Mary Lake
Mary Lake in Douglas County, Minnesota has a water quality grade of A (Excellent). 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.
- Clarity
- 10.5 ft
- Typical in August
- 69°F
- Max depth
- 40 ft
- Area
- 2,450 acres
- Type
- Mesotrophic
- Trend
- ↑ Improving
- Public access
- Yes
- Ice-out
- Apr 16
- Open water
- 225 days
Next lake in Douglas County
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). Water quality is graded A based on 2 sampling years through 2025. 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. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLow algae
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 10.5 ft. Phosphorus level: 20 µg/L. Chlorophyll-a: 5.5 µg/L. Trophic State Index: 46.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 10.5 ft | Not graded |
| Phosphorus | 20 µg/L | Good |
| Chlorophyll-a (Algae) | 5.5 µg/L | Good |
| Trophic State Index (TSI) | 46 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay 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.
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Water Temperature
Mary Lake typically sits around 69°F at the surface in August. 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.
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 ↗
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 40 ft |
| Average Depth | 17 ft |
| Surface Area | 2,450 acres |
| Shoreline Length | 11.7 mi |
| Littoral Zone | 45% |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Lake Mary (South)Public
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
15 surveys on file from MN DNR Fisheries. Most recent: 2025-08-11 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| BKS | 31.99 | — |
| Spottail Shiner | 29.68 | — |
| Bluntnose Minnow | 22.47 | — |
| IOD | 20.77 | — |
| Bluegill | 15.23 | 0.17 lb |
| Largemouth Bass | 14.95 | 1.03 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
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
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.193 m/yr | 6 |
| Phosphorus | ↑ Improving | -1.34 µg/L/yr | 6 |
| Chlorophyll-a | ↓ Declining | +0.17 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (73 observations).
Estimated open water season: 225 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2016-03-25 (2016) | 1979-05-02 (1979) |
| Ice-In | 1995-11-12 (1995) | 2001-12-19 (2001) |
Most recent ice-out: 2025-04-11
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 MN21-0092-00 · Official waterbody report
County Ranking
Ranked #11 of 37 lakes in Douglas County
Nearby Lakes in Douglas County
Frequently asked questions about Mary Lake
What is the water quality grade for Mary Lake?
- Mary Lake in Douglas County, Minnesota has a water quality grade of A (Excellent). 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?
- Excellent for swimming, crystal clear water with minimal algae. Mary Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 5.5 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Mary Lake?
- Mary Lake typically sits around 69°F at the surface in August, 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.
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.
Explore Mary Lake across LakeGrade
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Moderate nutrients, good water quality
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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)
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
Named boat-access points from the USGS National Boat Ramps inventory
Ice data from Minnesota DNR Climate