What is the cleanest lake in Idaho?

Black Cnyn Reservoir Ns of in Gem County has the cleanest nutrient and algae readings in Idaho, with 7.5 feet of measured water clarity.

How are these rankings calculated?

Nutrients and algae, total phosphorus, nitrogen, chlorophyll-a and turbidity, are read against EPA National Lakes Assessment 2022 condition classes for that lake’s ecoregion. Water clarity (Secchi depth) is measured and shown separately. Lakes without enough recent samples are listed without a condition class rather than ranked low. Source data comes from the EPA Water Quality Portal.

Cleanest Lakes in Idaho

Black Cnyn Reservoir Ns of in Gem County leads the rankings on measured clarity and nutrients and 7.5 ft of clarity. Below are the top 7 cleanest monitored lakes in Idaho, ranked on EPA nutrient and algae measurements.

Ranked List

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

Ranked List

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