Thomson Reservoir
Thomson Reservoir in Carlton County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 4 ft visibility. Maximum depth: 22 ft. Surface area: 389.96 acres.
- Clarity
- 4 ft
- Max depth
- 22 ft
- Area
- 390 acres
- Public access
- No
Next lake in Carlton County
Why Thomson Reservoir 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 Thomson Reservoir, no nutrient or algae measurement meets the evidence bar (at least 3 samples within 10 years). 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.
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. Grade methodology: LakeGrade methodology.
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Swimming Safety
Not ratedLimited data
Not rated for swimming — there is not enough monitoring data to judge this lake
Water quality measurements for Thomson Reservoir
These readings exist but do not meet the evidence bar for an A–F grade — see why.
Murky, only visible to about 4 ft. Trophic State Index: 57.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 57 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay 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
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Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 22 ft |
| Surface Area | 390 acres |
| Shoreline Length | 10.7 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 Thomson Reservoir
DNR Fisheries Survey Summary
2 surveys on file from MN DNR Fisheries. Most recent: 2002-08-26 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| White Sucker | 9.05 | 1.48 lb |
| Walleye | 2.40 | 0.68 lb |
| Yellow Perch | 1.55 | 0.52 lb |
| Smallmouth Bass | 1.23 | 1.39 lb |
| Black Bullhead | 0.87 | 0.38 lb |
| Northern Pike | 0.84 | 1.54 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.
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
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
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.
Graded lakes near Thomson Reservoir
Thomson Reservoir has no grade, but these lakes within a short drive do.
Nearby Lakes in Carlton County
Frequently asked questions about Thomson Reservoir
What is the water quality grade for Thomson Reservoir?
- Thomson Reservoir in Carlton County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 4 ft visibility. Maximum depth: 22 ft. Surface area: 389.96 acres.
Is Thomson Reservoir safe to swim in?
- Not rated for swimming — there is not enough monitoring data to judge this lake. Thomson Reservoir 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 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.
Explore Thomson Reservoir across LakeGrade
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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)
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder