Mary Lake
Mary Lake in Crow Wing County, Minnesota has a water quality grade of C (Fair). Water clarity: 4.9 ft visibility. Phosphorus: 20.5 µg/L. Chlorophyll-a (algae): 8.8 µg/L. Maximum depth: 34 ft. Surface area: 412.64 acres.
- Clarity
- 4.9 ft
- Typical in August
- 75°F
- Max depth
- 34 ft
- Area
- 413 acres
- Type
- Eutrophic
- Trend
- ↓ Declining
- Public access
- No
- Ice-out
- Apr 15
- Open water
- 239 days
Next lake in Crow Wing County
Mary Lake is a 413-acre productive, nutrient-rich eutrophic lake in Crow Wing County, Minnesota with a maximum depth of 34 feet. Water quality is graded C based on 2 sampling years through 2025. Overall water quality is declining over recent sampling years. The lake supports 13 documented fish species including black bullhead, black crappie, and bluegill. No public access — the lake is private or restricted. Compared to the 84 other monitored lakes in Crow Wing County, Mary Lake ranks #72 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-14. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyModerate
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.
Generally safe for swimming, moderate water quality
Water Quality Grade: C (Fair)
Murky, only visible to about 4.9 ft. Phosphorus level: 20.5 µg/L. Chlorophyll-a: 8.8 µg/L. Trophic State Index: 51.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 4.9 ft | Not graded |
| Phosphorus | 20.5 µg/L | Good |
| Chlorophyll-a (Algae) | 8.8 µg/L | Fair |
| Trophic State Index (TSI) | 51 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Mary Lake
Compare lakeside cabins, hotels, and vacation rentals near Mary Lake in Crow Wing 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 Mary Lake
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Planning a whole trip? See where to stay at Minnesota’s top lakes →
Water Temperature
Mary Lake typically sits around 75°F at the surface in August. Summer average is about 75°F. Usually warmest in Jul (about 78°F) and coldest in May (about 62°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 25 field readings (2021–2025) via the EPA Water Quality Portal. See the full Mary Lake water temperature breakdown →
Blue-Green Algae (Cyanobacteria)
There’s no recent EPA CyAN cyanobacteria estimate for Mary Lake — clouds, ice, or an off-season gap in coverage. The most recent reading was moderate (25,823 cells/mL) on December 6, 2025.
Blue-green algae can appear and clear within days, so check your state’s current advisory before swimming. EPA CyAN ↗
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 34 ft |
| Surface Area | 413 acres |
| Shoreline Length | 4.7 mi |
| Littoral Zone | 31% |
| 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 Mary Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
DNR Fisheries Survey Summary
7 surveys on file from MN DNR Fisheries. Most recent: 2024-07-01 (Targeted Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 16.76 | 0.22 lb |
| Black Crappie | 5.43 | 0.21 lb |
| Yellow Perch | 4.81 | 0.14 lb |
| JND | 2.42 | — |
| CNM | 2.40 | — |
| Fathead Minnow | 1.89 | — |
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
Black Crappie
Yellow Perch
From the 2024-07-01 survey
A targeted survey of nearshore fish species in Mary Lake was conducted beginning on July 1, 2024, by Index of Biological Integrity (IBI) Program staff. Sampling sites were evenly spaced around the lake, and each was sampled by backpack electrofishing and seining with a 50-foot or 15-foot seine, where possible.…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Mary Lake. 2 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — MaryFisheries Lake Survey · MN DNR LakeFinder
- —Fish Stocking History — MaryFish 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 | -0.047 m/yr | 6 |
| Phosphorus | → Stable | +0.41 µg/L/yr | 6 |
| Chlorophyll-a | ↓ Declining | +0.2 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (12 observations).
Estimated open water season: 239 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2016-03-18 (2016) | 2023-05-02 (2023) |
| Ice-In | 2002-11-14 (2002) | 2015-12-17 (2015) |
Most recent ice-out: 2023-05-02
EPA Impairment Status
Mary 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 MN18-0212-00 · Official waterbody report · Matched by proximity (2.48 km)
County Ranking
Ranked #72 of 85 lakes in Crow Wing County
Nearby Lakes With Higher Grades
Mary Lake holds Grade C. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Crow Wing County
Frequently asked questions about Mary Lake
What is the water quality grade for Mary Lake?
- Mary Lake in Crow Wing County, Minnesota has a water quality grade of C (Fair). Water clarity: 4.9 ft visibility. Phosphorus: 20.5 µg/L. Chlorophyll-a (algae): 8.8 µg/L. Maximum depth: 34 ft. Surface area: 412.64 acres.
Is Mary Lake safe to swim in?
- Generally safe for swimming, moderate water quality. Mary Lake has a water quality grade of C (Fair). Algae concentration (chlorophyll-a) is 8.8 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Mary Lake?
- Mary Lake typically sits around 75°F at the surface in August, from 25 EPA Water Quality Portal field readings (2021–2025). That is a typical value for the month, not a live reading.
How deep is Mary Lake?
- Mary Lake has a maximum depth of 34 feet. The lake covers 412.64 acres.
What fish are in Mary Lake?
- Fish species found in Mary Lake include black bullhead, black crappie, bluegill, bowfin (dogfish), brown bullhead, largemouth bass, northern pike, pumpkinseed, rock bass, walleye, white sucker, yellow bullhead, yellow perch. Mary Lake is located in Crow Wing County, Minnesota.
When does Mary Lake thaw?
- Mary Lake typically has ice-out around Apr 15. Ice typically forms around Dec 10. The open water season is approximately 239 days.
Does Mary Lake have blue-green algae?
- There is no recent EPA CyAN cyanobacteria estimate for Mary Lake (clouds, ice, or an off-season gap). The most recent reading was moderate (25,823 cells/mL). Blue-green algae can appear and clear within days, so check Minnesota's current advisory before swimming.
Explore Mary Lake across LakeGrade
Depth, room, public access, and the state wake-sport rules
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Species activity outlook by water temperature and season
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Where this data comes from
90 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
MNPCA90 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
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