Upper Nemahbin Lake

Upper Nemahbin Lake earns an A — water clarity, phosphorus, and chlorophyll-a all rate among the best monitored waters in Waukesha County. Sub-grades cluster within a single letter of each other, which usually means the lake is in stable trophic balance rather than fighting one specific stressor.

A TSI value of 32 puts Upper Nemahbin Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. At 60 ft of maximum depth, Upper Nemahbin Lake stratifies reliably through the summer — the hypolimnion stays cooler and clearer than the surface layer suggests. At 277 acres, Upper Nemahbin Lake fits the Wisconsin median for monitored lakes, with partial shoreline records. Within Waukesha County's 48 graded waters, Upper Nemahbin Lake sits at rank 10, near the top of the local distribution.

Zebra mussels have been documented at Upper Nemahbin Lake, which alters the filter-feeding balance and can paradoxically increase water clarity while disrupting the food web. Walleye are documented at Upper Nemahbin Lake, one of 5 fish species on record for the lake. Public access is available — the lake is on the Wisconsin DNR public-access list. 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.