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.
Lake Grade is a term from limnology — the scientific study of inland waters. The detailed explanation below covers the definition, why the concept matters to lake water-quality interpretation, and how it intersects with the LakeGrade rubric. Reading Lake Grade alongside the LakeGrade rubric helps make the rubric's structure intuitive — each grading parameter maps directly onto an established limnological concept.
The per-lake pages on LakeQuality always show the specific Lake Grade value for that lake, so you can move from the general concept to the specific lake without leaving the site.
What It Means for Your Lake
The LakeQuality system assigns every monitored lake it tracks 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 on its own threshold: Secchi depth (A = greater than 15 feet, F = less than 3 feet), phosphorus (A = less than 15 ug/L, F = greater than 60 ug/L), and chlorophyll-a (A = less than 5 ug/L, F = greater than 30 ug/L). Each metric grade is converted to a 0-to-4 score and the three are averaged with equal weight to produce the overall letter. The Carlson Trophic State Index is reported alongside the grade as a holistic classification but is not part of the letter. 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
What is 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.
Why does lake grade matter for lake health?
The LakeQuality system assigns every monitored lake it tracks 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, w...
Related Terms
A measurement of water clarity determined by lowering a black-and-white disk into the water until it disappears from view.
A nutrient that fuels algae growth in lakes, measured as total phosphorus in micrograms per liter.
A green pigment found in all photosynthetic organisms, used as a direct measure of algae concentration in lake water.
The Carlson Trophic State Index (TSI) is a numerical scale from 0 to 100 that classifies lakes by their nutrient and productivity levels.
The systematic collection and analysis of water samples and measurements to track lake health over time and detect changes in water quality.
Source: EPA National Aquatic Resource Surveys, 2026.