Cooper Lake
Cooper Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 17.1 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 3.1 µg/L. Maximum depth: 70 ft. Surface area: 132.53 acres.
- Clarity
- 17.1 ft
- Max depth
- 70 ft
- Area
- 133 acres
- Type
- Oligotrophic
- Trend
- ↓ Declining
- Public access
- No
- Ice-out
- Apr 17
- Open water
- 229 days
Next lake in Cass County
Cooper Lake is a 133-acre cold, clear oligotrophic lake in Cass County, Minnesota with a maximum depth of 70 feet. Water quality is graded A based on 2 sampling years through 2025. Overall water quality is declining over recent sampling years. The lake supports 12 documented fish species including black crappie, bluegill, and bowfin (dogfish). No public access — the lake is private or restricted. Compared to the 103 other monitored lakes in Cass County, Cooper Lake ranks #62 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-29. Grade methodology: LakeGrade methodology.
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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.
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A (Excellent)
Crystal clear, you can see 17.1 ft down. Phosphorus level: 13 µg/L. Chlorophyll-a: 3.1 µg/L. Trophic State Index: 40.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 17.1 ft | Not graded |
| Phosphorus | 13 µg/L | Good |
| Chlorophyll-a (Algae) | 3.1 µg/L | Good |
| Trophic State Index (TSI) | 40 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Cooper Lake
Compare lakeside cabins, hotels, and vacation rentals near Cooper Lake in Cass County, Minnesota — 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.
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Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 70 ft |
| Surface Area | 133 acres |
| Shoreline Length | 3 mi |
| Littoral Zone | 73% |
| Public Access | No |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Cooper Lake
DNR Fisheries Survey Summary
2 surveys on file from MN DNR Fisheries. Most recent: 1987-07-06 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 41.02 | 0.12 lb |
| Pumpkinseed | 18.97 | 0.15 lb |
| Tullibee (Cisco) | 17.34 | 0.57 lb |
| Yellow Perch | 4.89 | 0.13 lb |
| Rock Bass | 4.17 | 0.24 lb |
| Northern Pike | 2.64 | 3.45 lb |
CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE = more abundant in standardized sampling.
From the 1987-07-06 survey
ALTHOUGH BLACK CRAPPIE WERE NOT TAKEN DURING THE SURVEY, THE OWNER OF CAMP SECLUSION SAID IT WAS AN EXCELLENT WINTER AND SUMMER CRAPPIE FISHING LAKE. ANY WALLEYE THAT ARE CAUGHT ARE LARGE.THE FISH POPULATION LOOKS VERY SIMILAR TO WHAT WAS FOUND IN THE 1973 SURVEY WITH THE BULK OF THE POPULATION MADE UP OF CENTRARCHIDS…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Cooper Lake. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — CooperFisheries Lake Survey · MN DNR LakeFinder
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | +0.003 m/yr | 6 |
| Phosphorus | ↓ Declining | +1.37 µg/L/yr | 6 |
| Chlorophyll-a | → Stable | -0.06 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (8 observations).
Estimated open water season: 229 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2005-04-14 (2005) | 2008-04-30 (2008) |
| Ice-In | 2003-11-25 (2003) | 2004-12-04 (2004) |
Most recent ice-out: 2008-04-30
EPA Impairment Status
Cooper Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN11-0167-00 · Official waterbody report · Matched by proximity (2.92 km)
County Ranking
Ranked #62 of 104 lakes in Cass County
Nearby Lakes in Cass County
Frequently asked questions about Cooper Lake
What is the water quality grade for Cooper Lake?
- Cooper Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 17.1 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 3.1 µg/L. Maximum depth: 70 ft. Surface area: 132.53 acres.
Is Cooper Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Cooper Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 3.1 µg/L. Always check local health department advisories before swimming.
How deep is Cooper Lake?
- Cooper Lake has a maximum depth of 70 feet. The lake covers 132.53 acres.
What fish are in Cooper Lake?
- Fish species found in Cooper Lake include black crappie, bluegill, bowfin (dogfish), greater redhorse, hybrid sunfish, largemouth bass, northern pike, northern pike (silver phase), pumpkinseed, rock bass, yellow bullhead, yellow perch. Cooper Lake is located in Cass County, Minnesota.
When does Cooper Lake thaw?
- Cooper Lake typically has ice-out around Apr 17. Ice typically forms around Dec 2. The open water season is approximately 229 days.
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Where this data comes from
42 measurements from 1 monitoring station, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA42 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
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
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate