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LakeQuality

Data Report · Updated July 2026

The State of South Dakota’s Lakes 2026

Of 4 monitored lakes in South Dakota, 25% earn a water-quality grade of A or B and 75% score D or F (average grade D). Traverse Lake is the cleanest;

4
Monitored lakes graded
D
Average grade
25%
Grade A or B
0%
On impaired list

South Dakota grade distribution

A
D
F
A: 1 (25.0%)B: 0 (0.0%)C: 0 (0.0%)D: 2 (50.0%)F: 1 (25.0%)

Cleanest lakes in South Dakota

Highest combined water-quality score, 2026.

  1. 1.DTraverse Lake — Roberts County3.9 ft clarity · 173 µg/L P
  2. 2.FBig Stone Lake — Roberts County3.0 ft clarity · 109 µg/L P

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

  1. 1.FBig Stone Lake — Roberts County3.0 ft clarity · 109 µg/L P
  2. 2.DTraverse Lake — Roberts County3.9 ft clarity · 173 µg/L P

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

No qualifying lakes with sufficient data.

Trending lakes

Improving

  1. 1.DLake Andes — Charles Mix County

Declining

  1. 1.DTraverse Lake — Roberts County

Grades combine Secchi clarity, total phosphorus, and chlorophyll-a from the EPA Water Quality Portal and state agencies. See the full methodology, the national State of America’s Lakes report, or all South Dakota lakes.

Cite this report: LakeQuality, “The State of South Dakota’s Lakes 2026,” lakequality.org/report/sd.