How Lake Grades Are Calculated
LakeGrade assigns every monitored lake in Minnesota and Wisconsin a letter grade from A (excellent) to F (very poor) based on three key water quality indicators. Our grading methodology follows the standards established by the Metropolitan Council in the 1980s, which are widely used across Minnesota for lake assessment.
Data Source
All water quality measurements come from the EPA Water Quality Portal, which aggregates monitoring data from:
- Minnesota Pollution Control Agency (MPCA)
- Wisconsin Department of Natural Resources
- U.S. Geological Survey (USGS)
- Various county and tribal monitoring programs
Three Grading Metrics
1. Water Clarity (Secchi Disk Depth)
Measured by lowering a black-and-white disk into the water until it disappears. Deeper visibility means clearer water.
| Grade | Secchi Depth | Meaning |
|---|---|---|
| A | > 4.5 meters | Crystal clear |
| B | 3.0 - 4.5 meters | Good clarity |
| C | 2.0 - 3.0 meters | Moderate clarity |
| D | 1.0 - 2.0 meters | Poor clarity |
| F | < 1.0 meter | Very murky |
2. Phosphorus
Phosphorus is the primary nutrient that drives algae growth in lakes. Higher phosphorus levels lead to more algae blooms, reduced clarity, and potential health risks.
| Grade | Phosphorus | Meaning |
|---|---|---|
| A | < 20 µg/L | Very low nutrients |
| B | 20 - 30 µg/L | Low nutrients |
| C | 30 - 60 µg/L | Moderate nutrients |
| D | 60 - 90 µg/L | High nutrients |
| F | > 90 µg/L | Very high nutrients |
3. Chlorophyll-a (Algae Concentration)
Chlorophyll-a directly measures the amount of algae in the water. High levels indicate algae blooms which can produce toxins, odors, and make the water unsafe for swimming.
| Grade | Chlorophyll-a | Meaning |
|---|---|---|
| A | < 5 µg/L | Minimal algae |
| B | 5 - 10 µg/L | Low algae |
| C | 10 - 20 µg/L | Moderate algae |
| D | 20 - 30 µg/L | High algae |
| F | > 30 µg/L | Very high algae |
Composite Grade
The overall lake grade is the average of available parameter grades, converted to a numeric scale (A=4, B=3, C=2, D=1, F=0) and rounded to the nearest whole letter grade.
Lakes with only one measured parameter receive a grade based on that single metric, marked with a "limited data" badge. Lakes with no recent measurements are excluded.
Seasonal Filtering
Grades are based on summer season (June through September) measurements when available, as this is the most ecologically relevant and comparable period. If no summer data exists, all available measurements are used.
Neighbor Comparisons
Each lake page shows the 8 geographically nearest lakes with their grades, calculated using Haversine distance between monitoring station coordinates. This helps contextualize a lake's grade relative to its regional peers rather than an abstract statewide ranking.
Limitations
- Shallow lakes naturally have higher phosphorus and lower clarity, so they may receive lower grades than deep lakes even when healthy for their type.
- Some lakes have been monitored more frequently or recently than others. The "limited data" badge indicates when a grade is based on fewer metrics.
- The EPA Water Quality Portal aggregates data from multiple agencies with different monitoring protocols, which may introduce some variability.