St. Marys Lake
St. Louis County, Minnesota234 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for St. Marys Lake. Here is what has been measured.
- Oligotrophic
- Impaired · Mercury
- 44 ft max depth
- Limited data
St. Marys Lake in St. Louis County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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.
Minnesota DNR's Standard Survey on July 21, 2025 caught 1.17 walleye per gill net at St. Marys Lake. DNR survey details →
St. Marys Lake is a 234-acre cold, clear oligotrophic lake in St. Louis County, Minnesota with a maximum depth of 44 feet. 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. Methodology: how these readings are compared.
What St. Marys Lake is good for
From access and monitoring records, not editorial picks.
- Clear water13.1 ft of visibility
- Fishing14 species recorded
- Ice seasonIce-out around Apr 24
- Deep water44 ft surveyed max depth
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. St. Marys Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
Whether St. Marys Lake 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 St. Marys Lake
These are the measurements Minnesota PCA publishes for St. Marys Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 13.1 ft. Phosphorus 7.5 µg/L. Chlorophyll-a 2.1 µg/L. TSI 37.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 13.1ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 7.5µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 2.1µg/L | Good GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 37 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about St. Marys Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
St. Marys Lake typically sits around 60°F at the surface in October. 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.
Column height runs from this lake’s coldest month (50°F) to its warmest (63°F).
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 →
Minnesota DNR's Standard Survey on July 21, 2025 caught 1.17 walleye per gill net at St. Marys Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows St. Marys Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
6 surveys on file from MN DNR Fisheries. Most recent: 2025-07-21 (Standard Survey).
6 surveys on file · most recent 2025-07-21 (Standard Survey)
| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| Bluegill | 26.00 | 3–7 in 2025-07-21 | 0.12 lb |
| Largemouth Bass | 9.80 | 4–16 in 2025-07-21 | 0.76 lb |
| Pumpkinseed | 9.71 | 3–7 in 2025-07-21 | 0.19 lb |
| Green Sunfish | 5.67 | 3–5 in 2011-08-08 | 0.15 lb |
| Fathead Minnow | 5.40 |
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.
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
Lake Details
Max depth
44 ft
Surface area
234 acres
Shoreline
4.1 mi
Littoral zone
33%
Share shallower than 15 ft
Public access
No
Listed by Minnesota PCA
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Stays near St. Marys Lake
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Fish Species
Click a species to see every lake where it is documented.
Fishing St. Marys Lake
Water Quality Trend: Worsening
Based on 6 years of monitoring (2020–2025).
Water clarity
Worsening6 years
Ice season
Typical ice-out
Apr 24
Earliest Apr 9, 2017 · Latest May 5, 2023
Typical ice-in
Nov 22
Earliest Nov 18, 2018 · Latest Nov 29, 2015
Most recent ice-out
May 5
2023
17 observations · Minnesota PCA ice records
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
Designated uses not supporting: Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN69-0651-00 · Official waterbody report
Nearby Lakes in St. Louis County
Frequently asked questions about St. Marys Lake
Where is St. Marys Lake?
- St. Marys Lake is a lake in St. Louis County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of St. Marys Lake?
- St. Marys Lake in St. Louis County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. St. Marys Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
What is the water temperature of St. Marys Lake?
- St. Marys Lake typically sits around 60°F at the surface in October, 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.
What did the DNR fisheries survey find at St. Marys Lake?
- Minnesota DNR's Standard Survey on July 21, 2025 caught 1.17 walleye per gill net at St. Marys Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows St. Marys Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
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.
Can I fish at St. Marys Lake?
- Public access to St. Marys Lake 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.
Explore St. Marys Lake across LakeQuality
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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)
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
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate