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. Clarity, phosphorus, and chlorophyll-a all rate similarly, so there is no obvious single lever to pull on watershed management.

Eutrophic conditions are the baseline here: clarity drops noticeably in late summer, and dissolved oxygen near the bottom can become a concern. Mcmahon Lake is a shallow lake at 14 ft maximum depth, which means it warms quickly, mixes constantly, and amplifies nutrient inputs. The lake's 187 acres and 4.3 miles of shoreline put it in the mid-range bucket — large enough for varied use, small enough that watershed inputs reach the whole basin. Within the 17 graded lakes of Scott County, Mcmahon Lake sits at rank 16, near the bottom of the county list.

Eurasian watermilfoil has been documented at Mcmahon Lake, requiring shoreline management to keep dense mats from crowding native vegetation. The fishery includes walleye alongside the lake's other 9 documented species — a notable draw for Minnesota anglers. A documented public access point at Mcmahon Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. Ice-out has been recorded 12 times at Mcmahon Lake, with a median around Mar 25. Monitoring depth is thin here: the LakeGrade rubric is applied to a small number of sample years, and the grade will be revised as more data accumulates.