What is the cleanest lake in Idaho?

Black Cnyn Reservoir Ns of in Gem County is the highest-rated lake in Idaho with an EPA water quality grade of A and 7.5 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 Idaho

Black Cnyn Reservoir Ns of in Gem County leads the rankings with a water quality grade of A and 7.5 ft of clarity. Below are the top 7 cleanest monitored lakes in Idaho, ranked by EPA water quality score.

Ranked List

#	Lake	County	Grade	Clarity	Phosphorus	Algae
1	Black Cnyn Reservoir Ns of	Gem	A	7.5 ft	29.5 µg/L	3.1 µg/L
2	Palisades Lake	Bonneville	C	13.1 ft	20.5 µg/L	2.1 µg/L
3	Cas Reservoir Inlet Between Lf And	Valley	D	4.3 ft	40 µg/L	12.4 µg/L
4	Lake Walcott	Cassia	D	4.3 ft	50.5 µg/L	14.8 µg/L
5	Cascade R W of Sugarloaf Island Lake	Valley	D	8.2 ft	28.5 µg/L	7.5 µg/L
6	Littlewood Lake	Blaine	D	12.5 ft	28 µg/L	13.9 µg/L
7	Mountain View Lake	Owyhee	D	-	39 µg/L	14 µg/L

Ranked List

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