About LakeQuality
How clean is your lake?
What we do
LakeQuality grades every lake it has data for on clarity, nutrients, and safety so swimmers and anglers know before they go. We use the same Carlson Trophic State Index that limnologists have used for fifty years, applied uniformly across thousands of lakes, and translated into a letter grade a cabin owner can read in five seconds.
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 multi-year EPA Water Quality Portal samples (Secchi depth, total phosphorus, chlorophyll-a, E. coli, nitrate) for every lake with at least three reporting years, compute a Carlson Trophic State Index from the three core parameters, and assign a letter grade based on Metropolitan Council water-quality thresholds adapted for letter-grade communication. Full weighting and cutoffs are documented 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
Lakes without sufficient monitoring drop out of the scoring rather than being graded on partial data — roughly two-thirds of U.S. lakes have gaps at that threshold. Sample timing matters: late-summer algal readings can dominate the annual index. Two lakes with identical letter grades can still differ meaningfully on a specific dimension (e.g. clarity vs. nutrient load), so the page for each lake breaks the grade down into its parts rather than presenting a single number.
Why letter grades instead of TSI numbers
Limnologists already have a perfectly good scoring system — the Carlson Trophic State Index. A TSI value tells you whether a lake is oligotrophic (clear, nutrient-poor) or eutrophic (productive, nutrient-rich). But a TSI of 47 versus 53 means nothing to someone deciding where to take their kids swimming this weekend.
We translate TSI to a letter grade because that’s how the people who actually use lakes think about lakes. A grade is unambiguous: an A-grade lake is one of the cleanest you can find, an F-grade lake has measurable, persistent water-quality problems that show up in the visible water. The TSI number is still on every page — we don’t hide it — but the letter is what catches your eye first.
The trade-off is real: a single letter compresses information that could otherwise be presented as three sub-grades. We address that by always showing the three components (clarity, phosphorus, chlorophyll-a) alongside the headline grade, so a lake with high phosphorus but excellent clarity reads differently from one with the inverse problem.
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.
Why we built this
The data has been public for years. The EPA Water Quality Portal aggregates millions of lake-sample records from every state, every tribal nation, and every federal water-quality agency. The Minnesota DNR LakeFinder and Wisconsin DNR Surface Water datasets cover thousands of lakes in those two states with detail no national portal touches. None of it is hidden.
It is also, until you have spent an afternoon with it, completely opaque. The data lives in three different APIs that return tab-separated files keyed on station codes you have to cross-reference against a separate metadata file. Field codes are documented in PDFs from 2008. The same lake is recorded under three different spellings depending on which agency sampled it. We built LakeQuality to do that cross-referencing once, score the result against a published rubric, and put a single page for each lake online so anyone can look at it without learning the field codes.
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. Hosting is paid for by advertising — see our Privacy Policy for details — and we do not take paid placements, sponsored rankings, or "remove-my-entry" fees.
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.