Mcmahon Lake

On the LakeGrade rubric Mcmahon Lake fails the cutoffs — a TSI of 61 and persistent algal pressure put it in the bottom tier.

A TSI in the eutrophic range signals a lake doing what nutrient-loaded waters do — supporting an active algal community at the cost of clear water. Mcmahon Lake is a shallow lake at 14 ft maximum depth, which means it warms quickly, mixes constantly, and amplifies nutrient inputs. At 187 acres, Mcmahon Lake fits the Minnesota median for monitored lakes, with 4.3 miles of shoreline. Within the 17 graded lakes of Scott County, Mcmahon Lake sits at rank 16, near the bottom of the county list.

Mcmahon Lake carries an invasive watermilfoil record — boat-launch decontamination and shoreline care matter more here than at uninfested lakes. The fishery includes walleye among the lake's 9 documented species — a notable draw for Minnesota anglers. Public access is available — the lake is on the Minnesota PCA public-access list. The lake has a partial ice record — 12 observed ice-outs, centered near Mar 25. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.