Mary Lake
Douglas County, Minnesota2,450 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Mary Lake. Here is what has been measured.
- Mesotrophic
- Impaired · Mercury
- zebra mussel
- 40 ft max depth
- Public access
Mary Lake in Douglas County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 10.5 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 5.5 µg/L. Maximum depth: 40 ft. Surface area: 2450.43 acres.
Minnesota DNR's Standard Survey on August 11, 2025 caught 12.92 walleye per gill net at Mary Lake. DNR survey details →
Free interactive depth-contour map of Mary Lake, reaching a maximum surveyed depth of 30 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Minnesota DNR.
Mary Lake is a 2,450-acre mesotrophic lake in Douglas County, Minnesota with a maximum depth of 40 feet (17-foot average). Overall water quality is improving over recent sampling years. The lake supports 17 documented fish species including black bullhead, black crappie, and bluegill. Public access is available with 1 public boat landing. Compared to the 36 other monitored lakes in Douglas County, Mary Lake ranks #11 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-10-06. Methodology: how these readings are compared.
Mary Lake covers about 2,500 acres in Douglas County. It has one public boat landing. The median ice-out date is April 16, which is when the open-water season begins. Vrbo is where to look for cabins and lake homes around Mary Lake, and Booking.com and Expedia search hotels near the lake for your dates.
What Mary Lake is good for
From access and monitoring records, not editorial picks.
- Clear water10.5 ft of visibility
- Boat launch1 documented ramp
- Fishing17 species recorded
- Ice seasonIce-out around Apr 16
- Deep water40 ft surveyed max depth
- Big water2,450 acres
Swimming conditions
Readings onlyLow algae
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. Mary Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
Whether Mary 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 Mary Lake
These are the measurements Minnesota PCA publishes for Mary 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 10.5 ft. Phosphorus 20 µg/L. Chlorophyll-a 5.5 µg/L. TSI 46.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 10.5ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 20µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 5.5µg/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. | 46 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Mary Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Mary Lake typically sits around 67°F at the surface in September. Summer average is about 71°F. Usually warmest in Jul (about 76°F) and coldest in Sep (about 67°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (67°F) to its warmest (76°F).
Typical surface water temperature by month, from 18 field readings (2010–2021) 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 low (15,276 cells/mL) on June 6, 2026.
Blue-green algae can appear and clear within days, so check your state’s current advisory before swimming. EPA CyAN ↗
Minnesota DNR's Standard Survey on August 11, 2025 caught 12.92 walleye per gill net at Mary Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Mary Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is improving. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
15 surveys on file from MN DNR Fisheries. Most recent: 2025-08-11 (Standard Survey).
15 surveys on file · most recent 2025-08-11 (Standard Survey)
| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| Spottail Shiner | 29.68 | ||
| Bluntnose Minnow | 22.47 | ||
| Bluegill | 15.23 | 2–8 in 2025-08-11 | 0.17 lb |
| Largemouth Bass | 14.95 | 5–20 in 2025-08-11 | 1.03 lb |
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
From the 2025-08-11 survey
Lake Mary (2,450 acres) is located five miles southwest of Alexandria. Water clarity is moderate (8 feet) and aquatic plants are common in shallow areas. Average depth is 17 feet with a maximum depth of 40 feet. Walleye, Northern Pike, Largemouth Bass, and Bluegill are common. Lake Mary has two public accesses. One is…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Mary Lake. 4 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
- —Aquatic Plant Report — Mary (0932)Aquatic Plant Report · MN DNR LakeFinder
- —Fish Stocking History — MaryFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — MaryPublic Water Access Sites · MN DNR LakeFinder
Lake Details
Max depth
40 ft
Average depth
17 ft
Surface area
2,450 acres
Shoreline
11.7 mi
Littoral zone
45%
Share shallower than 15 ft
Public access
Yes
Listed by Minnesota PCA
Lake Size & Watershed
Watershed (drainage) area
27 sq mi
HydroLAKES / HydroSHEDS
Elevation
1,358 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Lake Mary (South)Public
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays near Mary Lake
Compare lakeside cabins, hotels, and vacation rentals near Mary Lake in Douglas 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
Prices for Oct 9–11, 4 adults. Change dates on the map below.
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Minnesota’s top lakes →
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Fish Species
Click a species to see every 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.
Water Quality Trend: Improving
Based on 6 years of monitoring (2020–2025).
Water clarity
Improving6 years
Phosphorus
Improving-1.34 µg/L/yr6 years
Chlorophyll-a
Worsening+0.17 µg/L/yr6 years
Ice season
Typical ice-out
Apr 16
Earliest Mar 25, 2016 · Latest May 2, 1979
Typical ice-in
Nov 27
Earliest Nov 12, 1995 · Latest Dec 19, 2001
Most recent ice-out
Apr 11
2025
73 observations · Minnesota PCA ice records
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
Designated uses not supporting: Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN21-0092-00 · Official waterbody report
Nearby Lakes in Douglas County
Frequently asked questions about Mary Lake
Where is Mary Lake?
- Mary Lake is a lake in Douglas County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Mary Lake?
- Mary Lake in Douglas County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 10.5 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 5.5 µg/L. Maximum depth: 40 ft. Surface area: 2450.43 acres.
Is Mary Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Mary 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 Mary Lake?
- Mary Lake typically sits around 67°F at the surface in September, from 18 EPA Water Quality Portal field readings (2010–2021). That is a typical value for the month, not a live reading.
How deep is Mary Lake?
- Mary Lake has a maximum depth of 40 feet and an average depth of 17 feet. The lake covers 2450.43 acres.
What fish are in Mary Lake?
- Fish species found in Mary Lake include black bullhead, black crappie, bluegill, bowfin (dogfish), brown bullhead, common carp, green sunfish, hybrid sunfish, largemouth bass, northern pike, pumpkinseed, rock bass, tullibee (cisco), walleye, white sucker, yellow bullhead, yellow perch. Mary Lake is located in Douglas County, Minnesota.
What did the DNR fisheries survey find at Mary Lake?
- Minnesota DNR's Standard Survey on August 11, 2025 caught 12.92 walleye per gill net at Mary Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Mary Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is improving. Catch rate is not a swim clearance and is not a weekly bite report.
When does Mary Lake thaw?
- Mary Lake typically has ice-out around Apr 16. Ice typically forms around Nov 27. The open water season is approximately 225 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 low (15,276 cells/mL). Blue-green algae can appear and clear within days, so check Minnesota's current advisory before swimming.
Are there boat ramps on Mary Lake?
- Mary Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Lake Mary (South). Public access is listed as yes.
Can I fish at Mary Lake?
- Mary Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. 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 Mary Lake across LakeQuality
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Where this data comes from
141 measurements from 3 monitoring stations, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA141 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 (3)
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 size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Named boat-access points from the USGS National Boat Ramps inventory
Ice data from Minnesota DNR Climate