Armstrong Lake
St. Louis County, MinnesotaLimited DataOligotrophic
Armstrong Lake earns an A — every scored parameter (clarity) rates among the best monitored waters in St. Louis County.
Trophically, Armstrong Lake reads as oligotrophic — nutrient-poor, clear, and ecologically more sensitive to disturbance than richer lakes. The lake bottoms out at 34 ft — a moderate depth that supports a warm-water fishery without the year-round cold refuge a deeper basin provides. Armstrong Lake covers 382 acres alongside 4.5 miles of shoreline — a mid-sized water that supports a working fishery without being so large that conditions diverge between basins. Within St. Louis County's 187 graded waters, Armstrong Lake sits at rank 13, near the top of the local distribution.
No invasive species are currently listed at Armstrong Lake — the lake remains off the Minnesota infested-waters roster. Walleye are documented at Armstrong Lake, one of 10 fish species on record for the lake. Public access is available — the lake is on the Minnesota PCA public-access list. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2023-09-03. Grade methodology: LakeGrade methodology.
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
Crystal clear, you can see 16.2 ft down. Trophic State Index: 37.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 16.2 ft | A |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 37 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 34 ft |
| Surface Area | 382.34 acres |
| Shoreline Length | 4.5 mi |
| Littoral Zone | 27% |
| Public Access | Yes |
Fish Species
Click a species to see all Minnesota and Wisconsin lakes where it is found.
→ Best fishing times for Armstrong Lake (14-day solunar calendar)
→ Is it safe to eat fish from Armstrong Lake? (mercury & PFAS guide)
→ Armstrong Lake fishing regulations (limits, seasons, special rules)
Water Quality Trend: ↓ Declining
Based on 4 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.12 m/yr | 4 |
Ice Season
Historical ice data from Minnesota DNR (3 observations).
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2010-03-31 (2010) | 2015-05-14 (2015) |
Most recent ice-out: 2015-05-14
Location
County Ranking
Ranked #13 of 187 lakes in St. Louis County
Nearby Lakes in St. Louis County
More Minnesota Lake Rankings
State Parks Near Armstrong Lake
Other Grade A Lakes in Minnesota
Oligotrophic Lakes in Minnesota
DNR Fisheries Survey Summary
8 surveys on file from MN DNR Fisheries. Most recent: 2021-08-09 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 24.60 | 0.17 lb |
| Largemouth Bass | 11.73 | 0.74 lb |
| IOD | 5.05 | — |
| Walleye | 3.43 | 3.21 lb |
| White Sucker | 3.34 | 2.27 lb |
| Northern Pike | 3.15 | 2.32 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.
Bluegill
Largemouth Bass
Walleye
White Sucker
From the 2021-08-09 survey
Armstrong Lake is a 382 acre lake located 10 miles west of Ely. It is accessed by a concrete ramp off Hwy 169. The lake has a maximum depth of 34 feet and clear water at the time of this survey. On August 9, 2021 the lake was thermally stratified with a surface temperature of 72 F and a bottom temperature of 54 F.…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Armstrong Lake. 4 reports on file.
- —Fisheries Lake Survey — ArmstrongFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Armstrong (1864)Aquatic Plant Report · MN DNR LakeFinder
- —Fish Stocking History — ArmstrongFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — ArmstrongPublic Water Access Sites · MN DNR LakeFinder
EPA Impairment Status
Armstrong 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 MN69-0278-00 · Official waterbody report
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Grading methodology based on Metropolitan Council standards
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate
Most recent sample: 2023-09-03
Monitoring stations: 4