How Lake Water Quality Readings Are Shown

What Is Measured, and What Is Not

Four measured quantities carry the water-quality story on a US lake page: total phosphorus, total nitrogen, chlorophyll-a and turbidity. Those describe how much nutrient a lake carries and how much algae it grows. Beside each reading the page shows the EPA condition class that value falls in for the lake's ecoregion: Good, Fair or Poor. Water clarity (Secchi depth) is shown as a fifth measurement, in feet, with no class.

None of these readings measures bacteria, cyanotoxins, mercury or PFAS. They are not a swimming clearance and they are not a fish-consumption clearance. A lake can have textbook nutrient chemistry and still be listed under the Clean Water Act for mercury in fish tissue, for PFAS, or for E. coli. Where a lake carries a 303(d) impairment listing in EPA ATTAINS, the lake page shows it alongside the readings, precisely so the two can be read together.

Whether a lake is safe to swim in today, or its fish safe to eat, is a call for the state water agency and the county health department. They post beach closures, algae advisories and fish-consumption guidance; every lake and state page links to them.

Reference Condition, by Ecoregion

A phosphorus concentration that would be alarming in a Rocky Mountain lake is unremarkable in a Nebraska Sand Hills lake, which was hypereutrophic before anyone farmed near it. Reading both against one number would call the second lake degraded for being what it has always been. EPA solves this by setting condition classes separately in each ecoregion, from the distribution of its own least-disturbed reference lakes. Quoting the Technical Support Document directly:

“The cutoff between good and fair condition class was set at the 75th percentile (Q3) of reference lakes, and the cutoff between fair and poor condition class was set at the 95th percentile (P95) of reference lakes.”

So a value at or below the Q3 cutpoint is Good, above it and at or below P95 is Fair, and anything higher is Poor. These are percentiles of reference lakes, not percentiles of the lakes on this site, so the class is criterion-referenced and never set on a curve. A year in which every lake improves is a year in which every class can improve.

The spread across ecoregions is the reason this matters. The Good/Fair cutpoint for total phosphorus runs from 16.0 µg/L in the Northern Appalachians to 486 µg/L in naturally hypereutrophic Southern Plains lakes, a thirty-fold range. The Western Mountains sit at 23.4 µg/L.

The cutpoint table

Each cell reads Good/Fair cutpoint / Fair/Poor cutpoint. Phosphorus, nitrogen and chlorophyll-a are in µg/L; turbidity is in NTU.

Source: U.S. EPA (2024), National Lakes Assessment 2022 Technical Support Document, EPA 841-R-24-006, Table 6-3, p. 88. The identical values appear as Table 6-3, p. 83 of the NLA 2017 TSD.

There are ten rows for nine ecoregions because EPA splits the Southern Plains by lake origin. The Nebraska Sand Hills natural lakes carry a reference phosphorus roughly sixteen times the man-made Southern Plains value. That split is EPA's own published evidence that naturally eutrophic lakes exist and must not be read against a man-made scale.

How a lake gets its ecoregion

Each lake's coordinate is run point-in-polygon against EPA's Omernik Level III ecoregion boundaries, then crosswalked to the nine NLA aggregated ecoregions using EPA's own NLA 2017 site information file, which publishes both codes for every sampled lake. Across 5,721 NLA site records, 84 Level III codes map to the 9 aggregated regions with zero ambiguity.

The assignment is checked against EPA's published answer rather than assumed: running EPA's own NLA coordinates back through the same procedure, 5,600 of 5,600 points agree (100.00%), with 24 points (0.43%) falling outside every polygon, the Great Lakes and coastal-bay pattern.

One entry is inferred rather than EPA-published, and is labelled as such: Level III code 49, the Northern Minnesota Wetlands, was never sampled by any NLA cycle. It is assigned to Upper Midwest on two grounds. It shares 470.5 km of border with Northern Lakes and Forests (Upper Midwest) against 329.1 km with the Lake Agassiz Plain (Temperate Plains), and its peatland lakes have Upper Midwest analogues while agricultural Temperate Plains reference conditions would be badly wrong for them.

Why Water Clarity Is Shown, Not Classified

EPA's National Lakes Assessment assigns condition classes to phosphorus, nitrogen, chlorophyll-a and turbidity, and none to Secchi depth. LakeQuality follows that, because a clarity reading on its own does not tell you whether a lake is polluted.

Dark water has three quite different causes, and only one of them is a water-quality problem:

• Algae. The water is green and opaque because it is full of phytoplankton. This is a real problem, and it is already captured by chlorophyll-a, which measures the algae directly. Classifying clarity too would count the same fact twice.
• Natural dissolved colour. Tannins leached from peat and conifer forest stain bog lakes brown. A Boundary Waters lake can be tea-coloured, nutrient-poor and pristine.
• Suspended sediment. Wind-resuspended clay or a silty inflow scatters light. A reservoir on clay soils can be permanently turbid with almost no algae in it.

A clarity reading cannot tell those three apart, so bog lakes and clay reservoirs would be marked down for their geology. That is why clarity carries no class here.

How clarity is read: the clarity driver

Clarity is still measured and still displayed in feet on every lake page. Behind the readings, where the record allows, each lake is classified by what is limiting its clarity, and that classification decides how its phosphorus reading is treated in the rankings below:

The 10-unit test comes from EPA's Nutrient Criteria Technical Guidance Manual, p. 3-9: “In turbid lakes, it is common to see a close relationship between the total phosphorus TSI and the Secchi depth TSI, while the chlorophyll index falls 10 or 20 units below the others.” See the Secchi depth guide for how the measurement itself works.

Why Phosphorus Can Mislead in Clay-Turbid Lakes

A clay-turbid reservoir can carry a Poor phosphorus class without growing much algae, because phosphorus binds to clay. EPA's Nutrient Criteria Technical Guidance Manual (Table 3.2, p. 3-10, after Carlson 1983/1992) states it plainly:

“Clay particles contain phosphorus, and therefore, lakes with heavy clay turbidity will have the phosphorus correlated with the clay turbidity while the algae may neither utilize all the phosphorus nor contribute significantly to the light attenuation.”

So when a lake is classified sediment-driven, its phosphorus reading is still shown, but it is not used to order the best lists. Chlorophyll-a, the algae the lake actually grows rather than the phosphorus riding on its clay, is the reading those lists use instead. If such a lake has no usable chlorophyll-a measurement, it stays out of the rankings rather than being placed on a number EPA's own guidance calls untrustworthy.

How Much Sampling a Summary Needs

A measurement has to clear three tests before it can stand for a lake's typical summer.

• Index period. Only June through September samples are used, matching the National Lakes Assessment's operative window; EPA's Consolidated Assessment and Listing Methodology is explicit that “only results from similar index periods [should] be compared”. There is no all-season fallback: winter or under-ice readings measure a different lake than the one people swim in, and are not folded into the summer medians.
• Minimum samples. A parameter needs at least 3 summer samples before a trophic class is stated from it. Every state assessment methodology we reviewed sets a floor of 4 to 10 samples over 2 to 3 years, and EPA CALM notes that good statistical power needs roughly 30. Three sits below the lowest adopted state floor, which is why every lake page also shows the sample counts and date range behind its readings.
• Recency. The most recent sample must fall inside a 10-year window. EPA CALM Table 4-2 places data more than 5 years old, “not reflective of current conditions”, in its lowest rigour tier.

What enters a ranking

The best lists (cleanest, clearest, best for swimming) are ordered on the agency readings themselves: Secchi clarity, total phosphorus and chlorophyll-a. A lake enters one only when at least one nutrient or chlorophyll-a parameter has 8 or more samples across 2 or more distinct years, with the most recent within 5 years, at or above the state assessment floors. A lake sampled less often than that still shows its readings on its own page, next to its sample counts, where they can be read for what they are; it is not allowed to carry a comparative claim against another lake.

Lakes With No Nutrient or Algae Reading on Record

Most lakes on this site have never been sampled for nutrients or algae, or not recently enough to count. Their pages say so in plain language and show what the record does hold. That matches EPA Integrated Reporting Category 3, “insufficient available data and/or information to make a use support determination”. It is a statement about the monitoring record, not a finding about the lake.

The reasons a page can give are:

• No nutrient or algae measurement clears the three tests above: fewer than 3 summer samples, or nothing inside the 10-year window.
• The only measurement available is water clarity, which is not a water-quality indicator on its own.
• Clarity and phosphorus are both driven by suspended sediment, and there is no chlorophyll-a to read instead.
• The lake falls outside every EPA Level III ecoregion polygon, so the NLA condition classes cannot be applied. These are chiefly Great Lakes and coastal-bay coordinates, and waters outside the coterminous United States.

Those lakes keep their pages. Name, location, depth, fish, ice-out, impairment status, boat access and neighbouring lakes are all still there.

Trophic State

Where the sampling floor is met, each lake carries a trophic-state classification. It comes from chlorophyll-a alone, using EPA's national bands, rather than from a Carlson index averaged across Secchi:

The classic Carlson Trophic State Index is still computed and displayed as a measurement, for continuity and for the clarity-driver test. See the trophic state guide for the formulas.

Canada and Other Countries

The EPA reference framework stops at the US border, so no EPA condition class is shown for a lake outside it. Ontario and Québec lakes carry the province's own measurements, attributed to the provincial programme that took them. Where an agency publishes a class of its own, that class is shown as the agency's: Québec's volunteer lake-monitoring network (RSVL) publishes a trophic class for the lakes it samples, and the European Environment Agency publishes a bathing-water class (Excellent, Good, Sufficient, Poor) built on bacteria counts rather than nutrients. Named lakes in other countries show the readings their national or regional agency published, with that agency named on the page. None of these are converted onto any other scale.

Provenance: Who Measured What

Every lake page names the organizations that actually submitted its samples, with their station identifiers, analytical methods, sample counts and date range, taken from the submitting-organization field the Water Quality Portal records on every row.

When provenance is missing, the page says so. It does not fall back to a plausible-sounding agency name. An earlier version filled the gap with a template that appended “DNR” to the state name, which cited agencies that do not exist: Texas's water agency is the Texas Commission on Environmental Quality, and Oklahoma's is the Department of Environmental Quality. “Source not recorded” is a true statement; a confident wrong attribution is not.

Data Sources

Licence, publisher, and attribution for every dataset are on the data sources page. US in-situ measurements come from the EPA Water Quality Portal, which aggregates monitoring data from:

• State environmental and natural-resource agencies across 49 US states, including the Minnesota Pollution Control Agency and the Wisconsin Department of Natural Resources
• U.S. Geological Survey
• EPA STORET legacy records
• County, tribal and volunteer monitoring programs

Condition classes for US lakes come from the EPA National Lakes Assessment; ecoregion boundaries from EPA Omernik Level III; impairment listings from EPA ATTAINS; lake morphometry from HydroLAKES and state DNR sources; invasive-species listings from the USGS Nonindigenous Aquatic Species database. Satellite-product caveats are on the data sources page.

Other Data on a Lake Page

Fisheries context comes from state survey archives: 5,243 DNR fisheries reports and resources indexed across Minnesota and Wisconsin, drawn from the Wisconsin DNR fisheries portal and its Minnesota counterpart. These add fish-community composition, stocking history and habitat notes. They are context beside the water-quality readings, not part of them.

Limitations

• Ecoregion boundaries are discontinuities. Because thresholds change at an ecoregion line, two lakes with identical chemistry on opposite sides of one can fall in different EPA classes. This was measured rather than assumed away: of 431 neighbouring lake pairs within 15 km of each other that straddle an ecoregion boundary, 137 (32%) fall in a different class. Osage Cove Lake in Oklahoma sits 5.3 km from a lake on the other side of an ecoregion line. Its chlorophyll-a of 16.9 µg/L is Poor against the Southern Plains reference condition and Fair against the Temperate Plains one. Using a single national threshold instead would mean applying a reference condition EPA never published, so the discontinuity is reported rather than hidden.
• The readings are blind to toxics and pathogens. Read them together with the impairment panel and the state and county advisories, never instead of them.
• Depth is not adjusted for. Shallow lakes have less volume to dilute incoming phosphorus, more wind-driven sediment resuspension and warmer summer water. Reference conditions vary by ecoregion but not by lake depth, so a shallow lake at natural levels for its type can still read higher in nutrients than a deep lake in the same region.
• Monitoring effort is uneven. Some lakes have decades of records; many have a handful of samples. Every page shows the sample counts behind its readings, and lightly sampled lakes are kept out of rankings, but a well-monitored lake and a barely-monitored one are not equally well understood, whatever their numbers.
• The Portal aggregates many protocols. Different agencies use different laboratory methods and detection limits. Methods are reported per lake and medians are used rather than means to limit the influence of outliers, but cross-agency variability is real.
• The condition classes are coterminous-US only. The NLA reference framework does not extend beyond the Level III ecoregion coverage, so lakes outside it show their readings with no EPA class.
• A reading is a multi-year summer summary, not today's water. It cannot tell you whether there is a bloom this week. Always check current advisories from your state water agency and the county health department before swimming, especially with young children, pregnant family members or pets.

Data Refresh

Water Quality Portal records are updated as state agencies upload new monitoring data, most commonly on a 6 to 12 month cycle. Every median, condition class, trophic class and ranking is recomputed from scratch whenever new data arrives. The current data was last updated August 2026.