What is the cleanest lake in South Dakota?

Big Stone Lake in Roberts County is the cleanest lake on the readings in South Dakota with an EPA water-quality readings of — and 3 feet of 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; 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 Readings in South Dakota

2 lakes in South Dakota 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	Clarity	Phosphorus	Algae
1	Big Stone Lake	Roberts	—	3 ft	108.5 µg/L	70.4 µg/L
2	Traverse Lake	Roberts	—	3.9 ft	172.5 µg/L	36.3 µg/L

Ranked List

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