Economists estimate that Flathead Lake boosts shoreline property values by $6-$8 billion, nature based tourism (which depends upon a healthy Flathead Lake-River System) accounts for roughly 20% of the $7.8 billion annual economy of Flathead and Lake Counties, and ecological services (e.g., water supply and purification, flood and drought mitigation) contribute another $20+ billion in benefits to human society.