Cooney Reservoir is a mesotrophic lake in Carbon County, Montana. Water quality is graded A based on 3 sampling years through 2025. Compared to the 7 other monitored lakes in Carbon County, Cooney Reservoir ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DNR, last sampled 2025-10-24. Grade methodology: LakeGrade methodology.
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
Good clarity, visible to about 12.5 ft. Phosphorus level: 19.5 µg/L. Chlorophyll-a: 2.6 µg/L. Trophic State Index: 43.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 12.5 ft | B |
| Phosphorus | 19.5 µg/L | A |
| Chlorophyll-a (Algae) | 2.6 µg/L | A |
| Trophic State Index (TSI) | 43 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Cooney Reservoir
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Fishing Cooney Reservoir
Location
County Ranking
Ranked #3 of 8 lakes in Carbon County
Nearby Lakes in Carbon County
Explore Cooney Reservoir across LakeGrade
More Montana Lake Rankings
Other Grade A Lakes in Montana
Rosebud County · Excellent
Lewis and Clark County · Clarity: 21.8 ft
Lewis and Clark County · Clarity: 15.6 ft
Lewis and Clark County · Clarity: 15.4 ft
Lewis and Clark County · Clarity: 19 ft
Lewis and Clark County · Clarity: 26.2 ft
Mesotrophic 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: 2025-10-24
Monitoring stations: 1