How big is Flathead Lake?

Nestled in the northwestern corner of Montana, Flathead Lake stands as one of the most remarkable natural freshwater lakes in the United States. Known for its crystal-clear waters, scenic mountain backdrop, and abundant recreational opportunities, this glacially-carved lake is a centerpiece of beauty and biodiversity in the region. But just how big is Flathead Lake? Let’s dive deep into its size, dimensions, and what makes it so special.

Size and Surface Area

Flathead Lake is the largest natural freshwater lake west of the Mississippi River in the contiguous United States. It covers an impressive surface area of approximately 197 square miles (510 square kilometers). This makes it larger than Lake Tahoe, which often draws comparisons due to its alpine setting and clarity, though Flathead Lake is not as deep.

To put its size into perspective:

◉ The lake stretches 27 miles (43 km) long from north to south.

◉ It is up to 15 miles (24 km) wide at its broadest point.

Its shoreline spans more than 185 miles (298 km), offering a diverse mix of pebble beaches, rocky outcrops, and forested coves.

Depth and Volume

While Flathead Lake may not be the deepest, it is still impressively deep. Its maximum depth is about 370 feet (113 meters), and its average depth hovers around 165 feet (50 meters). The lake holds approximately 5.5 trillion gallons of water, or about 22.5 cubic kilometers in volume.

This massive volume contributes to the lake’s legendary water clarity. The lake is fed by glacial melt and runoff from the surrounding mountains, especially the Mission Mountains to the east and the Salish Mountains to the west. Because of its depth and size, the lake is slow to warm, maintaining cool temperatures even in summer.

Elevation and Location

Flathead Lake sits at an elevation of 2,893 feet (882 meters) above sea level. Located in northwestern Montana, it straddles two counties—Lake County to the south and Flathead County to the north. The lake is part of the Flathead Valley and lies about 7 miles south of Kalispell and 30 miles southwest of Glacier National Park.

The Flathead River, which originates in the Canadian Rockies, flows into the north end of the lake near the town of Somers. The lake’s outlet is at Polson on the south end, where it feeds into the Lower Flathead River.

Islands and Unique Features

Flathead Lake is home to several islands, the largest of which is Wild Horse Island, spanning 2,165 acres. This island is a state park and wildlife haven, known for its population of wild horses, bighorn sheep, bald eagles, and mule deer.

Other notable islands include:

◉ Cromwell Island (a privately owned island once listed for sale for over $70 million)

◉ Melita Island (used as a Boy Scouts camp)

◉ Numerous smaller islets and rock outcroppings that enhance the lake’s scenic charm

Formation and Geology

Flathead Lake was formed more than 10,000 years ago during the last Ice Age. Glacial activity carved out the Flathead Valley, and as the glaciers retreated, they left behind a deep basin that eventually filled with meltwater. The lake is a remnant of a much larger prehistoric lake called Glacial Lake Missoula.

Today, the lake is dam-regulated. The Seli’š Ksanka Qlispe’ Dam (formerly Kerr Dam), located at the lake’s outlet near Polson, raises the water level by about 10 feet and helps control the lake’s seasonal water level fluctuations.

Ecological and Recreational Importance

Flathead Lake’s size not only makes it a geological marvel but also an ecological and economic treasure. The lake supports a variety of fish species, including lake trout, yellow perch, and whitefish. The surrounding environment is rich in birdlife, forests, and agricultural lands.

Recreationally, the lake draws thousands of visitors each year for boating, fishing, kayaking, swimming, and camping. Towns like Bigfork, Polson, and Lakeside provide charming access points to the water, complete with marinas, lakeside parks, and cultural festivals.

Conclusion

Flathead Lake’s sheer size—nearly 200 square miles of pristine, glacier-fed waters—makes it not just the largest natural freshwater lake west of the Mississippi, but also one of Montana’s most cherished landmarks. Whether you're exploring its islands, fishing in its deep blue depths, or simply admiring its vast surface reflecting the surrounding peaks, understanding how big Flathead Lake is adds a whole new dimension to appreciating its grandeur.

For anyone planning a trip to Montana or looking to explore one of the most spectacular freshwater lakes in the lower 48 states, Flathead Lake is a destination that promises both scale and serenity.