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Wetland delineation resources

Use only approved methods and guidance to delineate wetlands on your property.

What is wetland delineation?

Wetland delineation establishes the existence (location) and physical limits (size) of a wetland for purposes of federal, state, and local regulations.

Wetland delineation is also an element of a jurisdictional determination. This process identifies which water bodies within a project's boundaries meet the definition of "waters of the United States." For more information on this, see the U.S. Army Corps of Engineers (Corps) Regulatory Guidance Letter 16-01 Jurisdictional Determinations.

Important: The Corps, not applicants or their consultants, determines whether or not a wetland is a "water of the United States," which are regulated under the federal Clean Water Act. If the Corps determines a wetland is not subject to the Clean Water Act, the wetland may still be a "water of the state" and subject to regulation by Ecology and local jurisdictions. We do regulate wetlands determined by the Corps to be non-jurisdictional due to their isolation from navigable waters.

Does a wetland delineation expire?

Generally, any delineation done more than five years ago needs to be revisited. This is due to several factors:

  • Wetlands can change significantly in a five-year period due to changes in hydrology, land uses, and plant species composition.
  • Approved jurisdictional determinations by the Corps expire after five years (see the Corps' Regulatory Guidance Letter 05-02, Expiration of Geographic Jurisdictional Determinations).
  • The Corps' 1987 wetland delineation manual has a requirement for comprehensive determinations to "quantitatively describe the vegetation in the past five years" (page 41, Step 5).

Revisiting a wetland delineation that is five or more years old does not necessarily mean a new wetland delineation needs to be done. It means it may be necessary to revisit the site to determine whether the delineation is still accurate or needs to be redone based on current conditions.

Consult with the agencies to discuss your specific wetland delineation.

How do I conduct a wetland delineation?

Delineations should be done using the current approved federal manual and supplements as well as all applicable guidance:

The decision to use the Western Mountains, Valleys, and Coast Regional Supplement, or the Arid West Regional Supplement on a particular field site should be based on landscape and site conditions, and not solely on map location.
  • National Wetland Plant List (NWPL) — The current list should be used in any wetland delineation or determination. Make sure you are using the most current list and reference it in your documentation.
  • Field Indicators of Hydric Soils in the United States (Version 8.1, 2017)
    The soil field indicators presented in the Corps' regional supplements are a subset of the National Technical Committee for Hydric Soils (NTCHS) "Field Indicators of Hydric Soils in the United States" that are commonly found in the region. Any change to NTCHS field indicators represents a change to the subset of indicators for the regions. Check the NRCS hydric soils web site for updates to these indicators.

Wetland delineation should result in three things:

  1. A wetland boundary clearly marked in the field.
  2. A map that clearly identifies data-collection points and the boundaries of the delineated wetland (topographic and aerial site maps are very helpful).
  3. A report that explains how the boundary was determined, which should include:

See our delineation checklist for a detailed list of information that should be in your wetland delineation report, as well as a sample report outline.

NOTE: You may need to hire a qualified wetland professional to help you identify and delineate wetlands and prepare a delineation report. See our tips on hiring a qualified wetland professional.

Wetland delineations during a drought year

Delineations done during a drought year should be done using the methods in Chapter 5 of the regional delineation supplements, which address the following difficult wetland situations:

  • Periods of below-normal rainfall
  • Drought years
  • Years with unusually low winter snowpack

For delineations done during a drought year, we might request supplemental information if it was not conducted using appropriate methods. You can find links to drought monitoring resources on our water supply monitoring web page.