Water Quality

Lake Grade

An A-through-F letter grade assigned by LakeQuality to summarize overall lake water quality based on clarity, phosphorus, and chlorophyll-a measurements.

What It Means for Your Lake

The LakeQuality system assigns every monitored lake in Minnesota and Wisconsin a letter grade from A (excellent) to F (poor) based on three core water quality metrics: Secchi depth (water clarity), total phosphorus, and chlorophyll-a. The grading methodology uses thresholds established by the Metropolitan Council, which has monitored Twin Cities area lakes since the early 1980s and developed well-validated standards for Upper Midwest lake assessment. Each metric is graded independently: Secchi depth is weighted at 35% of the overall grade (A = greater than 15 feet, F = less than 3 feet), phosphorus at 30% (A = less than 15 ug/L, F = greater than 60 ug/L), and chlorophyll-a at 20% (A = less than 5 ug/L, F = greater than 30 ug/L). The remaining 15% is derived from the Carlson Trophic State Index, which provides a holistic assessment based on all three metrics. Grades are calculated from median summer-season (June through September) values using the most recent five years of monitoring data from the EPA Water Quality Portal, which aggregates results from the Minnesota Pollution Control Agency, Wisconsin DNR, USGS, and other monitoring programs. Lakes with fewer than three data points receive a "limited data" flag. The letter grade system deliberately simplifies complex water quality data into an intuitive format that lake visitors, property owners, and community members can immediately understand without technical training. An A grade does not guarantee perfect conditions on any given day, it indicates that the lake is consistently in excellent condition based on multi-year monitoring data.

Frequently Asked Questions