The Largest Assessed Lakes in the United States

Research question

Which US lakes have the largest assessed acreage, and how were they assessed?

Methodology

The ranking reads four fields for every lake-type assessment unit in the database, the display name, the state that filed it, the assessed size, and the count of listed impairment causes, orders them by assessed size from largest down, and shows the ten largest. The figures are read from the current dataset when the page is requested, so a revision filed by a state reaches this page the next time PlainLakes refreshes from EPA, without the page being rewritten by hand.

The ranked figure is a comparable area, and states file an assessment unit’s extent in whichever measure fits the water they assessed: 62,495 lake-type units are filed in acres, 68 in square miles, and 1,273 as a shoreline length in miles. Acres are ranked as filed and square miles are converted at 640 acres per square mile, the exact statutory figure, so both are ranked and each record page also shows the state’s own number. A shoreline length is not an area at all, so those units keep their own pages and their filed length and are not ranked here; ranking a mile against an acre would put a large number beside a small one and call the comparison a result.

One lake can reach the file more than once. Where several units share a state and a canonical name, a size ranking keeps the LARGEST of them, because keeping the unit with the fuller assessment instead would rank the lake by whichever segment the state studied hardest; Florida files Lake Okeechobee as nine units from 9,480 to 76,101 acres. Each row says how many units the state files under that name, so a row is read as the largest of N rather than as the whole water.

Filing categories are excluded. 2,354 units that would otherwise qualify for this ranking stand for a group of waters rather than for one water. Some say so in the state’s own words: New York records their location as “total area of both lakes”, which states outright that the acreage is a sum. The rest are identified from the filed name — a plural water word located by an area, qualified by a filing status, or naming its members outright as a list. Those rows are real source records and keep their own pages, but their reported area is the extent of a group, and a size ranking that admitted them would rank a bookkeeping entry against Lake Michigan.

Drinking-water intake zones are excluded. 36 units that would otherwise qualify for this ranking are the protection zone around a public water intake rather than the water they are named for. Michigan files them under the lake’s bare name — seventeen of them are called Lake Michigan — at a standard 1.5 square miles, and files each lake itself as separate units whose descriptions state that they do not include these zones. Those rows are real source records and keep their own pages, but ranking one by area would place a facility’s protection zone among lakes under a lake’s name.

The size shown is the extent the state filed for that assessment unit, not an independent survey of the lake's surface area. When a state splits a large lake into several assessed segments, each segment carries its own acreage and the lake's full surface area is spread across those rows; when a state assesses the whole lake as one unit, the acreage approaches the familiar published figure. This is why the assessed acreage on a page can differ from the surface area a reference work reports for the same lake, and why the number belongs to the assessment, not to the shoreline.

The assessment outcome beside each lake is the reporting authority's own Integrated Report verdict, carried across unchanged. PlainLakes computes no water-quality score of its own and reclassifies no filed decision. Across the full lake-type universe of 63,836 units, 37,394 carry no assessment verdict, which is the single most common condition in the file and the reason a ranking of the largest lakes is not a ranking of the most-studied ones. That figure has two parts worth separating: 31,428 units were filed as "Not Assessed", and 31,361 of those carry EPA integrated-reporting Category 3, meaning the unit was reviewed and recorded as having insufficient information to make a determination. The other 5,966 units carry nothing at all (no status, no category, no reporting cycle), so they are the ones for which the file is genuinely silent rather than explicitly undetermined.

Refresh cadence follows the sources, not this page. States report assessment decisions to EPA on their own reporting cycles, typically every two years, and some states file late, so the verdict shown for a lake can describe conditions from an earlier period. The database behind this page holds decisions from reporting cycles 1987 through 2026. PlainLakes reloads from the source when EPA publishes and shows the resulting data-vintage date on the page; there is no live connection to a state agency and no claim that a decision reaches this page within any fixed number of hours.

Where a unit has no comparable area, this page states what the state filed rather than reporting a gap the source does not have. 1,068 lake-type units are filed with a size of 0, which is a figure rather than a measurement; those are held out of every size ranking, percentile and distribution rather than sorted to the bottom as though the water had been measured and found small, and 1,273 are filed in a measure that is not an area and keep their own filed figure on their own pages instead. No unit in the register carries an empty size field, so nothing here is excluded for want of a filing; every unit this page cannot rank was reported by its state in a form a ranking of areas cannot use, and each says which. Where a value looks extreme against its neighbours it is shown as filed, without smoothing, and the lake's own page carries the record it came from.

Comparison across state lines has a limit worth naming. Each state writes its own water-quality standards, sets its own monitoring strategy, and decides how finely to divide its waters into assessment units, so the size distribution in one state reflects that state's unit-splitting convention as much as its geography. The ranking is a reliable answer to "which assessed lake units are largest as filed", and it is not a measure of which state has the most lake water.

Every figure here is traceable. Each row links to that lake's page, which names the assessment unit, its state, its reporting cycle, and its listed causes, and the citation block names the dataset and the vintage this page was built from. Readers who want to verify a number independently can follow the source link to EPA's own ATTAINS interface and look the unit up there; nothing on this page depends on an intermediate that a reader cannot reach.

PlainLakes is published by an organization, not by a newsroom of named reporters, and no individual byline or human review is claimed for these pages. Rankings are produced by documented queries against the source data rather than by editorial selection, which is why the ordering rules above are stated in full. Corrections are welcome: a reader or a state agency that believes a value is wrong can write to the correction address in the footer, and a substantive correction is reconciled against the upstream record and carried at the next source refresh.

See the methodology page for the complete data-update process, source vintage, and field definitions.