Open Water Data

Big Arm State Park -- Flathead Lake

Big Arm, Montana, United States

Data Sources
  • Water Quality Status
    Passed tests Thu, Sep 5, 2024 at 6:16pm
  • Air Temperature
    -0.3°C / 31.5°F (air) Sun, Dec 4, 2022 at 9:15pm
  • Wind
    15.1 - 9.7km/h NNE 30° Sun, Dec 4, 2022 at 9:15pm
  • Clear
    Clear Sun, Dec 4, 2022 at 9:15pm
  • Precipitation
    0mm Sun, Dec 4, 2022 at 9:15pm
  • Air Visibility
    2km Sun, Dec 4, 2022 at 9:15pm
  • Humidity
    63% Sun, Dec 4, 2022 at 9:15pm

Flathead Lake in northwest Montana is the largest natural freshwater lake west of the Mississippi; slightly larger than Lake Tahoe at 27.3 mi. long, 15.5 mi. wide, and max depth of 370.7 ft.

Located on the lake's Big Arm Bay, its long pebble beach is popular with sunbathers and swimmers. Camping under a stand of mature ponderosa pine and juniper is a major attraction. The hiking trail provides excellent vistas of surrounding mountain ranges and high peaks with abundant watchable wildlife opportunities. Other opportunities include: fishing for lake trout, board sailing, boating, swimming, camping, picnicking, bicycling, hiking the 2.5 mile trail, wildlife viewing, scuba diving, and water-skiing.

This site is 2,953 feet in elevation and covers 217 acres. The campground has 41 campsites, including one wheelchair accessible site; one group campsite; and three yurts. Maximum RV/trailer length is 40 feet. Bear resistant storage lockers are available.

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!

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.

Swim Guide shares the best information we have at the moment you ask for it. 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!

The description above was copied from or heavily based on Swim Guide, with permission.

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