Big Sky_deep Lake is a highly nutrient-loaded hypereutrophic lake in Missoula County, Montana. Water quality is graded F based on 1 sampling years through 2025. Compared to the 59 other monitored lakes in Missoula County, Big Sky_deep Lake ranks #60 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DNR, last sampled 2025-09-09. Grade methodology: LakeGrade methodology.
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Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F, Very Poor
No clarity data. Phosphorus level: 181 µg/L. Trophic State Index: 79.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 181 µg/L | F |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 79 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
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Location
County Ranking
Ranked #60 of 60 lakes in Missoula County
Cleaner Lakes Within 30 Miles
Big Sky_deep Lake holds Grade F. 3 nearby lakes hold higher grades.
Nearby Lakes in Missoula County
Explore Big Sky_deep Lake across LakeGrade
More Montana Lake Rankings
Other Grade F Lakes in Montana
Hypereutrophic Lakes in Montana
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: 2025-09-09
Monitoring stations: 1