Is Mary Lake Safe for Swimming?
Mary Lake in Douglas County, Minnesota has a water quality grade of A (Excellent). Excellent for swimming, crystal clear water with minimal algae. Water clarity: 10.5 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 5.5 µg/L. Always check local health department advisories before swimming.
Key Metrics
| Metric | Value |
|---|---|
| Overall Grade | A (Excellent) |
| Water Clarity (Secchi Depth) | 10.5 ft |
| Phosphorus | 20 μg/L |
| Chlorophyll-a (Algae) | 5.5 μg/L |
| Maximum Depth | 40 ft |
| Location | Douglas County, Minnesota |
How does Mary Lake compare?
With an overall water quality grade of A, Mary Lake matches or beats the Minnesota state average of Grade C. It ranks #11 of 37 lakes in Douglas County by water quality score.
Water clarity (Secchi depth) sits at 10.5 feet. That counts as exceptional clarity — sunlight penetrates deep, supporting submerged plants and limiting algae. Algae levels (chlorophyll-a) measure 5.5 µg/L — low and well within safe swimming guidelines.