What is the cleanest lake in Idaho?

Mann Creek Reservoir in Washington County is the highest-rated lake in Idaho with an EPA water quality grade of NR.

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.

Lakes With Algae Problems in Idaho

4 lakes in Idaho report elevated algae levels (chlorophyll-a above 20 µg/L). These lakes are most likely to develop blue-green algae blooms during warm, calm summer weather. Avoid swimming during visible blooms and rinse pets after contact.

Ranked List

#	Lake	County	Grade	Clarity	Phosphorus	Algae
1	Mann Creek Reservoir	Washington	NR	-	49 µg/L	179 µg/L
2	Little Wood Reservoir	Blaine	NR	-	70 µg/L	30.2 µg/L
3	Soldier Meadow Reservoir East Arm	Nez Perce	NR	4.6 ft	34 µg/L	29 µg/L
4	Soldiers Meadow Reservoir West Arm	Nez Perce	NR	3.6 ft	56.5 µg/L	28.5 µg/L

Ranked List

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