St. Marys Lake
St. Marys Lake in St. Louis County, Minnesota has a water quality grade of A (Excellent). Water clarity: 13.1 ft visibility. Phosphorus: 7.5 µg/L. Chlorophyll-a (algae): 2.1 µg/L. Maximum depth: 44 ft. Surface area: 234.16 acres.
- Clarity
- 13.1 ft
- Typical in August
- 60°F
- Max depth
- 44 ft
- Area
- 234 acres
- Type
- Oligotrophic
- Trend
- ↓ Declining
- Public access
- No
- Ice-out
- Apr 24
- Open water
- 212 days
Next lake in St. Louis County
St. Marys Lake is a 234-acre cold, clear oligotrophic lake in St. Louis County, Minnesota with a maximum depth of 44 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 14 documented fish species including black crappie, bluegill, and brown bullhead. No public access — the lake is private or restricted. Compared to the 67 other monitored lakes in St. Louis County, St. Marys Lake ranks #16 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-10-08. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLimited data
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)
Good clarity, visible to about 13.1 ft. Phosphorus level: 7.5 µg/L. Chlorophyll-a: 2.1 µg/L. Trophic State Index: 37.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 13.1 ft | Not graded |
| Phosphorus | 7.5 µg/L | Good |
| Chlorophyll-a (Algae) | 2.1 µg/L | Good |
| Trophic State Index (TSI) | 37 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near St. Marys Lake
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Water Temperature
St. Marys Lake typically sits around 60°F at the surface in August. Summer average is about 61°F. Usually warmest in Jul (about 63°F) and coldest in May (about 50°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 131 field readings (2019–2025) via the EPA Water Quality Portal. See the full St. Marys Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 44 ft |
| Surface Area | 234 acres |
| Shoreline Length | 4.1 mi |
| Littoral Zone | 33% |
| 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 St. Marys Lake
DNR Fisheries Survey Summary
6 surveys on file from MN DNR Fisheries. Most recent: 2025-07-21 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 26.00 | 0.12 lb |
| Largemouth Bass | 9.80 | 0.76 lb |
| Pumpkinseed | 9.71 | 0.19 lb |
| BNS | 5.70 | — |
| Green Sunfish | 5.67 | 0.15 lb |
| Fathead Minnow | 5.40 | — |
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
Pumpkinseed
Green Sunfish
From the 2025-07-21 survey
St. Mary's is a 234-acre lake located 2 miles south of Eveleth off the Miller Trunk Road. It shares much in common with Armstrong, Bass (Biwabik), and Ely lakes in the Tower Area. The water is clear, Secchi visibility is 13 feet, and the lake is up to 44 feet deep. St. Mary's was thermally stratified on 07/21, with a…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for St. Marys Lake. 3 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — St. Mary'sFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — St. Mary's (1772)Aquatic Plant Report · MN DNR LakeFinder
- —Fish Stocking History — St. Mary'sFish Stocking History · MN DNR LakeFinder
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.138 m/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (17 observations).
Estimated open water season: 212 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2017-04-09 (2017) | 2023-05-05 (2023) |
| Ice-In | 2018-11-18 (2018) | 2015-11-29 (2015) |
Most recent ice-out: 2023-05-05
EPA Impairment Status
St. Marys 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 MN69-0651-00 · Official waterbody report
County Ranking
Ranked #16 of 68 lakes in St. Louis County
Nearby Lakes in St. Louis County
Frequently asked questions about St. Marys Lake
What is the water quality grade for St. Marys Lake?
- St. Marys Lake in St. Louis County, Minnesota has a water quality grade of A (Excellent). Water clarity: 13.1 ft visibility. Phosphorus: 7.5 µg/L. Chlorophyll-a (algae): 2.1 µg/L. Maximum depth: 44 ft. Surface area: 234.16 acres.
Is St. Marys Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. St. Marys Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 2.1 µg/L. Always check local health department advisories before swimming.
What is the water temperature of St. Marys Lake?
- St. Marys Lake typically sits around 60°F at the surface in August, from 131 EPA Water Quality Portal field readings (2019–2025). That is a typical value for the month, not a live reading.
How deep is St. Marys Lake?
- St. Marys Lake has a maximum depth of 44 feet. The lake covers 234.16 acres.
What fish are in St. Marys Lake?
- Fish species found in St. Marys Lake include black crappie, bluegill, brown bullhead, cisco species, hybrid sunfish, largemouth bass, northern pike, pumpkinseed, rock bass, tullibee (cisco), walleye, white sucker, yellow bullhead, yellow perch. St. Marys Lake is located in St. Louis County, Minnesota.
When does St. Marys Lake thaw?
- St. Marys Lake typically has ice-out around Apr 24. Ice typically forms around Nov 22. The open water season is approximately 212 days.
Explore St. Marys Lake across LakeGrade
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Where this data comes from
80 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
MNPCA80 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