Green Lake

Green Lake comes in at a B on the grading rubric — a respectable showing for a Wisconsin lake of its size and depth. The two scored sub-grades — clarity and phosphorus — track close together, so neither one is dragging the average down.

Mesotrophic conditions dominate at Green Lake: enough nutrients for a healthy fishery, not so much that algal blooms become a chronic problem. At 236 ft of maximum depth, Green Lake stratifies reliably through the summer — the hypolimnion stays cooler and clearer than the surface layer suggests. Green Lake covers 7,920 acres alongside partial shoreline records, large enough that conditions can vary meaningfully between bays. Among the 13 graded lakes in Green Lake County, Green Lake ranks 1 — in the top quartile locally.

Zebra mussels have been documented at Green Lake, which alters the filter-feeding balance and can paradoxically increase water clarity while disrupting the food web. Walleye are documented at Green Lake, one of 7 fish species on record for the lake. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.