Published April 7, 2026 · Updated annually
Lake Water Quality Grades Explained: What A-F Means
LakeGrade assigns every monitored lake in Minnesota and Wisconsin a letter grade from A (excellent) to F (very poor) based on three scientific measurements: secchi depth (water clarity), total phosphorus (nutrient load), and chlorophyll-a (algae concentration). These grades are derived from 3,770 lakes using EPA Water Quality Portal data and Metropolitan Council grading standards.
The Three Measurements Behind Every Grade
1. Secchi Depth (Water Clarity)
A Secchi disk is a black-and-white disk lowered into the water until it disappears from sight. The depth at which it vanishes measures water clarity. Higher secchi depth means clearer water.
| Grade | Secchi Depth (meters) | What It Looks Like |
|---|---|---|
| A | > 4.5 m (15+ ft) | Crystal clear — see the bottom in shallow areas |
| B | 3.0-4.5 m (10-15 ft) | Clear water — good visibility while swimming |
| C | 2.0-3.0 m (6.5-10 ft) | Moderate — some haze, feet visible while treading |
| D | 1.0-2.0 m (3-6.5 ft) | Murky — limited visibility, greenish tint |
| F | < 1.0 m (< 3 ft) | Opaque — cannot see below the surface |
2. Total Phosphorus
Phosphorus is the primary nutrient that drives algae growth in freshwater lakes. It enters lakes through agricultural runoff, stormwater, septic systems, and natural erosion. The EPA identifies phosphorus as the leading cause of lake water quality degradation.
| Grade | Phosphorus (ug/L) | Impact |
|---|---|---|
| A | < 20 | Very low nutrients — algae blooms unlikely |
| B | 20-30 | Low nutrients — occasional mild algae |
| C | 30-60 | Moderate nutrients — algae blooms in warm weather |
| D | 60-90 | High nutrients — frequent algae blooms |
| F | > 90 | Very high nutrients — persistent algae, potential toxins |
3. Chlorophyll-a
Chlorophyll-a is the green pigment in algae. Measuring it tells us exactly how much algae is in the water. Unlike phosphorus (which predicts future algae), chlorophyll-a measures current algae levels.
| Grade | Chlorophyll-a (ug/L) | Algae Level |
|---|---|---|
| A | < 5 | Minimal algae — water appears clear |
| B | 5-10 | Low algae — slight green tint possible |
| C | 10-20 | Moderate algae — visible green color |
| D | 20-30 | Heavy algae — green, soupy appearance |
| F | > 30 | Severe algae — pea soup, potential scum layer |
How the Overall Grade Is Calculated
Each of the three measurements receives its own letter grade. The overall grade is a weighted average converted to a 0-4 numeric scale (A=4, B=3, C=2, D=1, F=0), then converted back to a letter grade. This approach follows the Metropolitan Council grading standards established in the 1980s.
Lakes with limited data (fewer than 3 sampling events) are flagged as having limited data confidence. The grade still appears but should be interpreted with caution.
Grade Distribution Across All Lakes
| Grade | Rating | Lakes | Percentage |
|---|---|---|---|
| A | Excellent | 1,697 | 45.0% |
| B | Good | 776 | 20.6% |
| C | Fair | 581 | 15.4% |
| D | Poor | 406 | 10.8% |
| F | Very Poor | 310 | 8.2% |
| Total | 3,770 | 100% | |
Trophic State: Another Way to Understand Lake Health
Scientists also classify lakes by trophic state — a measure of biological productivity based on the Carlson Trophic State Index (TSI). LakeGrade calculates TSI for every lake with sufficient data.
- Oligotrophic — Low nutrients, clear water, excellent for swimming
- Mesotrophic — Moderate nutrients, good water quality
- Eutrophic — High nutrients, frequent algae, reduced clarity
- Hypereutrophic — Very high nutrients, dense algae, poor clarity
Frequently Asked Questions
A grade of C means fair water quality. The lake has moderate clarity (6.5-10 ft visibility), moderate phosphorus (30-60 ug/L), and moderate algae levels. It is generally safe for swimming but may experience algae blooms during warm summer months.
All data comes from the EPA Water Quality Portal, which aggregates monitoring data from state agencies (MN Pollution Control Agency, WI DNR), tribal nations, and federal programs. Measurements are taken by trained field scientists using standardized protocols.
Lake grades are updated annually when new monitoring data becomes available from the EPA Water Quality Portal. Most lakes are sampled during the summer months (June-September) when conditions best represent recreational use.
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