This Ancient Montana Lake is So Clear, Boats Look Like They’re Floating on Air

Flathead Lake stretches endlessly beneath the vast Montana sky

Flathead Lake, along the mainline of Flathead River, is the largest natural freshwater lake in northern Montana. 

While the rugged mountains rise majestically in the distance, the shoreline curves gently, dotted with green forests and rocky outcroppings. It’s actually the 79th largest of the natural freshwater lakes in the world and one of the cleanest.

Nestled in the Flathead Watershed, here’s everything you need to know.

Flathead Lake is the largest natural freshwater lake west of the Mississippi

Situated in the Flathead Valley, the lake is bordered by the Swan and Mission mountain ranges. 

Covering about 197 square miles, it has a maximum depth of 370 feet. The lake’s shoreline stretches for 185 miles. Flathead Lake is 28 miles long and up to 15 miles wide. Experts estimate the lake’s volume at 18,776,897 acre-feet.

Did you know? Flathead Lake is so large that some of its water currents are affected by the Earth’s rotation.

Glacial activity formed the lake during the last ice age

The lake’s formation dates back approximately 15,000 years, during the last ice age. It’s leftover from the ancient glacial Lake Missoula. 

Glacial activity sculpted the Rocky Mountain Trench, leading to the lake’s formation. The Polson Moraine, marking the southernmost extent of the glacier, is located near the town of Polson, Montana. 

Fun fact: a bedrock hill buried under the Polson Moraine stopped the lake from draining completely. 

Flathead Lake is known for its exceptional water clarity

People call the lake one of the cleanest for its size and type in the populated world. 

The water in Flathead Lake fully refreshes every 2.2 years. Thanks to a deep basin and its water circulation patterns that help filter out pollutants. 

Additionally, due to its naturally low levels of nutrients, particularly phosphorus, algae growth is limited here explaining the lake’s clarity. 

In some areas, visibility can reach up to 25 feet. Local groups also closely monitor the lake’s water quality. 

The lake creates a unique microclimate in the surrounding area

The “lake effect” helps moderate temperatures in the region. 

In the summer, the warm sun heats the lake, with temperatures ranging in the mid-70s to mid-80s. But winters around Flathead Lake stay milder compared to nearby areas. The lake rarely freezes over completely because of its size and depth.

The microclimate allows the growth of cherry orchards along the eastern shore. This is because the cold temperatures from the lake keep the land cooler, preventing cherry trees from freezing. 

As a result, vineyards for wine production do well on the western shore of the lake. 

Flathead Lake is home to diverse wildlife and ecosystems

Flathead Lake is part of the larger Flathead Watershed ecosystem. It’s also home to lake trout, which was introduced to the lake in the late 1800s and early 1900s. Now it supports the native bull trout and west-slope cutthroat trout.

Wild Horse Island, the largest island in the lake, is home to bighorn sheep and wild horses. People often see bald eagles and osprey around the lake. The lake’s ecosystem includes different wetlands and riparian areas.

Native American tribes have a rich cultural history tied to the lake

The southern half of Flathead Lake lies within the Flathead Indian Reservation. The Confederated Salish and Kootenai Tribes have lived in the area for thousands of years.

With 1.3 million acres, the reservation is made up of diverse landscapes, including the Jocko Valley and the Bison Range. There are also numerous communities, like Polson, Ronan, Charlo, and St. Ignatius, to name a few, on the reservation.

Local tribal stories and legends claim the Salish people, also called the Flathead Indians, named the lake. Today, the tribes share responsibility for managing the lake’s fisheries with the state of Montana.

Flathead Lake offers numerous recreational opportunities

People love the lake for outdoor recreation.

The lake holds several annual fishing tournaments, including the “Mack Days” event, which is a months-long, semi-annual fishing derby sponsored by the Confederated Salish and Kootenai Tribes. This year, the Spring Mack Days event is scheduled from March 13th to May 10th, 2025.

Six state parks sit along the shores of Flathead Lake. Wildhorse Island State Park is only accessible by boat and has a 1.75-mile hiking trail that offers views of the lake and surrounding mountains. On the lake’s eastern shore near Bigfork, sits the main park that offers camping, picnicking, and water activities. There’s even a sandy beach area for swimming and a boat launch for easy access to the lake.

Scenic drives along U.S. Route 93 provide vistas of the Mission Mountains to the east and the Swan Mountains to the west. Following the eastern shore, Route 35 offers access to several state parks, including Flathead Lake State Park, and scenic views of the Swan Mountains as you pass through charming lakeside communities.

Introduced species have caused significant ecological changes in the lake

Lake trout, an invasive species in the region, has become the main predator in the lake.

This surge resulted in the decline of native fish species, including kokanee salmon, impacting local fishing businesses and seasonal angling. In the late 1970s, non-native Mysis shrimp were also introduced into the upper Flathead basin, but it became the primary food source for juvenile lake trout.

As a result, the Kokanee salmon population collapsed by 1989. In 2024, the Flathead Lake Biological Station conducted over 100,000 boat inspections to prevent the spread of invasive species, such as zebra and quagga mussels, into Flathead Lake.

In addition to this, fisheries managers also conduct regular “Mack Days” fishing derbies to reduce the population of lake trout.

Flathead Lake plays a crucial role in the region’s economy

Tourism around the lake is a major economic driver for the area. In 2022 and 2023, visitors spent an average of $165 million on products made in Montana. The presence of Flathead Lake has increased property values in the area, contributing between $12 million and $17 million.

The lake provides irrigation for farms in the Flathead Valley. People recognize Flathead Lake cherries for their sweetness and quality. This industry alone is a whopping $3.5 million contributor to Montana’s economy.

Today, sweet cherry varieties like Regina, Hudson, Attika, and Santina are being trialed in the area.

The lake serves as an important source of hydroelectric power

Kerr Dam, completed in 1938, helps control the lake’s water levels, by controlling the flow of the Flathead River. The dam’s hydroelectric facility consists of three units that generate electricity and manage water levels for various uses.

Constructed in 1938, Kerr Dam was initially built by the Montana Power Company with federal assistance during the Great Depression. The Confederated Salish and Kootenai Tribes now own the dam.

The dam causes the lake’s water level to go up and down by 10 feet each year. The dam’s operation balances power generation with recreational and environmental needs by regulating water levels to support activities like boating and fishing.

Flathead Lake contributes to the larger Columbia River hydropower system by regulating water flow into the Flathead River, providing flood control, irrigation, and power generation benefits across the region.

Scientific research on Flathead Lake provides valuable ecological insights

The University of Montana’s Flathead Lake Biological Station has been rigorously monitoring Flathead Lake’s water quality since the 1970s, serving as the “Sentinel of the Lake.”

Their studies encompass water quality monitoring, invasive species impacts, and climate change effects on freshwater systems. The lake is also a good example for studying the effects of introduced species on native ecosystems.

These scientific findings from Flathead Lake help guide conservation efforts in other large lakes.