Yellow Bay State Park - Flathead Lake

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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 10th through September 12th, 2023. We will resume testing in July 2024. 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. In 2022, the Flathead Lakers took over management of the program. Samples are processed 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.