NWI Wetlands Data in Early Site Diligence

The National Wetlands Inventory tells you, in its own metadata, not to use it for the thing most people use it for. The dataset record served with the wetlands layer states that the data are intended for use with base maps and digital aerial photography at a scale of 1:12,000 or smaller, and that because of that scale the primary intended use is regional and watershed data display and analysis rather than specific project data analysis (U.S. Fish and Wildlife Service, dataset metadata, publication date 1 May 2024). A single parcel is specific project data analysis. The layer says so before you open it.

That is not a reason to skip the layer. It is a reason to know exactly what a polygon is worth, because the answer is genuinely useful and genuinely narrow. A wetland flag on a site changes the sequence of a development, the length of a permitting timeline and the size of the buildable envelope, and it does so early enough that a lender who reads the layer properly at screening will scope the engagement correctly and one who reads it loosely will either kill a good site or fund a surprise.

What follows is one federal layer read properly: what the inventory is, how its classification code works, where the code goes blank, how old a given polygon is and how to find out, what the national record actually measures, and where the whole exercise stops. The stopping point goes at the top and will be repeated, because it is the single most misused fact in this subject area.

The National Wetlands Inventory is a mapping inventory. It is not a jurisdictional determination. Wetland jurisdiction under the Clean Water Act is determined by the U.S. Army Corps of Engineers through the approved jurisdictional determination process that supports Section 404 permitting, and the inventory's own metadata disclaims any regulatory reading. After Sackett v. Environmental Protection Agency in 2023 the question of how far federal jurisdiction reaches became a legal question about surface connection rather than a cartographic question about wetland presence. Nothing in this article is a delineation, a determination or advice on either. MMCG Analytics provides data and analytics with source and vintage provenance carried on displayed values; the decisions rest with the lender and the qualified professionals it engages.

This is the wetlands layer of the wider early pass described in public-records hazard screening, taken one layer deep, and it sits inside the same discipline as the rest of the public-data stack for commercial real estate analysis: every figure carries a named source and a date, and every negative result carries the scale it was negative at.

What the inventory is, and what it was built to do

The inventory exists because Congress told the Department of the Interior to build it. The Emergency Wetlands Resources Act of 1986 requires the Secretary of the Interior, through the Director of the Fish and Wildlife Service, to map the nation's wetlands and to produce national wetlands status and trends reports to Congress. The Geospatial Data Act of 2018 requires the Department to work with partners to collect, produce, maintain and disseminate the national Wetlands Data Layer, which the Service produces and maintains as the Wetlands Layer of the National Spatial Data Infrastructure, identified as a National Geospatial Data Asset by the Federal Geographic Data Committee.

Note what those mandates ask for and what they do not. They ask for an inventory of the resource. They do not ask for a map of anybody's regulatory reach, and the Service has never claimed to supply one.

The product is large. The Service states that the Wetlands Data Layer contains more than 37 million wetland and deepwater features, covering the conterminous United States, Hawaii, Puerto Rico, the Virgin Islands, Guam, the major Northern Mariana Islands and Alaska, growing at a rate of 50 to 100 million acres annually as data are updated, and released twice a year on the Wetlands Mapper and in the downloads (USFWS, Wetlands Data, 2026). The dataset metadata record itself, published 1 May 2024 with a metadata date of 1 October 2023, counts 36,091,275 polygons and records the maintenance update frequency as biannual. The gap between those two counts is not an error. It is the first practical lesson of the layer: the program page and the metadata record describe the same dataset at different moments, and a screen that cites a figure without citing its vintage has already made the mistake this article is about.

The standards behind the layer are federal ones. The program, working with federal and non-government partners, developed standards for both wetland classification and mapping that the Federal Geographic Data Committee adopted as federal standards, with the consequence that all federally funded wetlands mapping projects must follow them, while state and private projects are strongly encouraged to adopt the same guidance (USFWS, Wetlands Data, 2026). That matters for a lender reviewing a consultant's mapping product: if the work was federally funded, the classification is not a matter of house style.

Reading a wetland code, character by character

Every polygon carries an attribute the Service defines as an alphanumeric code identifying the wetland classification of the polygon, and the example the Service gives is PFO1A. Most people looking at a wetlands map never read it. It is the most information-dense thing on the screen.

The classification of record is the Federal Geographic Data Committee wetlands classification standard, FGDC-STD-004-2013, second edition, August 2013, adapted from the 1979 classification by Cowardin, Carter, Golet and LaRoe. The Service's own decode of the example is exact: PFO1A is Palustrine (P), Forested (FO), Broad-leaved Deciduous (1), Temporarily Flooded (A) (USFWS, Wetland Classification Codes, 2026).

Read that left to right as a lender. Palustrine means the non-tidal inland system, the one that covers the overwhelming majority of wetland acreage in the country. Forested means the canopy is trees, which on a development site means clearing, which means a visible, photographable, contestable change. Broad-leaved deciduous narrows the vegetation. Temporarily flooded is the water regime, and it is the character that most often surprises people: the standard defines temporarily flooded as surface water present for brief periods, from a few days to a few weeks, during the growing season, with the water table usually well below the ground surface for most of the season. A PFO1A polygon can be, and frequently is, dry ground when anyone visits it.

That single fact explains most of the arguments that happen on site. A broker walks a temporarily flooded palustrine forested wetland in August, finds firm ground and concludes the map is wrong. The map is not wrong. The map is describing a condition the standard explicitly says applies to average conditions over a period of years, and the standard warns that because many habitats are dynamic and subject to rapid changes in appearance, proper classification will frequently require data that span a period of years and several seasons in each of those years.

The structure behind the code is deeper than four characters suggest. The standard sets out five systems (Marine, Estuarine, Riverine, Lacustrine and Palustrine), eight subsystems distributed across the first four of those, eleven classes and forty-one subclasses, plus nineteen water regime modifiers grouped as tidal salt, nontidal and tidal fresh, seven salinity levels, three pH levels, two soil modifiers and eight special modifiers.

A second worked decode shows the tidal half of the system. E2EM1P reads as Estuarine (E), Intertidal (2), Emergent (EM), Persistent (1), Irregularly Flooded (P): a salt marsh whose vegetation stands through the winter and which the tide floods less often than daily. For a coastal site that code is a different conversation from PFO1A, because the estuarine intertidal category is where the national record shows sustained decline and where state coastal programs are typically most active.

The special modifiers repay attention on any site with a history. Eight exist, and seven of them describe human alteration: partly drained or ditched, farmed, managed, excavated, diked or impounded, artificial substrate and spoil. Only the beaver modifier describes a non-human agent. A polygon carrying an excavated or diked modifier is telling a reader that the feature is not a remnant of the original landscape, which is a materially different starting point for a conversation about mitigation than an untouched forested swamp.

Where the code goes blank, and why it matters to a budget

Here is the part of the classification that almost nobody reads, and it carries the finding of this article.

The standard is explicit about why soil belongs in a wetland classification at all. Soil, it says, through its depth, mineral composition, organic matter content, moisture regime, temperature regime and chemistry, exercises a strong influence over the types of plants that live on its surface and the organisms that dwell within it, and it adds that the nature of soil in a wetland, particularly the thickness of organic soil, is of critical importance to engineers planning construction of highways or buildings.

That is the classification standard naming, in its own words, the attribute that decides what a site costs to build on. Organic soil thickness is the difference between a slab on grade and a pile foundation, and it is the reason a wetland finding sometimes damages a pro forma far more through geotechnical cost than through permitting delay.

Now read the instruction four pages later. Water chemistry modifiers and soil modifiers, the standard states, generally are not used when classification data are obtained using remote sensing, and should be applied only when detailed supporting data have been gathered in the field or from reliable sources such as soil surveys.

The National Wetlands Inventory is built from remote sensing. The Service's own metadata says that the accuracy of image interpretation depends on the quality of the imagery, the experience of the image analysts, the amount and quality of the collateral data, and the amount of ground truth verification work conducted. So the two modifiers that speak directly to a construction budget are the two the national layer generally leaves blank, by design and by the standard's own instruction, and the levels it does populate describe vegetation and hydrology rather than what is under them.

The practical consequence for an early screen is precise. A wetland polygon tells you that a permitting and mitigation conversation is likely. It does not tell you whether the ground under the polygon is peat. Those are two separate risks with two separate consultants attached, and the map answers only the first. The soil question routes to the soil survey and eventually to a geotechnical scope, which is the same escalation logic that governs screening buildable land at national scale, where the product resolution under the parcel decides whether an earthwork estimate is worth anything.

Every polygon has a vintage, and it is not printed on the map

The Wetlands Mapper draws a seamless national layer. It is not a seamless national survey. It is an assembly of mapping projects flown at different times, from different imagery, by different interpreters, and the layer's currency varies from county to county in ways the rendered map does not show.

The Service publishes the answer at three levels. There is a Federal Geographic Data Committee content standard metadata record for each layer served on the Wetlands Mapper. There is project level metadata, recording the source imagery scale, type and date, together with collateral data, the inventory method, data limitations, geographic features and landforms, and wetland types for each mapping project. And there is a historic wetlands map information document recording wetland types, vegetation, regional and temporal conditions and geographic features. Project metadata is reached by selecting a wetland polygon in the Wetlands Mapper and clicking the Project Metadata link in the pop-up window, and the historic document through the Historic Map Info link in the same place.

Two cautions travel with that. The Service states plainly that not all areas have a Project Metadata document and not all areas have a Historic Wetlands Map Information document. So the vintage question has three possible answers on any given site: a documented source imagery date, a documented absence of project metadata, or a polygon whose provenance the Service does not publish. All three are legitimate findings for a credit file. Only the first supports a statement about how current the mapping is.

This is the single highest-value habit in the whole workflow, and it takes one extra click. A screen that records "NWI shows a palustrine forested wetland of about two acres in the north-east corner" is management information of unknown age. A screen that records "NWI shows PFO1A across roughly two acres in the north-east corner; project metadata gives source imagery of a stated date and scale; no historic map information document exists for this project area" is a dated observation someone can act on. The same provenance discipline is the subject of the provenance standard, and it applies here more sharply than almost anywhere else in the public-data stack, because the wetlands layer is the one whose rendered output hides its own age most completely.

What the national record actually measures

There is a second federal wetlands product, and confusing it with the first is a common error. The Status and Trends reports to Congress are not a summary of the Wetlands Mapper. They are a separate, sample-based statistical survey.

The sixth report in the congressionally mandated series, covering 2009 to 2019, rests on a survey-based approach carried out within 5,048 four square mile plots randomly distributed across the conterminous United States, with change measured over a 10.5-year study period, quality assured by regional and national experts, and field verification completed for 1,034 sample plots distributed across 46 states, or 20 percent of total plots. Statistical evaluation used paired t-tests with degrees of freedom set by 5,048 plots less 215 strata (Lang, Ingebritsen and Griffin, USFWS, 2024).

The headline estimates are these. There were an estimated 116.4 million acres of wetlands in the conterminous United States in 2019, accounting for under 6 percent of the conterminous United States by area. The vast majority were freshwater, 95 percent or 110.4 million acres, with saltwater wetlands occupying 6.1 million acres. Freshwater forested wetlands were the most abundant type overall at 52.4 million acres, followed by freshwater emergent at 30.0 million acres, scrub-shrub at 19.1 million acres and ponds at 6.9 million acres. The most common saltwater type was salt marsh at 4 million acres.

The change estimates are where the reading gets interesting. Net wetland loss increased substantially, by more than 50 percent, against the previous 2004 to 2009 study period, producing a net loss of 221,000 acres of wetlands between 2009 and 2019, primarily to uplands, at a rate of about 21,000 acres a year. But that net figure conceals a composition shift that matters far more than the total: vegetated wetlands fell by 670,000 acres while non-vegetated wetlands rose by 488,000 acres, with ponds alone gaining 455,000 acres, about 7 percent of total pond habitat.

The report is direct about what this does to a reader who watches only the total. The pattern of freshwater vegetated decrease alongside pond increase has persisted for about 70 years, and the report states that this pattern has obscured vegetated wetland losses. A headline that says the country lost 221,000 acres of wetland understates by a factor of three what happened to the wetlands that actually hold canopy, marsh and shrub.

What the survey can resolve, and what it cannot

The Service publishes a coefficient of variation beside every area and change estimate in the report, defined as the standard error divided by the mean, expressed as a percent. Reading those numbers is the difference between citing the report and understanding it.

Some estimates are tight. The change in all non-vegetated wetlands carries a coefficient of variation of 3.4 percent and the change in palustrine ponds 4.3 percent. Some are not. The change estimate for palustrine emergent wetlands carries a coefficient of variation of 160.2 percent, and for palustrine shrub wetlands 206.8 percent. When the coefficient of variation on a change estimate exceeds 100 percent, the estimated change is smaller than its own standard error, which is a formal way of saying the survey did not detect a change in that category at all.

This is not a criticism of the survey. A national sample of 5,048 plots resolves aggregate change well and category-level change unevenly, which is exactly what a sample of that size should be expected to do. It is a criticism of the way the survey is usually quoted. Palustrine emergent and palustrine shrub wetlands together cover 49.1 million acres, more than 40 percent of the national total, and for both of them the honest statement about 2009 to 2019 is that the national survey does not resolve their change. Anyone asserting a trend in marsh or shrub acreage from this report is reading past the coefficient of variation printed next to the number.

What actually converts a wetland

For a development audience the driver analysis is the least intuitive part of the report, and it is worth sitting with.

Between 2009 and 2019, conversion to upland was the dominant mechanism of net wetland loss, accounting for a total wetland reduction of 194,000 acres, with conversion to deepwater accounting for a further 27,000 acres. Looking only at vegetated freshwater wetlands, where net loss to upland reached 607,000 acres, the report attributes that loss to upland agriculture (211,000 acres), upland forested plantation (107,000 acres), upland other (86,000 acres), upland urban (64,000 acres) and upland rural development (51,000 acres).

Farming and silviculture together account for nearly three acres of vegetated wetland loss for every one attributable to urban and rural development combined. That is a reversal of the position in the late 1900s: for the 1986 to 1997 period the report records urban and rural development as associated with over half, 53 percent, of net wetland loss, followed by agriculture at 26 percent and silviculture at 23 percent.

Two things follow for a lender. First, the national wetland loss story is now largely an agricultural and forestry story, so a general claim that development is destroying the nation's wetlands is not what the current federal evidence says. Second, and more useful at a desk, the same report shows where new ponds came from: net pond gains of 184,000 acres from upland agriculture, 126,000 acres from upland other, and 106,000 acres converted out of vegetated wetlands. A pond appearing on a site between two imagery vintages is as likely to be a stormwater or agricultural feature as a natural one, and the special modifiers in the classification are the place where that distinction is recorded.

Jurisdiction is a legal question, and it narrowed in 2023

Everything above concerns whether a wetland is there. Whether the federal government regulates it is a separate question decided by a separate body under a separate definition, and the gap between the two has never been wider.

Start with the definitions, because they do not match and the mismatch is documented by the Corps itself. A Corps technical report on regionalizing the delineation manual sets the definitions side by side and states the conclusion plainly: the inventory definition is broader than either of the regulatory definitions used by the Corps and the Environmental Protection Agency or by the Natural Resources Conservation Service, in that it only requires a one-factor test, meaning an area must satisfy any one of three attributes to be considered a wetland for inventory purposes. The three attributes, from the classification, are that the land at least periodically supports predominantly hydrophytes, or that the substrate is predominantly undrained hydric soil, or that the substrate is nonsoil and is saturated or covered by shallow water at some time during the growing season each year (Wakeley, ERDC/EL TR-02-20, U.S. Army Engineer Research and Development Center, 2002).

The regulatory definition at 33 CFR 328.3 is narrower and requires vegetation: those areas inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. The Corps report notes that this requirement is further narrowed in the delineation manuals to mean the presence of vascular macrophytes, so the regulatory definitions exclude wet environments that are unvegetated. The Service made the same point in the 2024 Report to Congress, stating that wetlands were identified using a biological definition which differs from the federal regulatory definition and does not imply regulatory jurisdiction.

Then the reach narrowed again. In Sackett v. Environmental Protection Agency, decided 25 May 2023, the Supreme Court held that the Clean Water Act's waters refers only to geographic features described in ordinary parlance as streams, oceans, rivers and lakes, and that adjacent wetlands are covered only where they have a continuous surface connection to such waters so that there is no clear demarcation between the two. The regulation was conformed: 33 CFR 328.3 now covers wetlands adjacent to the enumerated waters, or to relatively permanent bodies of water with a continuous surface connection to those waters, and defines adjacent as having a continuous surface connection (88 FR 3142, 18 January 2023, as amended at 88 FR 61968, 8 September 2023).

The rulemaking has not settled. On 12 March 2025 the Environmental Protection Agency and the Department of the Army issued joint guidance to field staff on implementing continuous surface connection consistent with Sackett, and on 17 November 2025 they announced a proposed rule to clarify the definition of waters of the United States and fully implement the decision, with the comment period closing 5 January 2026 (EPA, Waters of the United States, page updated 25 June 2026).

Which means a mapped polygon and a jurisdictional wetland are now two answers to two different questions, and only one of them is an appealable federal document. Under 33 CFR 331.2 an approved jurisdictional determination is a Corps document stating the presence or absence of waters of the United States on a parcel, or a written statement and map identifying the limits of waters of the United States on a parcel, and it is an appealable action. A preliminary jurisdictional determination is a written indication that there may be waters of the United States on a parcel, or of their approximate location, and it is advisory in nature and may not be appealed.

Between the polygon and the determination sits the field work. A delineation is performed on the ground under the 1987 Corps of Engineers Wetlands Delineation Manual together with the applicable regional supplement, of which the Corps publishes ten covering Alaska, the Arid West, the Atlantic and Gulf Coastal Plain, the Caribbean Islands, the Eastern Mountains and Piedmont, the Great Plains, Hawaii and the Pacific Islands, the Midwest, the Northcentral and Northeast, and the Western Mountains, Valleys and Coast, each with its own wetland determination data sheet (U.S. Army Corps of Engineers, Regional Supplements to Corps Delineation Manual, 2026). Which supplement applies is a function of where the site is, and it changes the field indicators the delineator uses, which is why a delineation from one region does not read like a delineation from another. Beyond the delineation, filling a jurisdictional wetland requires a Section 404 permit, and the applicant must show that impacts were avoided, then minimised, then compensated for whatever remains (EPA, Section 404 Permit Program, page updated 17 February 2026).

The state layer did not narrow when the federal one did

A screen that stops at federal jurisdiction will be wrong in a large number of states, and the error runs in the direction that hurts: it will tell a borrower there is no constraint where there is one.

Two verified examples make the point. Wisconsin's Department of Natural Resources responded to the Sackett decision by stating that state wetland protections were unaffected and that the department would continue to implement the state's uniform wetland regulatory program for projects proposing to impact wetlands, citing 2001 Wisconsin Act 6, which established comprehensive regulations for isolated, non-federal wetlands (Wisconsin DNR, 2 June 2023). Wisconsin had already legislated for exactly the category that Sackett later removed from federal reach.

Colorado moved after the decision. House Bill 24-1379, approved and effective 29 May 2024, directs the Water Quality Control Commission to promulgate rules by 31 December 2025 establishing a state permitting programme for dredge and fill activities in state waters, clarifies that state waters include wetlands, and expressly covers isolated wetlands, isolated ponds and impoundments and isolated streambed reaches. The rules must prioritise avoidance and minimisation and incorporate the federal Section 404(b)(1) guidelines unless the commission determines those guidelines inadequately protect state waters, and a general authorisation for isolated waters cannot authorise projects exceeding one tenth of an acre of wetland impact or three hundredths of an acre of streambed impact.

Note the size of that Colorado threshold. One tenth of an acre is about 4,356 square feet, roughly the footprint of a modest single-storey building. A site screen that dismisses a small mapped wetland as immaterial because it would not attract federal attention has not screened Colorado at all. The national-scale version of this patchwork, and how to map it, is the subject of mapping wetlands constraints nationally.

The screen sequence, and when to escalate

Put the pieces in order and the workflow is short.

First, locate the parcel and read the polygons on and immediately around it. Adjacency is the point, not just the parcel boundary, because the post-Sackett test turns on continuous surface connection to a covered water, and because a wetland just off the line still constrains grading, drainage and access. Do not invent a radius. Record what you looked at and how far out you looked, and say so in the file.

Second, decode every code rather than noting that something is green. System, class, subclass and water regime each change the conversation, and a special modifier changes it more. A temporarily flooded palustrine forested polygon and a permanently flooded palustrine unconsolidated bottom polygon are not the same finding, and treating them as one is how screens lose credibility with borrowers.

Third, click through to project metadata and record the source imagery date and scale, or record that no project metadata document exists. This is the step that separates a dated observation from a guess.

Fourth, read the layer against its neighbours. Wetland, flood and terrain are the same physical story told three ways, and they cross-check each other: a wetland polygon in a mapped floodplain is a more robust signal than either alone, and a wetland polygon on a slope is worth a second look at the imagery. The flood side is set out in FEMA NFHL flood zones, and the way constraint layers combine into a usable envelope in parcel-to-buildable analysis.

Fifth, decide whether to escalate, and be explicit about the trigger. Any mapped wetland on a parcel where the development programme touches that ground warrants a delineation before the site plan is fixed. Any mapped wetland adjacent to the parcel warrants one where the programme involves grading, fill or stormwater discharge near the boundary. A site in a state running its own wetland permitting programme warrants one at a much lower threshold than the federal test would suggest. And a site with no mapped wetland at all warrants one when the imagery, the topography or the soil survey disagree with the map, which brings us to the last section.

Six ways this screen is wrong

Every one of these is a documented limitation rather than a hypothetical.

Mapped absence is the weakest signal the screen produces. The inventory is built by photointerpretation at a scale intended for use with base maps at 1:12,000 or smaller. Small wetlands, narrow features and drier-end wetlands that read as ordinary vegetation from the air are the ones that scale misses first, and a delineation applying the three field criteria of the Corps manual regularly finds wetland where the map shows none. A mapped presence is a question. A mapped absence is not an answer.

The inventory is broader than jurisdiction in the other direction. Because the inventory definition is a one-factor test and the regulatory definition requires vegetation, a polygon can be a wetland for inventory purposes and not a wetland under 33 CFR 328.3 at all, quite apart from whether it has the continuous surface connection Sackett requires. Both errors run in the same file at once, which is why the polygon is a routing instruction rather than a conclusion.

Known under-representation is documented. Because aerial imagery is the primary detection source, seagrasses and submerged aquatic vegetation in intertidal and subtidal zones and some deepwater reef communities may be under-represented or excluded, and by policy the Service also excludes certain farmed wetlands as defined by the Food Security Act or falling outside the classification definition.

Vintage is invisible on the rendered map. A seamless national layer is not a synchronised national survey, and the project metadata that dates a polygon does not exist everywhere.

The soil and water chemistry levels are generally blank. The two modifiers that would speak to construction cost are the two the standard says are generally not applied to remotely sensed data.

The layer defines nobody's jurisdiction, and says so. The use constraints in the dataset metadata are unambiguous: federal, state and local regulatory agencies with jurisdiction over wetlands may define and describe wetlands in a different manner than that used in this inventory, and there is no attempt, in either the design or products of this inventory, to define the limits of proprietary jurisdiction of any federal, state or local government or to establish the geographical scope of the regulatory programs of government agencies. The Service adds that persons intending to engage in activities involving modifications within or adjacent to wetland areas should seek the advice of appropriate federal, state or local agencies.

What the screen is worth

Read this way the wetlands layer earns its place in an early file, and the reason is not that it answers the wetland question. It is that it prices the wetland question before anyone spends on answering it.

A decoded polygon with a recorded vintage tells a credit officer three things a term sheet needs: that a delineation is probably required, roughly what kind of wetland the delineator will be arguing about, and whether the state layer is going to be the binding constraint rather than the federal one. That is enough to scope an engagement correctly the first time and to put a permitting allowance in the timeline before the borrower has committed to a site plan. It is not enough to conclude anything about jurisdiction, and a file that treats it as though it were has inherited a risk it did not price.

The discipline generalises. This layer belongs to the same early pass as the rest of the due diligence data stack and gets used at the same moment as the pre-term-sheet site screen, and in every one of those checks the honest output has the same shape: a dated observation, the scale it was made at, and an explicit statement of what it does not decide.

MMCG Analytics is a map-first commercial real estate analytics platform for lenders and investors, built by MMCG Invest, LLC of San Francisco on federal, state and public data sources with source and vintage provenance carried on displayed values. Wetlands is one of its analytical layers, sourced from public records. Jurisdiction is determined by the U.S. Army Corps of Engineers, delineation by qualified professionals, and the credit decision rests with the lender.

Frequently asked questions

What is the National Wetlands Inventory?

It is the U.S. Fish and Wildlife Service's national map of wetland and deepwater habitats, produced under the Emergency Wetlands Resources Act of 1986 and maintained as the Wetlands Layer of the National Spatial Data Infrastructure under the Geospatial Data Act of 2018. The Service states the layer contains more than 37 million wetland and deepwater features and is updated twice a year. It is an ecological inventory of the resource, not a map of regulatory jurisdiction.

Can you use the National Wetlands Inventory instead of a wetland delineation?

No. The dataset metadata states that the data are intended for use with base maps at a scale of 1:12,000 or smaller and that the primary intended use is regional and watershed display and analysis rather than specific project data analysis. A delineation is field work performed under the 1987 Corps of Engineers Wetlands Delineation Manual and the applicable regional supplement, of which the Corps publishes ten. The inventory tells you whether to commission that work; it does not replace it.

What does a wetland code like PFO1A mean?

It is the Cowardin classification carried on every polygon. The Fish and Wildlife Service decodes the example as Palustrine (P), Forested (FO), Broad-leaved Deciduous (1), Temporarily Flooded (A). The classification of record is FGDC-STD-004-2013, second edition, which sets out five systems, eight subsystems, eleven classes, forty-one subclasses, nineteen water regime modifiers and eight special modifiers. Temporarily flooded means surface water is present only for a few days to a few weeks in the growing season, so a mapped wetland can be dry ground on the day someone walks it.

Does a wetland on the National Wetlands Inventory mean the property is regulated?

Not by itself. Jurisdiction under the Clean Water Act is determined by the U.S. Army Corps of Engineers, and since Sackett v. EPA in 2023 adjacent wetlands are covered only where they have a continuous surface connection to a covered water. The inventory's own use constraints state that agencies with jurisdiction may define wetlands differently and that no attempt is made to define the limits of anyone's regulatory jurisdiction. Separately, many states regulate wetlands that federal law does not reach.

How do you find out how old a National Wetlands Inventory polygon is?

Select the polygon in the Wetlands Mapper and open the Project Metadata link in the pop-up window, which records the source imagery scale, type and date along with the inventory method and data limitations for that mapping project. A Historic Map Info link carries older map information where it exists. The Service states that not all areas have either document, so a documented absence of project metadata is itself a valid finding to record.

How much wetland is there in the United States, and is it shrinking?

The Fish and Wildlife Service estimated 116.4 million acres of wetland in the conterminous United States in 2019, under 6 percent of its land area, with 95 percent freshwater. Between 2009 and 2019 there was a net loss of 221,000 acres, but that total conceals the composition shift: vegetated wetlands fell by 670,000 acres while non-vegetated wetlands, mainly ponds, rose by 488,000 acres. The report notes this pattern has obscured vegetated wetland losses.

What is the difference between an approved and a preliminary jurisdictional determination?

Under 33 CFR 331.2 an approved jurisdictional determination is a Corps document stating the presence or absence of waters of the United States on a parcel, or a written statement and map identifying their limits, and it is an appealable action. A preliminary jurisdictional determination is a written indication that there may be waters of the United States on a parcel, or of their approximate location; it is advisory in nature and may not be appealed.

What is driving wetland loss in the United States?

For 2009 to 2019 the Fish and Wildlife Service attributes net loss of vegetated freshwater wetlands to upland agriculture (211,000 acres), upland forested plantation (107,000 acres), upland other (86,000 acres), upland urban (64,000 acres) and upland rural development (51,000 acres). Farming and forestry together account for nearly three acres for every one attributable to urban and rural development, a reversal of the 1986 to 1997 position when development was associated with 53 percent of net loss.

Sources

1. U.S. Fish and Wildlife Service, National Wetlands Inventory dataset metadata (FGDC Content Standard for Digital Geospatial Metadata record for the Wetlands Data Layer), publication date 1 May 2024, metadata date 1 October 2023; cited for the 1:12,000 and 1:250,000 scale statements, the regional-and-watershed intended use, the complete use constraints, the polygon count of 36,091,275, the biannual update frequency, the ATTRIBUTE definition and the PFO1A example, the WETLAND_TYPE domain, the image-interpretation accuracy statement and the farmed-wetland and submerged-vegetation exclusions. https://documentst.ecosphere.fws.gov/wetlands/data/metadata/FWS_Wetlands.xml
2. Federal Geographic Data Committee, 2013. Classification of wetlands and deepwater habitats of the United States. FGDC-STD-004-2013, Second Edition. Wetlands Subcommittee, Federal Geographic Data Committee and U.S. Fish and Wildlife Service, Washington, DC. Adapted from Cowardin, Carter, Golet and LaRoe (1979); cited for the five systems, the subsystem and class structure, Table 1 subclasses, the water regime, salinity, pH, soil and special modifiers, the minimum classification standard, the statement that soil thickness is of critical importance to engineers planning construction, and the statement that water chemistry and soil modifiers generally are not used when classification data are obtained using remote sensing. https://www.fws.gov/sites/default/files/documents/Classification-of-Wetlands-and-Deepwater-Habitats-of-the-United-States-2013.pdf
3. U.S. Fish and Wildlife Service, National Wetlands Inventory, Wetland Classification Codes, read 25 August 2026; cited for the official decode of PFO1A as Palustrine, Forested, Broad-leaved Deciduous, Temporarily Flooded, and for the code definitions download package and code interpreter tool. https://www.fws.gov/program/national-wetlands-inventory/classification-codes
4. U.S. Fish and Wildlife Service, National Wetlands Inventory, Wetlands Data, read 25 August 2026; cited for the statement of more than 37 million wetland and deepwater features, the geographic coverage list, the twice-yearly update, the 50 to 100 million acres of annual updates, and the FGDC adoption of the classification and mapping standards binding on federally funded wetland mapping projects. https://www.fws.gov/program/national-wetlands-inventory/wetlands-data
5. U.S. Fish and Wildlife Service, National Wetlands Inventory, Wetlands Data Metadata levels, read 25 August 2026; cited for the FGDC record, project level metadata contents, the historic wetlands map information document, the Wetlands Mapper access route and the statement that not all areas have either document. https://www.fws.gov/node/264592
6. Lang, M.W., Ingebritsen, J.C. and Griffin, R.K. 2024. Status and Trends of Wetlands in the Conterminous United States 2009 to 2019. U.S. Department of the Interior, Fish and Wildlife Service, Washington, D.C., 43 pp. Report to Congress; cited for the 116.4 million acre 2019 estimate, the type-level areas, Table 2 and Table 4 area and change values with their coefficients of variation, the 5,048 plot and 215 strata design, the 1,034 field-verified plots across 46 states, the driver attributions, the 1986 to 1997 driver shares, the pond gains by source and the statement that wetlands were identified using a biological definition that differs from the federal regulatory definition and does not imply regulatory jurisdiction. https://www.fws.gov/sites/default/files/documents/2024-04/wetlands-status-and-trends-report-2009-to-2019_0.pdf
7. Wakeley, J.S. 2002. Developing a "Regionalized" Version of the Corps of Engineers Wetlands Delineation Manual: Issues and Recommendations. ERDC/EL TR-02-20, Environmental Laboratory, U.S. Army Engineer Research and Development Center, Vicksburg, MS, August 2002; cited for the comparison of the inventory, Corps and EPA, and Food Security Act wetland definitions, the statement that the inventory definition is broader because it is a one-factor test, and the narrowing of the regulatory definition to vascular macrophytes in the delineation manuals. https://usace.contentdm.oclc.org/utils/getfile/collection/p266001coll1/id/7606
8. Code of Federal Regulations, 33 CFR 328.3, definition of waters of the United States including adjacent wetlands with a continuous surface connection, the definition of adjacent and the definition of wetlands; source line 88 FR 3142, 18 January 2023, as amended at 88 FR 61968, 8 September 2023; current text read 25 August 2026. https://www.law.cornell.edu/cfr/text/33/328.3
9. Code of Federal Regulations, 33 CFR 331.2, definitions of jurisdictional determination, approved jurisdictional determination and preliminary jurisdictional determination, including which is an appealable action and which is advisory; current text read 25 August 2026. https://www.law.cornell.edu/cfr/text/33/331.2
10. U.S. Environmental Protection Agency, Waters of the United States, page updated 25 June 2026; cited for the 12 March 2025 joint guidance with the Department of the Army on implementing continuous surface connection, and the 17 November 2025 proposed rule with its comment period closing 5 January 2026. https://www.epa.gov/wotus
11. U.S. Environmental Protection Agency, Section 404 Permit Program, page updated 17 February 2026; cited for the scope of Section 404, the Corps and EPA roles, individual and general permits and the avoid, minimise and compensate sequence. https://www.epa.gov/cwa-404/section-404-permit-program
12. Sackett v. Environmental Protection Agency, No. 21-454, decided 25 May 2023; slip opinion syllabus, cited for the holding that adjacent wetlands are covered only where they have a continuous surface connection to covered waters such that there is no clear demarcation between the two. https://www.supremecourt.gov/opinions/22pdf/21-454_4g15.pdf
13. U.S. Army Corps of Engineers, Regional Supplements to Corps Delineation Manual, read in the browser 25 August 2026 because usace.army.mil refuses non-browser clients; cited for the 1987 Corps Wetland Delineation Manual and the ten regional supplements with their wetland determination data sheets. https://www.usace.army.mil/Missions/Civil-Works/Regulatory-Program-and-Permits/reg_supp/
14. Wisconsin Department of Natural Resources, State Wetland Regulations Remain Intact After SCOTUS Sackett Decision, news release of 2 June 2023; cited for the department's statement that it would continue to implement the state's uniform wetland regulatory program and for 2001 Wisconsin Act 6 covering isolated, non-federal wetlands. https://dnr.wisconsin.gov/newsroom/release/77381
15. Colorado General Assembly, House Bill 24-1379, Regulate Dredge and Fill Activities in State Waters, approved and effective 29 May 2024; cited for the December 2025 rulemaking deadline, the inclusion of wetlands in state waters, the coverage of isolated wetlands, ponds and streambed reaches, the Section 404(b)(1) baseline and the one tenth of an acre and three hundredths of an acre general authorisation thresholds. https://leg.colorado.gov/bills/hb24-1379
16. MMCG Research, tabulation of the FGDC-STD-004-2013 classification structure (code values by level, imagery-assignable against field-dependent levels) and of the instrument comparison in emblem six, 25 August 2026. https://mmcganalytics.com