Big Arm Bay Lake is a cold, clear oligotrophic lake in Lake County, Montana. Water quality is graded A based on 1 sampling years through 2025. Compared to the 26 other monitored lakes in Lake County, Big Arm Bay Lake ranks #20 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DNR, last sampled 2025-08-26. Grade methodology: LakeGrade methodology.
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
No clarity data. Phosphorus level: 2 µg/L. Trophic State Index: 14.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 2 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 14 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Big Arm Bay Lake
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Fishing Big Arm Bay Lake
Location
County Ranking
Ranked #20 of 27 lakes in Lake County
Nearby Lakes in Lake County
Explore Big Arm Bay Lake across LakeGrade
More Montana Lake Rankings
Other Grade A Lakes in Montana
Oligotrophic 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: 2025-08-26
Monitoring stations: 1