Upper Nemahbin Lake

On the LakeGrade scale Upper Nemahbin Lake grades a B, with clarity at 12.0 ft and phosphorus readings still being added placing it above the Wisconsin median. The three sub-grades — clarity, phosphorus, and chlorophyll-a — track close together, so no single parameter is dragging the average.

A TSI near 41 places Upper Nemahbin Lake in the mesotrophic band — moderate productivity, with seasonal swings between clearer spring water and richer late-summer conditions. 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. The lake's 277 acres and partial shoreline records put it in the mid-range bucket — large enough for varied use, small enough that watershed inputs reach the whole basin. Upper Nemahbin Lake sits at rank 29 of 45 in Waukesha County, in the lower half 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. The fishery includes walleye alongside the lake's other 5 documented species — a notable draw for Wisconsin anglers. A documented public access point at Upper Nemahbin Lake makes the lake usable for shore fishing, paddle craft, and trailered boats. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.