What is the cleanest lake in Montana?

Tally Lake in Flathead County is the highest-rated lake in Montana with an EPA water quality grade of A and 15.2 feet of water clarity.

How are these rankings calculated?

Lakes are graded A through F on nutrients and algae — total phosphorus, nitrogen, chlorophyll-a, and turbidity — against EPA National Lakes Assessment 2022 condition classes for that lake’s ecoregion. Water clarity (Secchi depth) is measured and shown; it is not the letter. Lakes without enough recent samples are UNRATED, not given an F. Source data comes from the EPA Water Quality Portal.

Cleanest Lakes in Montana

Tally Lake in Flathead County leads the rankings with a water quality grade of A and 15.2 ft of clarity. Below are the top 22 cleanest monitored lakes in Montana, ranked by EPA water quality score.

Ranked List

#	Lake	County	Grade	Clarity	Phosphorus	Algae
1	Tally Lake	Flathead	A	15.2 ft	4.7 µg/L	0.8 µg/L
2	Whitefish Lake	Flathead	A	22 ft	2.8 µg/L	0.9 µg/L
3	Swan Lake North End	Lake	A	-	3.1 µg/L	1.1 µg/L
4	Little Bitterroot Lake Center	Flathead	A	-	4 µg/L	1 µg/L
5	Inez Lake	Missoula	A	-	7.1 µg/L	1.8 µg/L
6	Alva Lake	Missoula	A	-	4.7 µg/L	0.7 µg/L
7	Cooney Reservoir	Carbon	B	12.5 ft	20 µg/L	2.6 µg/L
8	East Bay Lake	Lake	B	-	3 µg/L	-
9	Big Arm Bay Lake	Lake	B	-	1.5 µg/L	-
10	Dickey Lake	Lincoln	C	-	11.3 µg/L	2.2 µg/L
11	Mcgilvray Lake	Flathead	C	-	5.9 µg/L	1.7 µg/L
12	Hanson-Doyle Lake	Flathead	C	-	7.3 µg/L	2.3 µg/L
13	Fish Lake	Lincoln	C	-	8.1 µg/L	2.3 µg/L
14	Bailey Lake	Flathead	C	-	6.9 µg/L	2 µg/L
15	Big Sky Lake	Missoula	C	-	10 µg/L	2.1 µg/L
16	Loon Lake	Lake	D	-	6.7 µg/L	2.2 µg/L
17	Mary Ronan Lake West	Lake	D	-	33.6 µg/L	4.7 µg/L
18	Foys Lake	Flathead	D	-	15.8 µg/L	4.1 µg/L
19	Rogers Lake	Flathead	D	-	11.6 µg/L	2.6 µg/L
20	Jette Lake	Lake	D	-	16.6 µg/L	4.7 µg/L
21	Mary Ronan Lake East	Lake	D	-	15.8 µg/L	3.5 µg/L
22	Lake Helena Outlet	Lewis and Clark	D	-	115.5 µg/L	-

Ranked List

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