Lake Butte des Morts

Lake Butte des Morts earns an F: phosphorus readings still being added and clarity at 2.3 ft signal heavy nutrient loading from the surrounding watershed. 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.

At a TSI of 65, Lake Butte des Morts reads as eutrophic — nutrient-rich enough that summer algal growth and reduced clarity are expected, not unusual. A maximum depth of just 9 ft means sediment-bound phosphorus releases back into the water column whenever wind stirs the bottom — a classic shallow-lake dynamic. At 8,581 acres, Lake Butte des Morts is one of the larger lakes in Winnebago County, with partial shoreline records supporting multiple distinct use patterns. Within Winnebago County's 5 graded lakes, Lake Butte des Morts ranks 3 — below the local median, though not at the bottom.

Zebra mussels have been documented at Lake Butte des Morts, 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 8 documented species — a notable draw for Wisconsin anglers. Public access is available — the lake is on the Wisconsin DNR public-access list. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.