Deep Mill Lake is a mesotrophic lake in Yellowstone County, Montana. Water quality is graded B based on 1 sampling years through 2025.
Source: EPA Water Quality Portal sampling records, Montana DNR, last sampled 2025-09-17. Grade methodology: LakeGrade methodology.
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
No clarity data. Phosphorus level: 20 µg/L. Trophic State Index: 47.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 20 µg/L | B |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 47 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Deep Mill Lake
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Location
County Ranking
Ranked #1 of 1 lakes in Yellowstone County
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More records, surveys & data sources
Data Sources
Water quality data from the EPA Water Quality Portal
Grading methodology based on Metropolitan Council standards
Most recent sample: 2025-09-17
Monitoring stations: 1