Duncan Lake
Duncan Lake in Cook County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 21.5 ft visibility. Maximum depth: 130 ft. Surface area: 476.95 acres.
- Clarity
- 21.5 ft
- Max depth
- 130 ft
- Area
- 477 acres
- Trend
- ↓ Declining
- Public access
- No
Next lake in Cook County
Why Duncan Lake has no grade
We grade a lake only when its monitoring record clears the evidence bar: at least three phosphorus or chlorophyll-a samples in the last ten years, scored against EPA National Lakes Assessment thresholds for this lake’s ecoregion. For Duncan Lake, the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity. Sampling here has recorded water clarity, but not phosphorus and algae (chlorophyll-a).
Not Rated is not an F. It is EPA Integrated Reporting Category 3 — insufficient information. The water here may be excellent or poor; nobody has measured enough of it to say. Everything below that has been measured is still reported on this page.
Sampling is run by MPCA and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Duncan Lake is a 477-acre lake in Cook County, Minnesota with a maximum depth of 130 feet (47-foot average). Overall water quality is declining over recent sampling years. The lake supports 5 documented fish species including cisco species, green sunfish, and lake trout. No public access — the lake is private or restricted.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-08-07. Grade methodology: LakeGrade methodology.
Watch Duncan Lake water quality
Subscribe for LakeQuality updates by email. No spam, unsubscribe anytime.
Swimming Safety
Not ratedLimited data
Not rated for swimming — there is not enough monitoring data to judge this lake
Water quality measurements for Duncan Lake
These readings exist but do not meet the evidence bar for an A–F grade — see why.
Crystal clear, you can see 21.5 ft down. Trophic State Index: 33.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 21.5 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 33 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Duncan Lake
Compare lakeside cabins, hotels, and vacation rentals near Duncan Lake in Cook 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.
Top-rated places to stay near Duncan Lake
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Minnesota’s top lakes →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 130 ft |
| Average Depth | 46.8 ft |
| Surface Area | 477 acres |
| Shoreline Length | 7.6 mi |
| Littoral Zone | 31% |
| 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 Duncan Lake
DNR Fisheries Survey Summary
7 surveys on file from MN DNR Fisheries. Most recent: 2025-08-11 (Targeted Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Tullibee (Cisco) | 12.04 | 0.74 lb |
| CIS | 11.75 | 0.78 lb |
| Green Sunfish | 3.40 | 0.11 lb |
| White Sucker | 3.33 | 2.41 lb |
| Yellow Perch | 3.25 | 0.31 lb |
| Lake Trout | 2.05 | 2.52 lb |
CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE = more abundant in standardized sampling.
Length Distributions
Number of fish caught at each inch class in the most recent survey that recorded lengths. Red dashed line marks an approximate trophy threshold for that species.
Tullibee (Cisco)
CIS
Green Sunfish
White Sucker
From the 2025-08-11 survey
Temperature and dissolved oxygen (DO) measurements were collected from the deepest basin in Duncan Lake on August 11th, 2025, to evaluate the quantity and quality of cold, oxygenated water (i.e., oxythermal habitat) available to Lake Trout, the most sensitive coldwater species present in this lake (Cisco, also known…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Duncan Lake. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — DuncanFisheries 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 | ↓ Declining | -0.29 m/yr | 6 |
| Phosphorus | ↓ Declining | +0.5 µg/L/yr | 4 |
| Chlorophyll-a | ↓ Declining | +0.08 µg/L/yr | 4 |
EPA Impairment Status
Duncan 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 MN16-0232-00 · Official waterbody report
Graded lakes near Duncan Lake
Duncan Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Cook County
Frequently asked questions about Duncan Lake
What is the water quality grade for Duncan Lake?
- Duncan Lake in Cook County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 21.5 ft visibility. Maximum depth: 130 ft. Surface area: 476.95 acres.
Is Duncan Lake safe to swim in?
- Not rated for swimming — there is not enough monitoring data to judge this lake. Duncan Lake is not rated for water quality — its monitoring record does not meet the evidence bar. Always check local health department advisories before swimming.
How deep is Duncan Lake?
- Duncan Lake has a maximum depth of 130 feet and an average depth of 46.8 feet. The lake covers 476.95 acres.
What fish are in Duncan Lake?
- Fish species found in Duncan Lake include cisco species, green sunfish, lake trout, smallmouth bass, white sucker. Duncan Lake is located in Cook County, Minnesota.
Explore Duncan Lake across LakeGrade
Depth, room, public access, and the state wake-sport rules
Fish species and top-graded lakes across Minnesota
Fish species and best lakes in Cook County
Species activity outlook by water temperature and season
Where to stay at Minnesota's top lakes, each with its grade
Lakes in the Lake Superior Basin (MN+WI), ranked by water quality
Minnesota's cleanest and most-polluted lakes, ranked
More Minnesota Lake Rankings
State Parks Near Duncan Lake
Other Lakes in Minnesota
Oligotrophic Lakes in Minnesota
Where this data comes from
5 measurements from 5 monitoring stations, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA5 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
Monitoring station identifiers (5)
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