Guides › Wetlands
A wetland is the part of a lot you may not be able to fill, drain, build on or run a drain field through, and a permit to do any of that is slow, expensive and sometimes refused. The U.S. Fish and Wildlife Service's National Wetlands Inventory is the free nationwide map of where they are. LandCheck draws it over every parcel. Here is what the polygons mean, what they cannot tell you, and who decides the rest.
The National Wetlands Inventory (NWI) was drawn mostly by interpreting aerial photographs, with field checks in some areas, and parts of it date from the 1980s. It maps wetlands by a biological definition, so it catches marshes, swamps, wet woods and ponds, but it misses small, forested and seasonally wet areas and draws every edge approximately. The agency's own caution is that the data should not be read as the presence, absence or extent of wetlands covered by any law. In plain words: a polygon on the lot means a wetland is likely; no polygon means nobody has mapped one yet. Only a delineation, flags set in the ground by a wetland scientist, says where the line really is.
Every NWI polygon carries a code from the Cowardin classification, such as PFO1A. The first letter is the system, the next two the class, and the rest describe the plants and how long water stays. The code decoder spells out any string; these are the ones that show up on rural land.
| PEM | Palustrine emergent: an inland freshwater marsh or wet meadow of grasses, sedges and cattails. Often the wettest ground on the lot and the easiest to mistake for a field in August. |
|---|---|
| PSS | Palustrine scrub-shrub: shrubs and young trees under about 20 feet, alders, willows, buttonbush. Frequently an old pasture going back to swamp. |
| PFO | Palustrine forested: a wooded swamp or bottomland hardwoods. Looks like any woods from the road; the giveaway is standing water in spring. |
| PUB / PAB | Palustrine unconsolidated bottom or aquatic bed: a pond, natural or dug. A trailing x means excavated, h means diked or impounded. |
| L | Lacustrine: a lake or large reservoir and its open-water shore, which usually carries its own shoreline setbacks. |
| R | Riverine: a river or stream channel and its banks. Stream crossings for a driveway are a permit of their own in most states. |
| Water regime letter | The last capital letter says how long water stays: A temporarily flooded, C seasonally flooded, F semipermanently, H permanently. An A or B wetland may be dry for most of the year and is still a wetland. |
Section 404 of the Clean Water Act (33 U.S.C. § 1344) requires a permit from the Army Corps of Engineers before dredged or fill material goes into "waters of the United States," which has long included some wetlands. In Sackett v. EPA (2023) the Supreme Court held that the Act reaches only wetlands with a continuous surface connection to a relatively permanent body of water that is itself connected to a traditional navigable water, so that the two are hard to tell apart. Many isolated wetlands and those separated from a stream by a berm or road lost federal coverage, and EPA and the Corps are still rewriting the definition to match the ruling. Two cautions follow. First, no map answers the question for a specific lot; the Corps district issues an approved jurisdictional determination on request, and that letter is what a lender or buyer can rely on. Second, losing federal coverage does not make a wetland free to fill, because the state and the county may regulate it on their own.
A hydric soil formed under saturation, and the USDA soil survey rates every map unit for it. A hydric unit with no NWI polygon is the classic unmapped wetland, and a delineator will walk it first. The reverse also matters: a polygon over a well-drained upland soil is often a photo-interpretation artifact. Turn on the Soil layer and click the wet corner; the soil map guide explains the drainage class and hydric rating in the detail panel.
A delineation is done by a wetland scientist or environmental consultant following the Corps manual and its regional supplement: three tests at each sample point, for wetland plants, hydric soil and signs of water, with flags on the boundary and a report with a map. Some states certify delineators and can review the flags in the field. Expect a few thousand dollars for a small residential lot and considerably more for a large or complicated site, plus survey work if the flags need to appear on a plat. A Corps jurisdictional determination based on the delineation is free to request and generally good for five years. If the seller already has one, ask for the report and its date before you order another.
The wetland itself is off the table for the house, the driveway and the drain field without a permit, and mitigation for permitted fill usually means buying credits from a wetland bank at a price that can exceed the land. The buffer is where most lots are won or lost. Counties set a septic setback from wetlands as part of the health code; Morrison County, Minnesota, for example, keeps tanks and drain fields 25 feet from a wetland and 75 feet from a protected one, while some Florida and Massachusetts towns require 100 feet. Building setbacks and no-disturb zones stack on top. On a narrow lot, a wetland along the back line plus a 100-foot buffer plus a well radius can leave no legal spot for the field, which is why the wetland question comes before the perc test.
Open the map with the wetlands layer on →
Wetland polygons are drawn live from the USFWS National Wetlands Inventory at the inventory's published scale and date. They screen a parcel; a delineation accepted by the Army Corps district and the state or county wetland program is the determination that counts for a permit. General information, not legal advice.