Impaired Lakes in Wisconsin

Reading the Wisconsin impairment landscape

270 of Wisconsin's 1,353 tracked lakes carry impaired-waters listings — 20%. That's a meaningful minority worth checking before specific trips, particularly for swimming, fishing, or drinking-water source identification. Most Wisconsin lakes are unimpaired; the impaired set tends to concentrate in agricultural, post-industrial, or high-development watersheds rather than being randomly distributed.

The leading impairment category in Wisconsin is Nutrients (phosphorus, nitrogen) — nutrient enrichment, typically from agricultural fertilizer runoff, septic discharge, or stormwater carrying lawn-care residue. Nutrient-impaired lakes can still be safe for contact recreation in many conditions, but they're prone to algal blooms (including potentially toxic cyanobacteria) during warm-water periods. Check the state's beach-monitoring or harmful-algal-bloom dashboard before swimming on nutrient-impaired waters in summer.

Impaired-waters listings come from the state environmental agency's §303(d) submission to EPA, refreshed on a roughly 2-year cycle. A listed lake stays on the list until either a TMDL is established and the cleanup completes, or subsequent monitoring shows the pollutant concentrations no longer exceed water-quality standards. The list below reflects the most recent available Wisconsin submission to the EPA Assessment, Total Maximum Daily Load Tracking and Implementation System (ATTAINS) dataset.