Big Arm State Park -- Flathead Lake

Big Arm, Montana

Our recreational water quality monitoring program is complete for the season. Please note that our beaches passed 95% of the tests this summer. For the most up to date water quality information from around the world, download the free app at www.TheSwimGuide.org. Water conditions can change quickly, so always use caution.

Thanks to Dalon at Explore Flathead Lake for his first-year adoption of Big Arm State Park. In addition to his real estate business, Dalan runs “Explore Flathead Lake” scenic tours with qualified guides out on Flathead Lake. If you are looking to move to the area or just explore, Dalan is your guy at www.exploreflatheadlake.com.

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!

COVID-19

Keep your distance from other people.

Practicing social distancing is still essential. Only go to the beach if you are able to keep 6 feet or 2 meters away from others. Follow the instructions provided by your local health authorities. If your community has asked that you remain indoors and away from others, do so. Spending a day in any crowded place is the worst thing we can do for our most vulnerable right now and will counter our efforts to curb the virus’s spread.

Water Quality
  • Passed water quality tests at least 95% of the time

  • Historical Status
  • This status is based on the latest sample, taken on August 31st, 2021. Flathead Lake Open Water Swimmers updates the status of this beach as soon as test results become available. These results were posted to Swim Guide on August 31st, 2021 at 6:09 PM.
For water quality icon legend, click:  
Monitoring Frequency

Big Arm State Park -- Flathead Lake is sampled weekly from June 29th to September 1st.

Source Information

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.. 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!

Water at Flathead Lake's bathing beaches is monitored for recreational water quality during effective swimming season: July 1st through September 1st.


In 2017 FLOW Swimmers (Flathead Lake Open Water Swimmers) launched a sampling program of three local beaches. In 2018, the program has grown to 10 beaches around the lake. Samples areprocessed by Flathead Lake Biological Research Station. The water at the bathing beaches will be tested for E.coli


Monitoring Status
Montana follows federal guidelines for the monitoring of its recreational waters.
The guidelines for freshwater levels of E. coli are a geometric mean of 100 cfu per 100 mL and an STV of 320 cfu per 100 mL, measured using EPA Method 1603, or any other equivalent method that measures culturable E. coli.

E. coli
A beach is marked Green when two consecutive single sample results are under a geometric mean of 100 cfu per 100 mL and an STV of 320 cfu per 100 mL


A beach is marked Red when two consecutive single sample results are equal to or above a geometric mean of 100 cfu per 100 mL or aSTV of 320 cfu per 100 mL


A beach is marked Grey if there is no current results or no available information.

Read more
Water Quality Graph

Big Arm State Park -- Flathead Lake

Big Arm, Montana

COVID-19

Keep your distance from other people.

Practicing social distancing is still essential. Only go to the beach if you are able to keep 6 feet or 2 meters away from others. Follow the instructions provided by your local health authorities. If your community has asked that you remain indoors and away from others, do so. Spending a day in any crowded place is the worst thing we can do for our most vulnerable right now and will counter our efforts to curb the virus’s spread.

Water Quality
  • Passed water quality tests at least 95% of the time
  • Historical Status
  • This status is based on the latest sample, taken on August 31st, 2021. Flathead Lake Open Water Swimmers updates the status of this beach as soon as test results become available. These results were posted to Swim Guide on August 31st, 2021 at 6:09 PM.
For water quality icon legend, click:  

Our recreational water quality monitoring program is complete for the season. Please note that our beaches passed 95% of the tests this summer. For the most up to date water quality information from around the world, download the free app at www.TheSwimGuide.org. Water conditions can change quickly, so always use caution.

Thanks to Dalon at Explore Flathead Lake for his first-year adoption of Big Arm State Park. In addition to his real estate business, Dalan runs “Explore Flathead Lake” scenic tours with qualified guides out on Flathead Lake. If you are looking to move to the area or just explore, Dalan is your guy at www.exploreflatheadlake.com.

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!

Monitoring Frequency

Big Arm State Park -- Flathead Lake is sampled weekly from June 29th to September 1st.

Source Information

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.. 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!

Water at Flathead Lake's bathing beaches is monitored for recreational water quality during effective swimming season: July 1st through September 1st.


In 2017 FLOW Swimmers (Flathead Lake Open Water Swimmers) launched a sampling program of three local beaches. In 2018, the program has grown to 10 beaches around the lake. Samples areprocessed by Flathead Lake Biological Research Station. The water at the bathing beaches will be tested for E.coli


Monitoring Status
Montana follows federal guidelines for the monitoring of its recreational waters.
The guidelines for freshwater levels of E. coli are a geometric mean of 100 cfu per 100 mL and an STV of 320 cfu per 100 mL, measured using EPA Method 1603, or any other equivalent method that measures culturable E. coli.

E. coli
A beach is marked Green when two consecutive single sample results are under a geometric mean of 100 cfu per 100 mL and an STV of 320 cfu per 100 mL


A beach is marked Red when two consecutive single sample results are equal to or above a geometric mean of 100 cfu per 100 mL or aSTV of 320 cfu per 100 mL


A beach is marked Grey if there is no current results or no available information.

Read more
Water Quality Graph

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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.

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