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.

Safest Lakes to Swim In in Idaho

1 lakes in Idaho qualify as safest for swimming, all hold grade A or B and report chlorophyll-a (algae) under 10 µg/L. Always check local advisories on the day you swim.

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

Ranked List

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