Yellow Bay State Park - Flathead Lake

Join the Flathead Lakers to protect clean water and healthy ecosystems in the Flathead Watershed

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.

The park offers all services and includes a protect swimming beach, bathrooms, and picnic table. There is also a public boat ramp with plenty of parking.

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!

Get directions to this beach

Water Quality

Passed water quality tests 60-95% of the time
Historical Status
This status is based on the latest sample, taken on September 6th, 2022. Flathead Lakers updates the status of this beach as soon as test results become available. These results were posted to Swim Guide on September 27th, 2022 at 1:23 PM.

For water quality icon legend, click:

Report Pollution

Monitoring Frequency

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 5th through September 5th.

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.

Water Quality Graph