Guide · Behind the data
How Lake Data Gets Collected
From a sampling crew on a boat to a page on this site: the two federal pipelines lake data travels through, and the lags and gaps it picks up along the way.
The short answer
Lake data reaches you through two pipelines with very different clocks: assessment verdicts move on a two-year regulatory cycle from state programs through EPA ATTAINS, while lake levels flow from USGS gauges as daily values.
- 60,590
- lake waterbodies filed by 50 state programs and 16 sovereign tribal nations across 50 states
- 2 pipelines
- state assessments via EPA ATTAINS, gauge records via USGS NWIS
- 2-year cycle
- the rhythm of Clean Water Act assessment reporting
Knowing each pipeline's clock is what lets you judge how current any number on a lake page is.
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.
Pipeline One: State Monitoring to EPA ATTAINS
The assessment side of PlainLakes begins with state monitoring programs: field crews collecting water samples, fish tissue, and biological surveys across each state's priority waters. The path from sample to database runs in stages:
- Monitoring. States (with tribal, federal, volunteer, and university partners) sample chemistry, bacteria, fish tissue, and habitat. No state can monitor everything; each publishes a monitoring strategy that decides which waters get visited and how often.
- Assessment. Agency staff compare accumulated data against the state's water quality standards, use by use, waterbody by waterbody, applying documented assessment methodologies.
- The integrated report. On the two-year cycle, the state compiles verdicts, impairment causes, and its 303(d) priority list into one submission, with public comment along the way.
- EPA review and ATTAINS. EPA approves or disapproves the impaired-waters list and loads the state's decisions into ATTAINS, the national assessment database that PlainLakes reads.
What that pipeline produces, and what each verdict means, is covered in the assessment status guide.
Pipeline Two: The USGS Gauge Network
Lake levels travel a shorter, faster path. USGS operates a national network of monitoring stations, funded jointly with state and local cooperators, that measure water surface elevation, gage height, and reservoir storage with automated sensors. Readings flow into the National Water Information System (NWIS), where each series is published twice over: an instantaneous real-time feed, and a curated daily-values record in which recent figures are marked provisional until quality review approves them.
PlainLakes loads the daily-values service in batches and links each gauge to its lake's assessment record, which is what lets one page show a lake's regulatory verdicts beside its trailing 365 days of levels. How to read those series, including the datum caveats, is the subject of the lake level guide.
Why the Data Lags Reality
| Stage | Typical lag | Why |
|---|---|---|
| Sampling to assessment | Months to years | Verdicts draw on multi-year data windows, and many lakes are visited only once in several cycles |
| Assessment to ATTAINS | Up to the biennial cycle, plus review | Integrated reports are compiled, taken through public comment, submitted, and EPA-reviewed before loading |
| Gauge to NWIS daily value | About a day, provisional | Daily values post quickly but remain provisional until USGS quality review approves them |
| Source to PlainLakes | Daily batch | This site re-ingests on a schedule rather than streaming, trading immediacy for stable, checkable snapshots |
The practical rule: read level data as days old and assessment data as a regulatory snapshot whose underlying samples may be years old. Each PlainLakes page carries the data vintage it is serving, and the methodology documents the ingest schedule in detail.
The Coverage Footprint by State
Counting reported lake-type waterbodies per state shows how uneven the reporting footprint is, and each bar carries how many of those units carry a verdict. The counts reflect program design as much as geography: how a state divides lakes into assessment units, how broadly it monitors, and how much of its own register it rules on, all shape the totals.
States with the most reported lake-type waterbodies
Lake-type units filed with EPA ATTAINS from each state, by its own program and by any tribal nation reporting inside it, whatever the filer then said about them, top 10 by count
- WI
Wisconsin
19,871 waterbodies
- MN
Minnesota
13,176 waterbodies
- MI 4,307
Michigan
4,307 waterbodies
- NH 2,786
New Hampshire
2,786 waterbodies
- NY 2,231
New York
2,231 waterbodies
- IN 1,588
Indiana
1,588 waterbodies
- FL 1,475
Florida
1,475 waterbodies
- OR 1,431
Oregon
1,431 waterbodies
- MA 1,191
Massachusetts
1,191 waterbodies
- WA 832
Washington
832 waterbodies
What this shows Wisconsin holds the most reported lake-type waterbodies (19,871), of which 4,718 carry a verdict. A large register is a subdivision and monitoring choice, not a finding: totals reflect how each program divides its waters and how much of that register it rules on, not simply how many lakes a state has.
Per-state footprints, with impairment counts and top causes, are on the state pages.
Frequently Asked Questions
Why does an assessment from years ago still show as the current status?
Because it is the current regulatory status. A verdict stands until the state revisits that waterbody in a later integrated report, and states cannot re-monitor every lake every cycle. The cycle-last-assessed field on each PlainLakes lake page is there precisely so you can see how old the underlying decision is and weigh it accordingly.
Why do neighboring states look so different in the data?
Because the data is produced by 50-plus separate programs. States differ in monitoring budgets, in which waters they prioritize, in the standards they adopt, and in how they subdivide lakes into assessment units. A state with few listed lakes may have clean water or a small monitoring program, and the data alone cannot always distinguish the two. Cross-state comparisons are comparisons of programs as much as of lakes.
How fresh is the lake level data on PlainLakes?
Level data comes from the USGS NWIS daily-values service, which publishes one reviewed value per day rather than a live feed, so a gauge reading here describes a recent day, not the current minute. Each lake page shows the trailing 365 days as of the data-vintage date printed on the page, and that window moves forward when PlainLakes reloads the source. For instantaneous readings, USGS real-time services are the right source.
Does every lake in America appear in this data?
No. A lake appears on the assessment side only if its state created an assessment unit for it and reported it to EPA, and on the level side only if a monitoring site in the USGS National Water Information System reports daily values for it. That inventory includes sites other federal and state agencies operate, not only USGS’s own. Small, private, and remote lakes are routinely absent from both systems. Absence from PlainLakes means absence from the source federal datasets, not that the lake does not exist or has no problems.
Can the data change retroactively?
Yes, in both sources. States correct and revise assessment records, and EPA loads updates to ATTAINS between cycles. USGS marks recent daily values as provisional and may revise them during quality review before approving the record. PlainLakes re-ingests from both sources, so figures can shift as upstream corrections land; the as-of stamp on each page reflects the data vintage being served.
Sources
What to do with this
Now that you know each pipeline's clock, use the vintage fields on every page.
- Open a lake and check its cycle-last-assessed and level-data dates before trusting either number. Browse lakes
- Compare assessment footprints and impairment counts across state programs. See state pages
- Read the full ingest and processing pipeline for this site. Read the methodology
Coverage differences between states reflect program design; the counts here compare programs, not lake inventories.
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 coverage counts in this guide are computed at render time from per-state assessment tallies in the PlainLakes database; 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.