Wetlands are one of the most common environmental constraints on US development projects. If your site has wetlands - or even if it might have wetlands - you need to understand the delineation process before you can move forward with permitting.
This guide covers the practical side of wetland delineation: what it is, when you need one, how to screen your site using desktop data, what happens in the field, and what the results mean for your project.
What Is a Wetland Delineation?
A wetland delineation is the process of identifying and mapping the boundaries of wetlands on a property. It's performed in accordance with the US Army Corps of Engineers (USACE) Wetland Delineation Manual (1987) and the applicable Regional Supplement.
The delineation determines whether areas on your site meet the federal definition of a wetland and therefore fall under the jurisdiction of the Clean Water Act. If they do, any proposed impacts to those wetlands will require a Section 404 permit from the USACE.
A wetland delineation is not a regulatory determination on its own. The delineation report is submitted to the USACE, which then makes an Approved Jurisdictional Determination (AJD) or Preliminary Jurisdictional Determination (PJD) confirming which features on the site are regulated.
The Three-Parameter Approach
Under the USACE methodology, an area must exhibit all three of the following parameters to be classified as a wetland:
1. Hydrophytic Vegetation
The plant community must be dominated by species that are adapted to saturated or flooded soil conditions. The USACE uses the National Wetland Plant List (NWPL) to classify plants by their wetland indicator status:
- OBL (Obligate): Almost always found in wetlands (>99% of the time)
- FACW (Facultative Wetland): Usually found in wetlands (67-99%)
- FAC (Facultative): Equally likely in wetlands or uplands (34-66%)
- FACU (Facultative Upland): Usually found in uplands (67-99%)
- UPL (Upland): Almost always found in uplands (>99%)
The dominance test, prevalence index, and morphological adaptations are used to determine whether the vegetation parameter is met. In practice, if the dominant plants at a sampling point are rated OBL, FACW, or FAC, the vegetation parameter is likely met.
2. Hydric Soils
The soil must show characteristics of prolonged saturation or flooding. Hydric soils develop distinctive features due to the anaerobic conditions created by waterlogging:
- Gleyed (gray) matrix colors
- Redoximorphic features (rust-colored mottles and concentrations)
- Organic accumulation
- Depleted matrix
- Hydrogen sulfide odor
The USACE uses the Field Indicators of Hydric Soils in the United States guide to identify hydric soil indicators. Soil cores are typically taken to a depth of 20 inches at each sampling point.
3. Wetland Hydrology
The area must experience inundation or soil saturation for a sufficient duration during the growing season to create anaerobic conditions. Direct indicators include:
- Surface water
- High water table (within 12 inches of the surface)
- Soil saturation (within 12 inches of the surface)
- Water marks on trees or structures
- Drift deposits
- Sediment deposits
Indirect indicators include oxidized root channels, water-stained leaves, drainage patterns, and geomorphic position.
All three parameters must be present at a sampling point for it to be classified as wetland. If even one parameter is absent, the area is classified as upland at that point.
Desktop Screening: Before You Go to the Field
Before deploying a field crew, smart consultants screen their sites using desktop data. This step takes minutes and can save significant time and money by telling you where to focus your field effort.
National Wetlands Inventory (NWI)
The NWI maps wetland features across the US using aerial photography interpretation. NWI data tells you where wetlands have been mapped and their Cowardin classification (e.g. PEM1C = palustrine emergent, seasonally flooded).
What NWI tells you: Approximate location, extent, and type of mapped wetlands.
What NWI doesn't tell you: NWI maps are not delineations. They don't capture all wetlands (especially small, forested, or newly formed ones), and the boundaries shown are approximate. You cannot submit NWI data as a substitute for a field delineation.
USDA Web Soil Survey
The NRCS Web Soil Survey shows soil map units and their hydric soil ratings. If the soils mapped on your site include hydric components, wetlands may be present even if the NWI doesn't show any.
Topographic and Aerial Imagery
USGS topographic maps, Google Earth historical imagery, and NAIP aerial photography can reveal landscape features associated with wetlands: low-lying areas, drainage patterns, darker vegetation, standing water, and proximity to streams or rivers.
EcoCheck US
EcoCheck US includes NWI wetland data in every site search, showing wetland types, Cowardin codes, and acreage within your search buffer. For corridor projects, wetland data is reported per corridor point, so you know exactly which sections of a linear route cross wetland areas.
This desktop screening won't replace a field delineation, but it tells you whether wetlands are likely present and where your field crew should focus their sampling effort.
What Happens in the Field
A typical wetland delineation field visit involves:
Paired data points. The field scientist establishes sampling points in areas suspected to be wetland and adjacent upland areas. At each point, they evaluate all three parameters - vegetation, soils, and hydrology.
Data forms. USACE Wetland Determination Data Forms are completed at each sampling point, documenting the plant community, soil characteristics, and hydrological indicators observed.
Boundary flagging. The delineation boundary - the line between wetland and upland - is marked with survey flagging or GPS points. This boundary may be surveyed by a licensed land surveyor for precise mapping.
Streams and other waters. If streams, ditches, or other waterways are present, these are also mapped and may be jurisdictional under the CWA. The USACE defines Ordinary High Water Mark (OHWM) for non-wetland waters.
Regional Supplements. The field methodology varies by region. The USACE has published Regional Supplements for different parts of the country that modify the national manual to account for regional differences in soils, vegetation, and hydrology. Make sure your field scientist is using the correct supplement for your project location.
The Delineation Report
The delineation report is submitted to the USACE and typically includes:
- Project description and location
- Methodology (USACE manual version, Regional Supplement)
- Completed Wetland Determination Data Forms
- Map showing wetland boundaries, data point locations, and other waters
- Photographs of sampling points and wetland features
- Soil boring logs
- Plant species lists
- Summary of findings
The report recommends whether the mapped features are likely jurisdictional under the CWA. However, the final determination of jurisdiction rests with the USACE.
What Happens After the Delineation
Jurisdictional Determination
The USACE reviews the delineation report and makes a jurisdictional determination:
Approved Jurisdictional Determination (AJD): A definitive statement of which waters and wetlands on the site are jurisdictional. Valid for 5 years.
Preliminary Jurisdictional Determination (PJD): A non-binding statement that treats all aquatic features as potentially jurisdictional. Faster to obtain but doesn't provide the same regulatory certainty.
Section 404 Permitting
If your project will impact jurisdictional wetlands, you'll need a Section 404 permit:
Nationwide Permit (NWP): For projects with minor wetland impacts that fit within one of the USACE's pre-authorized categories. Faster and cheaper than an individual permit. Most NWPs limit impacts to 0.5 acres or less.
Individual Permit (IP): For projects with impacts exceeding NWP thresholds or that don't fit any NWP category. Requires a full public interest review and typically takes 6-12 months or longer.
Compensatory Mitigation
Any permitted wetland loss typically requires compensatory mitigation - replacing the lost wetland function and area. Options include:
- Purchasing credits from a wetland mitigation bank
- Making an in-lieu fee contribution
- Permittee-responsible mitigation (on-site or off-site wetland creation, restoration, or preservation)
Mitigation banks are the USACE's preferred option because they provide ecological functions before the impact occurs and are managed by experienced operators.
Cost and Timeline Estimates
A basic wetland delineation for a small site (under 10 acres) typically costs $3,000-8,000 including fieldwork and the report. Larger sites, sites with complex hydrology, or sites requiring multiple field visits can run $10,000-25,000 or more.
The USACE review of a delineation report and issuance of a jurisdictional determination typically takes 60-120 days, depending on the district's workload. NWP verifications may add another 30-45 days. Individual permits can take 12-24 months.
Planning for wetlands early in the project timeline is critical. A wetland delineation should be one of the first things you commission, not an afterthought.
Start With the Desktop Data
Before you commit to a field delineation, check the desktop data. NWI mapping, soil surveys, and topographic analysis can tell you whether wetlands are likely present and where to expect them. This informs your field scope and can save significant survey costs if the site is clearly upland.
EcoCheck US includes NWI data in every search, alongside Critical Habitat, protected areas, species records, and flood zones. Run a search on your site to see the full environmental constraints picture before scoping fieldwork.
Patrick O'Connor is a Freelance Ecologist at Kinterra Consulting and the developer of EcoCheck US - an instant ecological desktop assessment tool for any US location. Try it free at ecocheckus.com.