Guide · Reading the data
Understanding Lake Assessment Status
What Fully Supporting and Not Supporting actually mean, who makes the decision, and why a lake with no verdict is not the same as a clean lake.
The short answer
An assessment status is a state regulatory verdict against water quality standards, made under the Clean Water Act and reported to EPA, and it only covers what the state actually assessed.
- 60,590
- lake waterbodies with reported assessment records on PlainLakes
- Categories 1 to 5
- the EPA integrated reporting scale
- 303(d) + 305(b)
- the Clean Water Act sections behind every verdict
Verdicts are periodic findings, not real-time water safety statements.
According to the U.S. Environmental Protection Agency's ATTAINS program, state agencies and tribal nations report 60,590 lake-type waterbodies nationwide in the August 2026 extract, and PlainLakes renders those filed assessment decisions alongside linked U.S. Geological Survey daily gauge records where a conservative name match exists. See our methodology for reporting-unit limits, filer attribution, and refresh cadence.
What a Lake Assessment Actually Is
Every state assigns its waters a set of designated uses: the purposes the water is supposed to serve, such as supporting aquatic life, recreation, fish consumption, or drinking water supply. Each use comes with water quality standards, which are numeric or narrative criteria the water must meet for that use to be considered healthy. An assessment is the state's periodic comparison of monitoring data against those standards.
The result is a verdict per designated use, and a rolled-up overall status for the waterbody. This is the vocabulary you see on every PlainLakes lake page: the overall status near the top, and the individual use verdicts below it. The unit being assessed is not always the whole lake as you know it. States divide waters into assessment units, and a large lake can be split into several units with different verdicts.
Who Decides, and Under What Law
Two sections of the federal Clean Water Act drive the whole process. Section 305(b) requires every state to report on the condition of its waters. Section 303(d) requires states to identify the waters that fail to meet standards, the impaired waters, and to prioritize them for cleanup plans. Most states now combine both duties into a single integrated report submitted to EPA on a two-year cycle.
The decisions are made by state environmental agencies, using their own monitoring data and their own adopted standards. EPA's role is review: it approves or disapproves each state's impaired-waters list and stores the assessment decisions in a national database called ATTAINS. PlainLakes renders those ATTAINS records directly. When a page says a lake is Not Supporting, that is the state's regulatory finding, passed through unchanged. How a listing leads to a cleanup plan is covered in the 303(d) listing guide.
The Status Labels, Defined
| Status | What it means | What it does not mean |
|---|---|---|
| Fully Supporting | The evidence the state used met the standards for the assessed use(s) in the reporting cycle | A guarantee of current, real-time safety |
| Not Supporting | At least one standard was exceeded; the use is impaired | That the lake is unusable for everything |
| Insufficient Information | Some data exists but not enough to support a verdict | A clean bill of health |
| Not Assessed | The state did not evaluate this waterbody or use this cycle | Anything at all about actual water quality |
The overall status is a roll-up. A lake that fully supports recreation and aquatic life but fails its fish-consumption standard will usually carry a Not Supporting overall status. The use-level detail matters more than the headline: check which use failed before drawing a conclusion about your own use of the lake.
EPA Integrated Reporting Categories 1 to 5
Alongside the plain-language status, EPA guidance sorts every assessed waterbody into one of five integrated reporting (IR) categories. They answer a slightly different question: not just "is it impaired?" but "what happens next?"
| Category | What it means | Lake waterbodies |
|---|---|---|
| 1 | All designated uses assessed and supported | 2,450 |
| 2 | Assessed uses supported; some uses not assessed | 11,093 |
| 3 | Insufficient information to determine | 31,358 |
| 4A | Impaired; a TMDL cleanup plan is already approved | 5,279 |
| 4B | Impaired; another enforceable control is in place | 26 |
| 4C | Impaired by something that is not a pollutant (habitat, flow) | 508 |
| 5 | Impaired and still needing a TMDL; these are the 303(d) list | 6,865 |
| 5A | A Category 5 variant filed by individual state programs; PlainLakes reports the code as filed and does not interpret it (state-specific code) | 2 |
| 5R | A Category 5 variant filed by individual state programs; PlainLakes reports the code as filed and does not interpret it (state-specific code) | 117 |
The shape of that table is the story. Category 3, insufficient information to determine, holds 54% of every categorized lake waterbody in the database. The regulatory pipeline most people picture, impaired water moving toward a cleanup plan, describes a minority of the inventory; the majority sits in a category that records how little is known. A further 2,892 waterbodies carry no IR category at all.
The category also tells you where a lake sits in that pipeline. Two lakes can both be impaired for the same pollutant, with one in Category 4A because its cleanup plan is approved and the other in Category 5 still waiting for one.
The National Mix on PlainLakes
Summing the state-level assessment counts in the PlainLakes database shows how the three outcomes are distributed across all lake waterbodies on this site. The "no overall determination" group, waterbodies whose records carry neither verdict, is a direct picture of how much lake water remains outside the verdict system.
Lake waterbodies by overall assessment outcome
All lake waterbodies in the PlainLakes database, grouped by overall status; counts summed from reported assessment records
- Not Supporting 12,831
Not Supporting
12,831 waterbodies
- Fully Supporting 13,596
Fully Supporting
13,596 waterbodies
- No overall determination
No overall determination
34,163 waterbodies
What this shows Of 60,590 lake waterbodies with reported assessment records, 12,831 carry a Not Supporting overall status and 13,596 are Fully Supporting; the remaining 34,163 have no overall determination on record.
You can see the same breakdown per state on the state assessment pages, and filter individual lakes by status in the lake browser.
How to Read a Verdict on a Lake Page
A practical reading order for any assessment verdict:
- Start with the use-level verdicts, not the headline. Which specific use failed? A fish-consumption impairment and a recreation impairment imply very different things for a visitor.
- Check the cause. Mercury, nutrients, and pathogens each have their own story; see the impairment causes guide for what each one means in practice.
- Check the monitoring year, not just the cycle. The reporting cycle is when the state submitted the finding, not when it measured. Where the state filed a monitoring year, that year is shown beside the cycle on the lake page, and it is the one to read for how current the evidence is: the two can be decades apart.
- Remember the scope. The verdict covers an assessment unit, which may be one arm or basin of a larger lake.
Frequently Asked Questions
Who actually decides whether a lake is "Fully Supporting" or "Not Supporting"?
State environmental agencies make the call. Under Clean Water Act sections 305(b) and 303(d), each state assesses its own waters against the water quality standards it has adopted, then reports the decisions to EPA. EPA reviews and approves or disapproves the impaired-waters list, but the assessment verdict on a PlainLakes page is a state decision, reported through the EPA ATTAINS system, not an EPA measurement and not a PlainLakes judgment.
Does "Fully Supporting" mean the water is safe to drink or swim in right now?
No. Fully Supporting means that, for the uses the state assessed during its reporting cycle, the evidence the state used met the applicable water quality standards. That evidence is usually monitoring data, and ATTAINS records per verdict what it was: on some records the state files the basis as extrapolated rather than monitored, and where it filed one the lake page names it. ATTAINS also carries the state’s own confidence in a verdict (Low, Fair, Good or Excellent), and where the state filed that rating the lake page prints it beside the verdict, including where it rated a monitored finding Low. It is a periodic regulatory finding, not a real-time safety statement. Conditions change between assessment cycles, and short-lived events like algal blooms or sewage overflows are handled by health departments and beach programs, not by the assessment status.
What does "Not Assessed" mean, and is an unassessed lake clean?
Not Assessed means the state did not evaluate that waterbody, or that designated use, in the reporting cycle. It says nothing about water quality in either direction. States have finite monitoring budgets and prioritize where they sample, so many lakes have never been formally assessed. Absence of an impairment listing is absence of evidence, not evidence of clean water.
What is the difference between a designated use and an overall status?
A designated use is one specific purpose a state has assigned to a waterbody, such as fish consumption, recreation, aquatic life, or drinking water supply. Each use gets its own supporting or not-supporting verdict. The overall status rolls those use-level verdicts into one summary: if any assessed use is not supported, the waterbody as a whole is typically reported as Not Supporting, even when other uses are fine.
How often are these assessments updated?
The Clean Water Act integrated report runs on a two-year cycle: states compile monitoring data and submit updated assessments to EPA in even-numbered years, though submission and EPA approval dates vary by state. A verdict you see today may rest on data collected several years ago. The reporting cycle is when the state submitted the finding, not when it measured. Where the state filed a monitoring year, that year is shown beside the cycle on the lake page, and it is the one to read for how current the evidence is: the two can be decades apart.
Sources
What to do with this
Now that the labels have meaning, read them in context on live data.
- Look up any lake and read its use-level verdicts, causes, and last-assessed cycle. Browse lakes
- Compare how much of each state's assessed lake water is impaired. See state pages
- Understand how an impairment becomes a listing and a cleanup plan. Read the 303(d) guide
Status definitions follow EPA's integrated reporting guidance; verdicts are state agency decisions.
PlainLakes is rendered directly from state Clean Water Act assessment decisions in EPA ATTAINS and lake level records from USGS NWIS, no number is typed in by an editor. The status counts in this guide are summed at render time from the assessment records in the PlainLakes database, as filed by the state agencies and tribal nations that report them; no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error. Data current as of Reporting cycles 1987–2026. Primary sources: EPA ATTAINS and USGS National Water Information System.