Mcmahon Lake
Scott County, Minnesota187 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Mcmahon Lake. Here is what has been measured.
- Eutrophic
- Impaired · Mercury
- Eurasian watermilfoil
- 14 ft max depth
- Public access
Mcmahon Lake in Scott County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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.
Minnesota DNR's Standard Survey on June 23, 2025 caught 5.40 walleye per gill net at Mcmahon Lake. DNR survey details →
Mcmahon Lake is a 187-acre productive, nutrient-rich eutrophic lake in Scott County, Minnesota with a maximum depth of 14 feet. 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. Methodology: how these readings are compared.
Mcmahon Lake covers about 190 acres in Scott County. It has one public boat landing. The median ice-out date is March 25, which is when the open-water season begins. Vrbo is where to look for cabins and lake homes around Mcmahon Lake, and Booking.com and Expedia search hotels near the lake for your dates.
What Mcmahon Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
- Fishing9 species recorded
- Ice seasonIce-out around Mar 25
Swimming conditions
Readings onlyAlgae risk
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. Mcmahon Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
Whether Mcmahon 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 Mcmahon Lake
These are the measurements Minnesota PCA publishes for Mcmahon Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 2.3 ft of visibility. Phosphorus 71 µg/L. Chlorophyll-a 36 µg/L. TSI 65.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 2.3ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 71µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 36µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 65 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Mcmahon Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Mcmahon Lake typically sits around 72°F at the surface in September. 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.
Column height runs from this lake’s coldest month (67°F) to its warmest (83°F).
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 →
Minnesota DNR's Standard Survey on June 23, 2025 caught 5.40 walleye per gill net at Mcmahon Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Mcmahon Lake’s water-quality measurements separately, and the water-quality trend over 3 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
9 surveys on file from MN DNR Fisheries. Most recent: 2025-06-23 (Standard Survey).
9 surveys on file · most recent 2025-06-23 (Standard Survey)
| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| Bluegill | 81.66 | 3–9 in 2025-06-23 | 0.17 lb |
| Black Crappie | 21.58 | 4–10 in 2025-06-23 | 0.22 lb |
| Black Bullhead | 12.49 | 10–17 in 2025-06-23 | 1.18 lb |
| Northern Pike | 10.64 | 15–35 in 2025-06-23 | 1.67 lb |
| Yellow Perch | 9.71 | 4–9 in 2025-06-23 | 0.13 lb |
| Golden Shiner | 4.78 | 5–7 in 2025-06-23 | 0.16 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
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
Lake Details
Max depth
14 ft
Surface area
187 acres
Shoreline
4.3 mi
Littoral zone
85%
Share shallower than 15 ft
Public access
Yes
Listed by Minnesota PCA
Lake Size & Watershed
Watershed (drainage) area
10 sq mi
HydroLAKES / HydroSHEDS
Elevation
958 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- McMahon LakePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays 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 readings cover nutrients and algae, not lodging. Cabins and rentals around Mcmahon Lake are listed below.
Top-rated places to stay near Mcmahon 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 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.
Water quality readings over time
3 seasons of samples on record (2023–2025). Calling a direction takes at least 4 years of the same measurement.
Water clarity
Worsening3 years
Phosphorus
Worsening+9.5 µg/L/yr2 years
Chlorophyll-a
Worsening+23.5 µg/L/yr2 years
Ice season
Typical ice-out
Mar 25
Earliest Mar 7, 2017 · Latest Apr 30, 2018
Typical ice-in
Dec 11
Earliest Dec 1, 2025 · Latest Jan 7, 2024
Most recent ice-out
Mar 26
2026
15 observations · Minnesota PCA ice records
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
Designated uses not supporting: Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN70-0050-00 · Official waterbody report
Nearby Lakes in Scott County
Frequently asked questions about Mcmahon Lake
Where is Mcmahon Lake?
- Mcmahon Lake is a lake in Scott County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Mcmahon Lake?
- Mcmahon Lake in Scott County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. Mcmahon 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 Mcmahon Lake?
- Mcmahon Lake typically sits around 72°F at the surface in September, 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.
What did the DNR fisheries survey find at Mcmahon Lake?
- Minnesota DNR's Standard Survey on June 23, 2025 caught 5.40 walleye per gill net at Mcmahon Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Mcmahon Lake’s water-quality measurements separately, and the water-quality trend over 3 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
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.
Can I fish at Mcmahon Lake?
- Mcmahon 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 Mcmahon Lake across LakeQuality
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Fish species and best lakes in Scott County
Species activity outlook by water temperature and season
Where to stay at Minnesota's top lakes, each with its readings
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Minnesota's cleanest lakes, ranked on agency measurements
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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)
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