Flathead Lake Water Temperature by Month
Flathead Lake's average surface water temperature runs from about 36°F in mid-January to about 56°F in mid-June and about 68°F in mid-August, according to the Flathead Lake Biological Station (FLBS), the University of Montana's research station on the lake. Those are the only averages FLBS publishes, so this page gives those three figures, the seasonal pattern around them, and where to check today's reading. Even at its August average, the lake is cold enough for the National Weather Service's cold-water warnings to apply.
Flathead Lake water temperature through the year
| Time of year | What the lake is doing | Published average |
|---|---|---|
| Winter (Dec to Feb) | Coldest water. The whole lake rarely freezes, but bays and shoreline areas can (FLBS). | About 36°F (2.3°C) in mid-January |
| Spring (Mar to May) | Lake level is low and rising. Spring runoff, generally May 15 to June 15, can send a visible sediment plume into the lake. | None published |
| Early summer (June) | The lake reaches full pool by June 15 when runoff allows (FLBS weather center). | About 56°F (13.5°C) in mid-June |
| Midsummer (July) | Warming between the June and August averages. | None published; check the live reading |
| Late summer (August) | The warmest of the three published averages. | About 68°F (20.3°C) in mid-August |
| Fall (Sept to Nov) | Lake level is kept within about 1 foot of full pool through Labor Day, then lowered. FLBS live readings run until about September. | None published; check the live reading |
What is the Flathead Lake water temperature today?
Check the FLBS weather center. FLBS measures surface water temperature at the end of Yellow Bay Point and reports it every 15 minutes. The sensor only reports when the lake is at or near full pool, roughly May through September. The same page shows air temperature and wind from stations around the lake, including Somers, and the current lake level from the USGS gauge at Polson.
Is Flathead Lake warm enough to swim?
Swimming is one of the popular activities on Flathead Lake, according to Montana FWP, but plan for cold water. The National Weather Service says cold shock can be as dangerous in water of 50 to 60°F as in water of 35°F, and that the gasp reflex can be triggered by water as warm as 77°F (NWS cold water safety). Flathead Lake's mid-June average (about 56°F) sits in that first range, and even its mid-August average (about 68°F) is below 77°F.
Three habits for cold water:
- Wear a life jacket on boats and personal watercraft. NWS says it significantly increases the chance of survival.
- Ease in rather than jumping in, so the gasp reflex NWS describes does not happen with your head underwater.
- Watch the wind. Check the NWS Flathead Lake forecast for wind and waves before you paddle or boat.
When is the best time to swim in Flathead Lake?
Mid-July through August, based on FLBS's published averages, with the warmest published figure in mid-August. Two other reasons favor summer:
- Lake level. The lake is typically at or within about a foot of full pool from mid-June through Labor Day (FLBS weather center), so shoreline access is at its summer state.
- Water-quality checks. The Flathead Swim Guide, a project of the Flathead Lakers, FLBS and the Confederated Salish and Kootenai Tribes, posts weekly water-quality updates for 16 swimming locations around the lake.
Early June falls inside the spring runoff window FLBS describes (generally May 15 to June 15), and mid-June's average is the coldest of the summer figures.
Where to swim near Lakeside
The Tamarack has no private lake access. Guests swim from public beaches and parks. Nearby options from Montana FWP:
- West Shore unit, Flathead Lake State Park: 6 miles south of Lakeside on U.S. Highway 93, with a shoreline FWP calls excellent for picnicking and swimming. Check the park's official page for current concessions.
- Somers Beach State Park: on the north shore, day use only, with swimming listed as an activity. Its wide sandy beach is exposed when the lake is below full pool, from fall through spring.
- Yellow Bay unit: on the east shore, with a wide, gravelly beach popular for swimming and kayaking.
Plan a summer stay at The Tamarack
The Tamarack is a six-bedroom home in Lakeside, above Flathead Lake, with nine beds and 4.5 baths. Sleeps 16+. After a cold-water swim, the private hot tub and sauna are waiting back at the house. Rates and availability change by date; the booking page shows live availability and the total for your dates.
Check summer dates at The Tamarack
FAQ
What is the water temperature of Flathead Lake in summer?
The Flathead Lake Biological Station puts the average surface temperature at about 56°F in mid-June and about 68°F in mid-August.
What month is Flathead Lake warmest?
Of the averages FLBS publishes, mid-August is the warmest, at about 68°F. FLBS does not publish an average for every month.
How cold is Flathead Lake in winter?
About 36°F in mid-January, on average. The whole lake rarely freezes; FLBS says it has not completely frozen since the winters of 1988-89 and 1989-90.
Is Flathead Lake too cold to swim?
Swimming is popular there, but the water stays cold. The National Weather Service says cold shock is a risk at 50 to 60°F and the gasp reflex can start in water as warm as 77°F, so ease in and wear a life jacket on the water.
Where can I check Flathead Lake's water temperature today?
On the Flathead Lake Biological Station weather center, which reports the surface temperature at Yellow Bay Point every 15 minutes from roughly May through September.
Does The Tamarack have lake access?
No. The Tamarack has no private dock, slip, deeded or direct lake access. Guests use nearby public beaches, boat ramps, marinas and parks.