What is the current water temperature at Flathead Lake?

Current water temperature for Flathead Lake is shown in the live forecast above. Most Montana lakes have no public real-time water-temperature sensor, so we estimate from recent and forecast air-temperature trends weighted by the lake's depth (370ft maximum).

Where is Flathead Lake located?

Flathead Lake is in Montana (Flathead / Lake County), with public access near Polson, Bigfork, Lakeside, Somers. Surface area is 122,500 acres.

How deep is Flathead Lake?

Flathead Lake's maximum depth is 370 feet. Deeper lakes carry more thermal inertia and warm more slowly through summer; shallower lakes track the air temperature faster.

How cold is Flathead Lake?

Flathead Lake is the largest natural freshwater lake west of the Mississippi and one of the deepest in the US (370ft max), so it runs cold even in summer. Peak surface temperatures are typically in the low to mid 70s°F in July and August.

When is Flathead Lake warm enough to swim?

Flathead Lake generally becomes swimmable by mid-June and stays comfortable through August. The shallow bays warm faster than the main basin; check the live reading for current conditions.

Montana · Flathead / Lake County

Flathead Lake

Polson · Bigfork · Lakeside · Somers

Loading live conditions for Flathead Lake…

Surface area122,500acres

Max depth370feet

CountyFlathead / Lake

Data sourceEWMA modelAir-temp trend

Things to do

Tours & activities on Flathead Lake

Boat cruises, fishing trips, kayak rentals, and guided adventures from local operators.

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Places to stay

Lake houses & rentals

Vacation rentals right on the water.

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"The largest natural freshwater lake west of the Mississippi (in the lower 48). Surrounded by the Mission and Salish Mountain ranges, 30 minutes south of Glacier National Park. Famously clear, cold, and home to Flathead cherry orchards lining the east shore. Mostly Confederated Salish and Kootenai Tribes territory."

Seasonal pattern

When does Flathead Lake warm up?

Flathead Lake is deep (max depth 370ft) and carries significant thermal inertia, so it warms slowly — surface temperatures typically don't reach comfortable swimming range until late June or early July. Spring-fed or alpine lakes stay cooler all summer. The live forecast above shows current conditions.

Field notes

About Flathead Lake

Largest natural freshwater lake in the lower 48 west of Mississippi
30 minutes south of Glacier National Park
East shore is Montana's cherry-growing region
Mostly within the Flathead Indian Reservation

Surface area: 122,500 acres
Max depth: 370 ft
County: Flathead / Lake
Water data: Modeled estimate from air-temp trends