About LakeQuality
We grade the water, not the view.
What we do
A–F letters are US. Ontario and Québec are listed with 0 letters. 38 country hubs publish official ratings that are not LakeQuality A–F grades. LakeQuality grades lakes on nutrients and algae so swimmers and anglers know what the monitoring record actually says. Each lake is measured against the least-disturbed reference lakes of its own EPA ecoregion, because what counts as clean water in the Nebraska Sand Hills is not what counts as clean water in the Sierra Nevada. Where the evidence will not support a letter, we publish the data and say so rather than guess.
We focus on U.S. lake and freshwater quality. Every page on lakequality.org is built from the EPA Water Quality Portal, the Minnesota DNR LakeFinder, and the Wisconsin DNR Surface Water dataset, cited and linkable so readers can trace any number back to its source.
Who runs this
LakeQuality is built and maintained by the Lake Quality Team. We're a small group working on making public U.S. lake and freshwater quality data easier for non-specialists to read. If you have a correction, a data tip, or a question about how a number was derived, the contact email below reaches us directly.
Who this is for
LakeQuality is built for swimmers, anglers, cabin owners, lake-association members, public-health staff, and state environmental agencies.
Why this exists
Public data on U.S. lake and freshwater quality is technically free, but practically locked behind file formats, acronyms, and paywalled dashboards. LakeQualityexists to close that gap: take the raw federal and public-sector data, and turn it into pages a normal person can read in thirty seconds.
How we work
- Primary source only. We pull from the EPA Water Quality Portal, the Minnesota DNR LakeFinder, and the Wisconsin DNR Surface Water dataset and cite the exact dataset and version on every page.
- No invented numbers. If a figure is not in the underlying public data, it does not appear on lakequality.org. We never generate synthetic statistics to fill gaps.
- Methodology, in plain English. We ingest EPA Water Quality Portal samples from the June-September index period — chlorophyll-a, total phosphorus, total nitrogen, turbidity, true colour and Secchi depth — and grade each lake against the condition thresholds EPA publishes for its ecoregion in the National Lakes Assessment 2022 Technical Support Document (EPA 841-R-24-006, Table 6-3). Those thresholds are the 75th and 95th percentiles of least-disturbed reference lakes within each ecoregion, so a lake is compared with its own regional peers rather than a national average. Chlorophyll-a is the primary indicator because it measures algal biomass directly; phosphorus and nitrogen are the causal drivers. Water clarity is NOT graded — EPA assigns no condition class to Secchi depth, and low clarity is either redundant with chlorophyll-a or caused by natural dissolved colour or suspended sediment. We report clarity and explain what is driving it. Full thresholds, the composite rule, and the sampling a grade requires are all on our methodology page.
- Refreshed on a schedule. Refreshed every four months as new state-agency samples are pushed to the EPA Water Quality Portal. Ice-out observations, fish-species records, and invasive-species listings refresh on the cadence each state’s DNR publishes them — typically annually for ice data and quarterly for invasive-waters updates.
- Corrections welcome. Readers flag issues all the time. When the source fixes a record, LakeQuality follows.
Known limitations
A lake only receives a letter when the monitoring record supports one. Where it does not, we publish the measurements with an explicit reason and no grade — this is EPA Integrated Reporting Category 3, insufficient information, and it is not a failing grade. Every grade carries a confidence level; only high-confidence grades are used in rankings and comparisons. The grade covers nutrients and algae. It does NOT cover mercury, PFAS, bacteria or other contaminants, so it is not a swimming or fish-consumption clearance — check your state environmental and health agencies for current advisories. Ecoregion boundaries are a classification, not a wall: two lakes with similar chemistry on opposite sides of one can receive different letters. Sample timing matters, and depth figures marked as estimates are model output rather than survey measurements.
Why letter grades, and why clarity is not one of them
A letter is unambiguous in a way a number is not. Someone deciding where to take their kids swimming does not want to weigh a Carlson TSI of 47 against 53 — they want to know whether this lake is in good shape for its region.
The grade is built from EPA National Lakes Assessment condition thresholds, which are set per ecoregion from the 75th and 95th percentiles of least-disturbed reference lakes. That regional framing is the point: reference phosphorus varies roughly thirtyfold across US ecoregions, and the naturally hypereutrophic lakes of the Nebraska Sand Hills would fail any national scale while being exactly as EPA expects them to be.
Water clarity is deliberately NOT part of the grade. EPA assigns condition classes to phosphorus, nitrogen, chlorophyll-a and turbidity, and none to Secchi depth — because low clarity is either redundant with chlorophyll-a (the water is dark because of algae) or misleading (it is dark because of natural dissolved colour, or because clay is suspended in it). Grading clarity means failing bog lakes and clay-bottomed reservoirs for being what they naturally are. We show clarity on every page and name what is driving it instead.
The trade-off of one letter is real, so each page also shows the per-parameter condition classes behind it, plus how many samples over how many years support the grade — and where the record does not support a letter, no letter is shown.
What this data can and cannot tell you
A lake grade is a synthesis of a few specific measurements taken on a few specific days in a specific season. It is good for the things it is good for and silent on the things it is silent on.
What the grade does well: it identifies which lakes in your county or state have persistent water-clarity, nutrient-loading, or algal-growth problems versus which are running clean. If you are choosing a cabin to rent, a lake to fish, or a beach to swim at, the grade is a defensible starting point.
What the grade does not capture: short-term blooms that happen between sample dates, contamination events from upstream sources that flushed before our last sample window, fish-consumption advisories tied to mercury or PFAS (those live on state “safe to eat” pages and are surfaced separately on our site), beach-specific E. coli spikes (those are tracked in real-time by BEACON dashboards, not annual sampling), and invasive-species impacts beyond the binary “documented” flag.
If you are making a decision where any of those matter — a swim with small kids during a hot week, a fishing trip in a county with a recent fish-advisory update, or a property purchase — the lake grade is one input, not the entire picture. We link out to the upstream state and federal sources on every page so you can do that follow-up research without us in the loop.
How a public record becomes a lake page
The measurements are public. EPA's Water Quality Portal holds samples from state, tribal, and federal agencies. Minnesota DNR LakeFinder and Wisconsin DNR Surface Water add the detail those two states keep in their own systems. A single lake can appear under different spellings in each source, and the raw files are keyed on station codes rather than lake names. Each LakeQuality page matches those records to one lake and scores the result against the published EPA rubric, so you can read the grade without assembling the files yourself.
Independence
LakeQuality is an independent publication. We are not funded, owned, or directed by any of the agencies, companies, or organizations that appear in our data. The site is free to read and is supported by display advertising; advertising is served by third parties and is kept separate from our editorial and data decisions — see our Privacy Policy for how data and advertising cookies are handled. and we do not take paid placements, sponsored rankings, or “remove-my-entry” fees.
Editorial standards
We hold every page to a stated, repeatable process rather than asking readers to take our word for it. How our content is produced and the standards behind it are documented in our Editorial Policy; how we verify figures before publishing is in our Fact-Checking Policy; and how we handle mistakes is in our Corrections Policy.
History
LakeQuality launched in 2025 as part of a small portfolio of independent public-data sites. It has been maintained and updated continuously since.
Contact
Tips, corrections, data-partnership questions, and press inquiries: hello@lakequality.org. More options on our contact page.