Cooney Reservoir
Carbon County, Montana1,078 acresView mapDirections
Montana DEQ publishes the water-quality measurements for Cooney Reservoir. Here is what has been measured.
- Mesotrophic
- Public access
Cooney Reservoir in Carbon County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. Water clarity: 12.5 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 2.6 µg/L. Surface area: 1078 acres.
Cooney Reservoir is a 1,078-acre mesotrophic lake in Carbon County, Montana. 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. Methodology: how these readings are compared.
Cooney Reservoir covers about 1,100 acres in Carbon County. It has 2 public boat landings, so a lakefront rental is not the only way to get a boat on the water. Vrbo is where to look for cabins and lake homes around Cooney Reservoir, and Booking.com and Expedia search hotels near the lake for your dates.
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 conditions
Readings onlyLow algae
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Cooney Reservoir is safe to swim in today is a call for the Montana Department of Environmental Quality and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Cooney Reservoir
These are the measurements Montana DEQ publishes for Cooney Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 12.5 ft. Phosphorus 20 µg/L. Chlorophyll-a 2.6 µg/L. TSI 43.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 12.5ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 20µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 2.6µg/L | Measured, not scored The green pigment in algae — a direct measure of how much algae is in the water. |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 43 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Cooney Reservoir
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
1,078 acres
Public access
Yes
Listed by Montana DEQ
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
Stays via Stay22
Stays near Cooney Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Cooney Reservoir in Carbon County, Montana — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near Cooney Reservoir
Prices for Oct 30–Nov 1, 4 adults. Change dates on the map below.
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Fishing Cooney Reservoir
Water quality readings over time
3 seasons of samples on record (2023–2025). Calling a direction takes at least 4 years of the same measurement.
Water clarity
Worsening3 years
Phosphorus
Worsening+3 µg/L/yr3 years
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
Nearby Lakes in Carbon County
Frequently asked questions about Cooney Reservoir
Where is Cooney Reservoir?
- Cooney Reservoir is a lake in Carbon County, Montana. Its water-quality measurements are published by Montana DEQ and shown on this page as reported.
What is the water quality of Cooney Reservoir?
- Cooney Reservoir in Carbon County, Montana has water-quality measurements published by Montana DEQ, shown here as reported. 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?
- Nutrient and algae readings are 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.
Can I fish at Cooney Reservoir?
- Cooney Reservoir has 2 documented boat landings in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Montana's fish and wildlife agency, so check its current regulations before you go.
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
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from 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