Mcmahon Lake
Mcmahon Lake in Scott County, Minnesota has a water quality grade of F (Algae-prone). Water clarity: 2.3 ft visibility. Phosphorus: 71 µg/L. Chlorophyll-a (algae): 36 µg/L. Maximum depth: 14 ft. Surface area: 186.61 acres.
- Clarity
- 2.3 ft
- Typical in August
- 82°F
- Max depth
- 14 ft
- Area
- 187 acres
- Type
- Eutrophic
- Trend
- ↓ Declining
- Public access
- Yes
- Ice-out
- Mar 25
- Open water
- 261 days
Grade F measures nutrients and algae, not whether this is a good place for a cabin trip. Cabins and rentals near Mcmahon Lake →
Next lake in Scott County
Mcmahon Lake is a 187-acre productive, nutrient-rich eutrophic lake in Scott County, Minnesota with a maximum depth of 14 feet. Water quality is graded F based on 2 sampling years through 2025. Overall water quality is declining over recent sampling years. The lake supports 9 documented fish species including black bullhead, black crappie, and bluegill. Public access is available with 1 public boat landing. Compared to the 20 other monitored lakes in Scott County, Mcmahon Lake ranks #18 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-10. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyAlgae risk
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.
Algae and nutrients are often high — check current advisories before swimming. Skip the water if you see scum or a paint-like bloom
Water Quality Grade: F (Algae-prone)
Very murky, less than 2.3 ft of visibility. Phosphorus level: 71 µg/L. Chlorophyll-a: 36 µg/L. Trophic State Index: 65.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 2.3 ft | Not graded |
| Phosphorus | 71 µg/L | Poor |
| Chlorophyll-a (Algae) | 36 µg/L | Poor |
| Trophic State Index (TSI) | 65 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Mcmahon Lake
Compare lakeside cabins, hotels, and vacation rentals near Mcmahon Lake in Scott 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. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Mcmahon Lake are listed below.
Top-rated places to stay near Mcmahon Lake
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Planning a whole trip? See where to stay at Minnesota’s top lakes →
Water Temperature
Mcmahon Lake typically sits around 82°F at the surface in August. Summer average is about 80°F. Usually warmest in Jul (about 83°F) and coldest in May (about 67°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 38 field readings (2010–2025) via the EPA Water Quality Portal. See the full Mcmahon Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 14 ft |
| Surface Area | 187 acres |
| Shoreline Length | 4.3 mi |
| Littoral Zone | 85% |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- McMahon LakePublic
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Mcmahon 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
9 surveys on file from MN DNR Fisheries. Most recent: 2025-06-23 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 81.66 | 0.17 lb |
| Black Crappie | 21.58 | 0.22 lb |
| Black Bullhead | 12.49 | 1.18 lb |
| Northern Pike | 10.64 | 1.67 lb |
| Yellow Perch | 9.71 | 0.13 lb |
| GOS | 4.78 | 0.16 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
Black Crappie
Black Bullhead
Northern Pike
From the 2025-06-23 survey
McMahon is a 187-acre (158 littoral acres) lake with a maximum depth of 14 feet. McMahon is located south of the City of Prior Lake in southeast Scott County. McMahon is primarily managed for Bluegill and Largemouth but includes catchable populations of Black Crappie, Northern Pike, and Walleye. Public water access…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Mcmahon Lake. 3 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — McMahonFisheries Lake Survey · MN DNR LakeFinder
- —Fish Stocking History — McMahonFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — McMahonPublic Water Access Sites · MN DNR LakeFinder
Water Quality Trend: ↓ Declining
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.105 m/yr | 3 |
| Phosphorus | ↓ Declining | +9.5 µg/L/yr | 2 |
| Chlorophyll-a | ↓ Declining | +23.5 µg/L/yr | 2 |
Ice Season
Historical ice data from Minnesota DNR (15 observations).
Estimated open water season: 261 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2017-03-07 (2017) | 2018-04-30 (2018) |
| Ice-In | 2025-12-01 (2025) | 2024-01-07 (2024) |
Most recent ice-out: 2026-03-26
EPA Impairment Status
Mcmahon 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 MN70-0050-00 · Official waterbody report
County Ranking
Ranked #18 of 21 lakes in Scott County
Nearby Lakes With Higher Grades
Mcmahon Lake holds Grade F. 3 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Scott County
Frequently asked questions about Mcmahon Lake
What is the water quality grade for Mcmahon Lake?
- Mcmahon Lake in Scott County, Minnesota has a water quality grade of F (Algae-prone). Water clarity: 2.3 ft visibility. Phosphorus: 71 µg/L. Chlorophyll-a (algae): 36 µg/L. Maximum depth: 14 ft. Surface area: 186.61 acres.
Is Mcmahon Lake safe to swim in?
- Algae and nutrients are often high — check current advisories before swimming. Skip the water if you see scum or a paint-like bloom. Mcmahon Lake has a water quality grade of F (Algae-prone). Algae concentration (chlorophyll-a) is 36 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Mcmahon Lake?
- Mcmahon Lake typically sits around 82°F at the surface in August, from 38 EPA Water Quality Portal field readings (2010–2025). That is a typical value for the month, not a live reading.
How deep is Mcmahon Lake?
- Mcmahon Lake has a maximum depth of 14 feet. The lake covers 186.61 acres.
What fish are in Mcmahon Lake?
- Fish species found in Mcmahon Lake include black bullhead, black crappie, bluegill, green sunfish, hybrid sunfish, largemouth bass, northern pike, walleye, yellow perch. Mcmahon Lake is located in Scott County, Minnesota.
When does Mcmahon Lake thaw?
- Mcmahon Lake typically has ice-out around Mar 25. Ice typically forms around Dec 11. The open water season is approximately 261 days.
Are there boat ramps on Mcmahon Lake?
- Mcmahon Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include McMahon Lake. Public access is listed as yes.
Explore Mcmahon Lake across LakeGrade
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Where this data comes from
87 measurements from 1 monitoring station, 2023–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA87 samples · 2023–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Colorimetry
- Total Phosphorus After Block Digestion
- Analysis of Chlorophyll in Periphyton and Plankton
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
Named boat-access points from the USGS National Boat Ramps inventory
Ice data from Minnesota DNR Climate