Updated April 2026 · EPA Water Quality Portal & WI DNR
How Lake Grades Are Calculated
LakeQuality 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, and is consistent with Wisconsin DNR water quality classification. Every reading is sourced from the public EPA Water Quality Portal, supplemented with WI DNR Fisheries survey reports for fish-community context.
Why a Letter Grade?
Lake water quality reporting in the limnology literature is quantitative — Secchi disk readings in meters, phosphorus and chlorophyll-a in micrograms per liter, trophic state in Carlson TSI units. Those measurements are precise, but they are not the way a curious homeowner or prospective buyer thinks about a lake. A letter grade is a familiar shorthand that lets you compare a 32-acre lake in Itasca County to a 4,800-acre lake in Vilas County without doing unit conversions. The thresholds underneath are still the same numeric cutoffs Metropolitan Council and MPCA have used for decades; the grade is just a UI affordance.
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)
- EPA STORET legacy records
- Various county and tribal monitoring programs
Lake physical characteristics (depth, surface area, shoreline length, fish species, invasive species, and public access) come from the Minnesota DNR LakeFinder for Minnesota lakes. Fish community and fisheries-management context for Wisconsin lakes comes directly from the WI DNR Fisheries Survey report archive.
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. All clarity measurements are displayed in feet.
| Grade | Secchi Depth | Meaning |
|---|---|---|
| A | > 14.8 ft (4.5 m) | Crystal clear |
| B | 9.8 - 14.8 ft (3.0 - 4.5 m) | Good clarity |
| C | 6.6 - 9.8 ft (2.0 - 3.0 m) | Moderate clarity |
| D | 3.3 - 6.6 ft (1.0 - 2.0 m) | Poor clarity |
| F | < 3.3 ft (1.0 m) | 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.
Trophic State Index (TSI)
In addition to letter grades, LakeQuality calculates the Carlson Trophic State Index for each lake. TSI classifies lakes into four categories based on nutrient productivity:
| TSI Range | Classification | Meaning |
|---|---|---|
| < 40 | Oligotrophic | Low nutrients, clear water, excellent for swimming |
| 40 - 50 | Mesotrophic | Moderate nutrients, good water quality |
| 50 - 70 | Eutrophic | High nutrients, frequent algae, reduced clarity |
| > 70 | Hypereutrophic | Very high nutrients, dense algae, poor clarity |
TSI is computed from three formulas developed by Robert Carlson (1977) using Secchi depth, phosphorus, and chlorophyll-a. See our Trophic State Guide for the full formulas and interpretation.
Lake Physical Data
For Minnesota lakes, LakeQuality enriches water quality data with physical characteristics from the Minnesota DNR LakeFinder, including:
- Maximum and average depth, in feet
- Surface area, in acres
- Shoreline length, in miles
- Fish species, from DNR survey data
- Invasive species, DNR-reported infestations
- Public access, whether a public boat launch is available
All measurements are displayed in US customary units (feet, miles, acres).
Wisconsin DNR Fisheries Surveys
For Wisconsin lakes, LakeQuality integrates 681 WI DNR Fisheries Survey reports drawn directly from the Wisconsin DNR fisheries portal. Each report adds fish community composition, walleye stocking history, year-class strength estimates, and habitat notes — context that ties directly back to the same nutrient and habitat conditions driving the water quality grade.
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, measured in miles using Haversine distance between monitoring station coordinates. This helps contextualize a lake's grade relative to its regional peers.
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.
- Lake physical data (depth, area, fish) is currently available for Minnesota lakes only. Wisconsin lake details are added as data sources become available.
- Day-of swim safety is not captured by long-term grades. Always check current advisories from the Minnesota Department of Health and Wisconsin DNR before swimming, especially with young children, pregnant family members, or pets.
Data Refresh
EPA Water Quality Portal records refresh as state agencies upload new monitoring data — most commonly on a 6–12 month cycle. LakeQuality re-runs every grade and ranking against the new file when each refresh ships. The current dataset was last refreshed April 2026.
Frequently Asked Questions
Why grade lakes A through F?
Letter grades are an accessible shorthand for a quantitative threshold system that already exists in the limnology literature. Metropolitan Council Lake Water Quality Standards in Minnesota set numeric cutoffs on Secchi depth, phosphorus, and chlorophyll-a; LakeQuality maps those to letter grades so a homeowner, a lake association volunteer, or a prospective buyer can compare lakes at a glance without doing a unit-conversion exercise.
Why do shallow lakes often get lower grades than deep lakes?
Shallow lakes have less water volume to dilute incoming phosphorus, more wind-driven sediment resuspension that lowers clarity, and warmer summer temperatures that accelerate algae growth. As a result, shallow lakes naturally trend toward higher phosphorus and chlorophyll-a readings even with strong stewardship. The grading thresholds do not adjust for depth, so a shallow lake at "natural" levels for its type may still grade lower than a deep lake. The "Limitations" section flags this explicitly.
Are summer measurements really representative?
For Midwestern lakes, summer-season measurements (June–September) are the most ecologically relevant and most comparable across years and lakes — they capture the period when algae growth, swimming use, and recreational impact are highest. Spring readings tend to be cleaner because the lake has not yet warmed and stratified; fall readings reflect post-stratification mixing. The Metropolitan Council, MPCA, and WI DNR all use summer-period thresholds for the same reason.
What if a lake only has one parameter measured?
A lake with only one of the three water quality parameters measured receives a grade based on that single metric, marked with a "limited data" badge. This is most common for smaller or less-frequently-monitored lakes where the EPA Water Quality Portal record is sparse. Lakes with no recent measurements at all are excluded from the dataset rather than grade-substituted.
Where does the underlying data come from?
Every measurement on LakeQuality comes from the EPA Water Quality Portal (waterqualitydata.us), which aggregates monitoring records from the Minnesota Pollution Control Agency, Wisconsin Department of Natural Resources, U.S. Geological Survey, and various county and tribal monitoring programs. Lake physical attributes come from the Minnesota DNR LakeFinder. WI DNR Fisheries Survey reports (681 reports integrated) come directly from the Wisconsin DNR fisheries portal. Last refreshed April 2026.
Sources: EPA Water Quality Portal (waterqualitydata.us), Minnesota Pollution Control Agency, Wisconsin DNR Fisheries, U.S. Geological Survey, EPA STORET legacy database, Minnesota DNR LakeFinder. Robert Carlson, "A trophic state index for lakes," Limnology and Oceanography, 1977. All data is government public domain. Cite as: "LakeQuality, April 2026 reading. Data: EPA WQP & WI DNR."
Last updated 2026-04-08 · 4,564 lakes graded.