Thomson Reservoir
Carlton County, Minnesota390 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Thomson Reservoir. Here is what has been measured.
- Impaired · Mercury
- 22 ft max depth
- Limited data
Thomson Reservoir in Carlton County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 4 ft visibility. Maximum depth: 22 ft. Surface area: 389.96 acres.
Minnesota DNR's Standard Survey on August 26, 2002 caught 4.22 walleye per gill net at Thomson Reservoir. Length samples in the latest survey that recorded them ran 7–18 inches. DNR survey details →
Thomson Reservoir is a 390-acre lake in Carlton County, Minnesota with a maximum depth of 22 feet. The lake supports 10 documented fish species including black bullhead, black crappie, and channel catfish. No public access — the lake is private or restricted.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2021-10-02. Methodology: how these readings are compared.
Thomson Reservoir covers about 390 acres in Carlton County. Vrbo is where to look for cabins and lake homes around Thomson Reservoir, and Booking.com and Expedia search hotels near the lake for your dates.
What Thomson Reservoir is good for
From access and monitoring records, not editorial picks.
- Fishing10 species recorded
- Camping1 public campground nearby
Swimming conditions
No readingLimited data
Nutrient and algae readings are not a swimming clearance. Thomson Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
Whether Thomson Reservoir is safe to swim in today is a call for the Minnesota Pollution Control Agency and the Minnesota DNR and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Thomson Reservoir
These are the measurements Minnesota PCA publishes for Thomson Reservoir, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Murky, only visible to about 4 ft. TSI 57.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 4ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | No reading The nutrient that most limits algae growth in freshwater. More phosphorus, more algae potential. | |
| Chlorophyll-a (algae) | No reading 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. | 57 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Thomson Reservoir
Answers come straight from the public measurements and survey records on this page.
Minnesota DNR's Standard Survey on August 26, 2002 caught 4.22 walleye per gill net at Thomson Reservoir. Length samples in the latest survey that recorded them ran 7–18 inches.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Thomson Reservoir’s water-quality measurements separately. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
2 surveys on file from MN DNR Fisheries. Most recent: 2002-08-26 (Standard Survey).
2 surveys on file · most recent 2002-08-26 (Standard Survey)
24 years old| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| White Sucker | 9.05 | 9–19 in 2002-08-26 | 1.48 lb |
| Walleye | 2.40 | 7–18 in 2002-08-26 | 0.68 lb |
| Yellow Perch | 1.55 | 5–11 in 2002-08-26 | 0.52 lb |
| Smallmouth Bass | 1.23 | 6–17 in 2002-08-26 | 1.39 lb |
| Black Bullhead | 0.87 | 6–8 in 2002-08-26 | 0.38 lb |
| Northern Pike | 0.84 | 12–24 in 2002-08-26 | 1.54 lb |
CPUE = catch per unit effort, fish caught per net per set in a standardised survey. It measures abundance, not size, and is not a weekly bite report.
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.
White Sucker
Walleye
Yellow Perch
Smallmouth Bass
From the 2002-08-26 survey
Thomson is a 339 acre reservoir located immediately north of Thomson, MN. It is operated as a water storage reservoir by Minnesota Power for the Thomson Hydro Station further down stream and is subject to water level manipulation. Maximum depth is 22 feet and 83% of the reservoir is shallower than 15 feet. Access is…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Thomson Reservoir. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — Thomson ReservoirFisheries Lake Survey · MN DNR LakeFinder
Lake Details
Max depth
22 ft
Surface area
390 acres
Shoreline
10.7 mi
Littoral zone
73%
Share shallower than 15 ft
Public access
No
Listed by Minnesota PCA
Lake Size & Watershed
Watershed (drainage) area
3,534 sq mi
HydroLAKES / HydroSHEDS
Elevation
1,050 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
Public campgrounds near Thomson Reservoir
- Jay Cooke State Park Campground2.4 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 Thomson Reservoir
Compare lakeside cabins, hotels, and vacation rentals near Thomson Reservoir in Carlton 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 Thomson Reservoir
Prices for Oct 9–11, 4 adults. Change dates on the map below.
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Planning a whole trip? See where to stay at Minnesota’s top lakes →
Follow Thomson Reservoir’s readings
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Fish Species
Click a species to see every lake where it is documented.
Fishing Thomson Reservoir
- → Thomson Reservoir fishing: DNR survey catch rates, species & stocking
- → Thomson Reservoir fishing forecast & best times (species outlook + 14-day solunar calendar)
- → Thomson Reservoir fishing regulations (limits, seasons, special rules)
- → Is it safe to eat fish from Thomson Reservoir? (mercury & PFAS guide)
Reservoir Info (USACE NID)
Thomson Reservoir is a man-made reservoir impounded by the Thomson (completed 1907), built primarily for hydroelectric on the St Louis River; earth-type dam, 15 ft tall and 1,600 ft long.
- Surface area
- 375 ac
- Normal storage
- 4,352 ac-ft
- Max storage
- 4,352 ac-ft
- Drainage area
- 3,548 sq mi
- Hazard class
- High
- Owner
- Allete, Inc.
All listed purposes: Hydroelectric;Recreation.
Source: USACE National Inventory of Dams, NID ID MN00604 · Operator website
EPA Impairment Status
Thomson Reservoir is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Designated uses not supporting: Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN09-0001-00 · Official waterbody report
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Thomson Reservoir under Clean Water Act (NPDES) discharge permits.
- Conocophillips Co-WrenshallWrenshall · 2.9 mi away · NPDES MN0060976
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
Nearby Lakes in Carlton County
Frequently asked questions about Thomson Reservoir
Where is Thomson Reservoir?
- Thomson Reservoir is a lake in Carlton County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Thomson Reservoir?
- Thomson Reservoir in Carlton County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 4 ft visibility. Maximum depth: 22 ft. Surface area: 389.96 acres.
Is Thomson Reservoir safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Thomson Reservoir is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
How deep is Thomson Reservoir?
- Thomson Reservoir has a maximum depth of 22 feet. The lake covers 389.96 acres.
What fish are in Thomson Reservoir?
- Fish species found in Thomson Reservoir include black bullhead, black crappie, channel catfish, northern pike, rock bass, shorthead redhorse, smallmouth bass, walleye, white sucker, yellow perch. Thomson Reservoir is located in Carlton County, Minnesota.
What did the DNR fisheries survey find at Thomson Reservoir?
- Minnesota DNR's Standard Survey on August 26, 2002 caught 4.22 walleye per gill net at Thomson Reservoir. Length samples in the latest survey that recorded them ran 7–18 inches. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Thomson Reservoir’s water-quality measurements separately. Catch rate is not a swim clearance and is not a weekly bite report.
Can I fish at Thomson Reservoir?
- Public access to Thomson Reservoir is listed as no on the lake record, so confirm before you make the trip. Seasons, size and bag limits, and whether you need a license are set by Minnesota's fish and wildlife agency, so check its current regulations before you go.
Are there campgrounds on Thomson Reservoir?
- Thomson Reservoir has 1 public campground from USGS The National Map within 5 km, including Jay Cooke State Park 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 Thomson Reservoir across LakeQuality
Depth, room, public access, and the state wake-sport rules
Fish species and the cleanest lakes across Minnesota
Fish species and best lakes in Carlton County
Species activity outlook by water temperature and season
Where to stay at Minnesota's top lakes, each with its readings
Lakes in the Lake Superior Basin (MN+WI), ranked by water quality
Minnesota's cleanest lakes, ranked on agency measurements
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State Parks Near Thomson Reservoir
Other Lakes in Minnesota
Eutrophic Lakes in Minnesota
Where this data comes from
3 measurements from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA3 samples · 2021
Analytical methods
- Field measurement/observation, generic method
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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Public campgrounds from USGS The National Map Structures, within about 3 miles