Reference guides
Lake data guides
Plain-language explainers for the EPA ATTAINS assessment register and the USGS daily lake-level records behind every lake page.
- 8
- guides in this hub
- 60,590
- named waterbodies
- 49%
- ruled verdicts not supporting a use
What these guides cover
PlainLakes publishes 8 reference guides to the EPA ATTAINS assessment vocabulary and USGS lake-level records behind 60,590 named waterbodies. 49% of ruled verdicts in this extract are Not Supporting at least one designated use.
- 8
- topic guides
- 49%
- not supporting at least one use
- 60,590
- lake records in ATTAINS
- 50 states
- with filed assessments
A definitive verdict is Fully Supporting or Not Supporting. Not Assessed and Insufficient Information are excluded from the impaired share.
Ruled verdicts in this extract
Every waterbody counted here carries a definitive overall verdict. The guides below explain the vocabulary behind these two buckets; they do not change the partition.
These guides reference records compiled for 60,590 lake and reservoir waterbodies reported to EPA ATTAINS across 50 states, filed by 50 state programs and 16 sovereign tribal nations. The full pipeline is documented in our methodology.
- Understanding Lake Assessment StatusWhat Fully Supporting and Not Supporting mean, who makes the call, EPA integrated reporting categories 1 to 5, and why an unassessed lake is not a clean lake.
- Can You Eat the Fish? A Consumption Status GuideWhat the fish-consumption use verdict means, why mercury and PCBs dominate the listings, and why your state fish advisory is the guidance that actually governs.
- How Impairment Listing Works: 303(d) ExplainedThe Clean Water Act 303(d) list, impairment causes, first-listed cycles, TMDL cleanup plans and priorities, and how a lake gets delisted.
- Reading Lake Level DataElevation, gage height, and reservoir storage: the three USGS parameters, datum caveats, and how to read a 365-day trend without misreading drawdown as drought.
- Common Lake Impairment Causes ExplainedMercury, nutrients, pathogens, low dissolved oxygen, pH, and invasive species: what each assessment cause means for people who use the lake.
- Trophic Status ExplainedOligotrophic to hypereutrophic: what the trophic scale measures, what ATTAINS trophic codes mean, and what they imply for algae and water clarity.
- Lake Recreation Safety: What Assessment Data Tells YouRecreation-use verdicts, pathogens, and harmful algal blooms, plus why assessment data is background context and never a substitute for current beach advisories.
- How Lake Data Gets CollectedThe pipeline from state monitoring crews to EPA ATTAINS reporting cycles and the USGS gauge network, including why the data lags reality and where coverage gaps sit.
How these guides work
PlainLakes publishes these guides to explain the vocabulary of official water data, not to replace it. Assessment verdicts on this site are state agency decisions reported to EPA under Clean Water Act sections 303(d) and 305(b); lake level figures are USGS gauge records refreshed in daily batches. Where a term has a precise regulatory meaning, the guide uses the official definition and says where it comes from.
Every count or figure inside a guide is computed from the same database that powers the rest of the site, so the numbers in a guide match what you see when you browse lakes, open a state page, or check the national rankings. Health-related decisions, such as whether to eat fish from a specific lake, belong to your state advisory program, and the guides say so explicitly.
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.