Wisconsin Lake Water Quality
Water quality grades for 754 monitored lakes across 71 counties.
About 39% of Michigan's lakes pull an A — 293 top-bracket waters. That high A-share comes mostly from the cold, deep, kettle-formed lakes in the north of the state, which buffer themselves against nutrient inputs.
Most Michigan lakes sit in the mesotrophic band — moderate productivity, decent clarity, capable of supporting both warm-water and cool-water fisheries. This is the band where well-managed working lakes tend to land. The cleanest county on average is Presque Isle (grade A); the most-stressed is Berrien. The gap between the top and bottom counties is large enough that "Michigan lake water quality" hides real local variation.
Lake Map
Reservoirs in Wisconsin
View all 62 reservoirs →62 of 754 Wisconsin lakes (8%) are man-made reservoirs in the USACE National Inventory of Dams. 25 carry a High or Significant hazard rating. The oldest matched dam dates to 1829.
EPA-Impaired Lakes in Wisconsin
View all 307 impaired lakes →307 of 754 Wisconsin lakes (41%) are officially listed as impaired under Clean Water Act §303(d) in the most recent EPA ATTAINS reporting cycle. 264 have an approved Total Maximum Daily Load (TMDL) restoration plan.
Counties
Top 25 Cleanest Lakes
25 Lakes Needing Attention
Source: EPA National Aquatic Resource Surveys, 2026.