What is the cleanest lake in South Dakota?

Big Stone Lake in Roberts County has the cleanest nutrient and algae readings in South Dakota.

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.

Lakes With Algae Readings in South Dakota

5 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	Indian Slough	Roberts	-	-	136.1 µg/L
2	Lake Isabella	Marshall	-	-	86.9 µg/L
3	Rehfeldt Slough	Minnehaha	-	-	82.4 µg/L
4	Meiers Lake	Bon Homme	-	-	66.9 µg/L
5	Big Stone Lake	Roberts	3 ft	116 µg/L	23.6 µg/L

Ranked List

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