Mary Lake vs Mill Lake
Water quality, depth, fish species, and recreation comparison.
Mary Lake has a higher water quality grade (A, Excellent) than Mill Lake (C, Fair). Both are in Douglas County, Minnesota.
Both Mary Lake and Mill Lake sit in Minnesota. A same-state comparison strips out the state-level water-quality regime as a variable: any grade differences here are about the lakes themselves, not the agencies grading them. Mary Lake (A) is materially cleaner than Mill Lake (C). A gap that wide is unlikely to close in a single year of remediation work; it reflects multi-decade differences in the lakes themselves.
For a recreational visitor, the wider grade gap is decisive — Mary Lake is the better water-quality choice. For a researcher, the gap is the interesting part: what is different about the two watersheds, and which of those differences is mutable?
Mary Lake
Good clarity, visible to about 10.5 ft.
Mill Lake
Good clarity, visible to about 11 ft.
Side-by-Side Metrics
| Metric | Mary Lake | Mill Lake |
|---|---|---|
| Overall Grade | A (Excellent) Better | C (Fair) |
| Water Clarity | 10.5 ft | 11 ft Better |
| Phosphorus | 20 µg/L Better | 23 µg/L |
| Chlorophyll-a (Algae) | 5.5 µg/L Better | 9 µg/L |
| Maximum Depth | 40 ft | 40 ft |
| Surface Area | 2,450 acres | 451 acres |
| Public Access | Yes | Yes |
| Fish Species | 17 Better | 15 |
| Trophic State | mesotrophic | mesotrophic |
A green ✓ marks the better value for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better). Surface area and trophic state carry no tick — a bigger lake is not a better lake.
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Compare Mary Lake or Mill Lake against a different lake.
Verdict
Mary Lake wins on overall water quality with a Grade A versus Mill Lake's Grade C. Water clarity: 10.5 ft vs 11 ft. For fishing diversity, Mary Lake also leads with 17 species.
Source: EPA National Aquatic Resource Surveys, 2026.