Cooney Reservoir
Carbon County, Montana1,078 acresView mapDirections
Clarity, phosphorus, and algae sit in the clean band for this region.
↓ down from A at the last regrade
- Mesotrophic
- #2 of 5 in the county
- Public access
Cooney Reservoir in Carbon County, Montana has a water quality grade of B (Good). Water clarity: 12.5 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 2.6 µg/L. Surface area: 1078 acres.
- Clarity
- 12.5 ft
- Area
- 1,078 acres
- Type
- Mesotrophic
- Phosphorus
- 20 µg/L
- Chlorophyll-a
- 2.6 µg/L
- Trend
- ↓ Declining
- Public access
- Yes
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Watch Cooney Reservoir’s letter
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Cooney Reservoir is a 1,078-acre mesotrophic lake in Carbon County, Montana. Water quality is graded B based on 2 sampling years through 2025. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available with 2 public boat landings. Compared to the 4 other monitored lakes in Carbon County, Cooney Reservoir ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Montana DEQ, last sampled 2025-10-24. Grade methodology: LakeQuality methodology.
What Cooney Reservoir is good for
From access and monitoring records, not editorial picks.
- Clear water12.5 ft of visibility
- Boat launch2 documented ramps
- Camping2 public campgrounds nearby
- Big water1,078 acres
Swimming Safety
Grade onlyLow algae
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The B is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: B (Good)
Good clarity, visible to about 12.5 ft. Phosphorus 20 µg/L. Chlorophyll-a 2.6 µg/L. TSI 43. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 12.5 ft | Not graded |
| Phosphorus | 20 µg/L | Good |
| Chlorophyll-a (Algae) | 2.6 µg/L | Not graded |
| Trophic State Index (TSI) | 43 | Mesotrophic |
Moderate nutrients, good water quality
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 1,078 acres |
| Public Access | Yes |
No bathymetric survey has been published for Cooney Reservoir, so its depth is not documented here.
Recreation & Access
- Cooney Reservoir State Park - LakeshorePublic
- Cooney Reservoir State Park - NorthshorePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Public campgrounds near Cooney Reservoir
- Marshall Cove Campground0.4 km
- Cottonwood Campground1.0 km
Nearest public campgrounds in the USGS National Map inventory, within about 3 miles. An empty list means USGS records no campground that close, which is not the same as camping being unavailable. Where the source gives an official reservation link, it is used; the cabin and hotel search does not cover campsites. More camping lakes
Fishing Cooney Reservoir
Water Quality Trend: ↓ Declining
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -1.865 m/yr | 3 |
| Phosphorus | ↓ Declining | +3 µg/L/yr | 3 |
Reservoir Info (USACE NID)
Cooney Reservoir is a man-made reservoir impounded by the Cooney (completed 1937), built primarily for irrigation on the Red Lodge Creek; earth-type dam, 102 ft tall and 2,369 ft long.
- Surface area
- 1,078 ac
- Normal storage
- 28,140 ac-ft
- Max storage
- 28,230 ac-ft
- Drainage area
- 206 sq mi
- Hazard class
- High
- Owner
- STATE WATER PROJECTS
Source: USACE National Inventory of Dams, NID ID MT00001 · Operator website
County Ranking
Ranked #2 of 5 lakes in Carbon County
Nearby Lakes in Carbon County
Frequently asked questions about Cooney Reservoir
Where is Cooney Reservoir?
- Cooney Reservoir is a lake in Carbon County, Montana. LakeQuality grades it B for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Cooney Reservoir?
- Cooney Reservoir in Carbon County, Montana has a water quality grade of B (Good). Water clarity: 12.5 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 2.6 µg/L. Surface area: 1078 acres.
Is Cooney Reservoir safe to swim in?
- The B is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Are there boat ramps on Cooney Reservoir?
- Cooney Reservoir has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Cooney Reservoir State Park - Lakeshore, Cooney Reservoir State Park - Northshore. Public access is listed as yes.
Are there campgrounds on Cooney Reservoir?
- Cooney Reservoir has 2 public campgrounds from USGS The National Map within 5 km, including Marshall Cove Campground, Cottonwood Campground. This is a location join, not a booking. Cabins and hotels are listed separately; campsite reservations use the official link when the source provides one.
Explore Cooney Reservoir across LakeQuality
More Montana Lake Rankings
Other Grade B Lakes in Montana
Mesotrophic Lakes in Montana
Where this data comes from
229 measurements from 1 monitoring station, 2023–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Montana DEQ WQPB
MDEQ_WQ_WQX229 samples · 2023–2025
Analytical methods
- Phosphorus by Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 4500 N C ~ Persufate Method for Total Nitrogen
Monitoring station identifiers (1)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Named boat-access points from the USGS National Boat Ramps inventory
Public campgrounds from USGS The National Map Structures, within about 3 miles