Wetland mitigation

Wetland mitigation

This page contains technical information to complete every stage of wetland mitigation for your project.

Mitigation must follow the Federal Rule on Compensatory Mitigation for losses of aquatic resources that governs compensatory mitigation for activities authorized by permits issued by the Department of the Army (2008 Federal Rule), and State and Local government requirements. Refer to Environmental Manual Chapter 431: Wetlands for policies driving wetland mitigation.

On this page:

After using this page, go to our Mitigation Proposals page for information on how to develop your compensatory mitigation proposal.

Plan for mitigation

Understand the watershed context of the area and availability of existing mitigation resources in the watershed when planning for mitigation needs. Mitigation credits may be available to compensate for unavoidable impacts including mitigation bank credits, in-lieu fee credits, and credits from approved advance mitigation sites.

Federal, tribal, state, and local regulations require mitigation sequencing to protect wetlands.  Mitigation sequencing requires the applicant to:

  1. Avoid impacts the project may have on wetlands and other aquatic resources.
  2. Minimize unavoidable impacts.
  3. Compensate for unavoidable impacts through required compensatory mitigation.

You must apply mitigation sequencing to identify unavoidable impacts and explain the proposed compensatory mitigation for all projects. If long-term planning for development of mitigation resources occurs in advance of project development, you still must apply mitigation sequencing. The agencies won’t determine the adequacy of proposed compensatory mitigation until they review a complete project proposal.

Complete proposals include:

  • Details of impact avoidance.
  • Details of minimization.
  • Proposed compensatory mitigation for unavoidable impacts.  

Part 1 of Wetland Mitigation in Washington , on the Washington State Department of Ecology’s (Ecology) webpage, explains the requirements of mitigation sequencing in detail.


Identify existing aquatic resources

Identify existing conditions of aquatic resources to quantify impacts. See the Wetland and stream reconnaissance and assessment page to determine presence of wetlands or other aquatic resources in the work area. Prepare a wetland and stream assessment report.


Research compensatory mitigation options

Begin researching mitigation options early to allow time to coordinate with other planning efforts and forecast mitigation needs. Early coordination can aid in planning sustainable and effective watershed-based solutions and may expand the range of mitigation options for project impacts. Identify mitigation opportunities and needs during environmental planning, corridor studies, and scoping phases.

When planning for compensatory mitigation, consider:

  • Quantity and type of potential wetland and aquatic resource impacts
  • Anticipated compensatory mitigation needs
  • Available mitigation credits
  • Timing of potential impacts

Compensatory mitigation must comply with regulatory requirements. Refer to Environmental Manual Chapter 431: Wetlands for policies driving wetland mitigation, and follow the 2008 Federal Rule order of preference for compensatory mitigation:

  1. Wetland mitigation banks (established by WSDOT or others)
  2. In-lieu fee programs (established by a non-profit entity or a government agency involved in natural resource management)
  3. Advance mitigation established by WSDOT (permittee-responsible mitigation)
  4. New WSDOT compensation site (permittee-responsible mitigation)

Minimize the effect of wetland impacts over time by using mitigation credits established prior to project impacts. Mitigation banks and advance mitigation projects establish the compensation resource before project impacts occur, acting to reduce temporal impacts. In-lieu fee programs promise to deliver compensation at a future date. Agencies must approve in-lieu fee program instruments, including identification of initial sites, before any credits are released for use. Each approach has specific requirements to meet before credits can be used.

If credits aren’t available from mitigation banks, in-lieu fee programs, or advance mitigation sites, then plan permittee-responsible mitigation. Find Guidance on preparing Mitigation Plans on Ecology’s Wetland Mitigation in Washington State page.

Consider opportunities to develop partnerships in mitigation development with other natural resource management entities or local jurisdictions. If possible, establish willing partners to transfer WSDOT-owned wetlands to long-term management. These qualified entities must agree to restrict the use of the property to preserve the natural and beneficial values of the wetland (RCW 47.12.370). Work with your region biologists and mitigation specialists to develop a compensatory mitigation project that meets the mitigation needs and regulatory requirements of your project.

Develop a mitigation strategy that results in the greatest ecological benefit while making efficient use of financial resources. Compensatory mitigation should make ecological sense in the landscape context in which it occurs. Prioritize compensation projects that restore environmental processes at the site scale or at a larger landscape scale. Find more information about evaluating landscape and site scale environmental processes on Ecology’s Watershed Characterization page.

To incorporate high quality wetland as preservation into a mitigation plan, refer to Mitigation Tools for Special Circumstances: Preservation of High Quality Wetlands (pdf 124 kb) and the Wetland Mitigation in Washington State page.


Identify impacts to aquatic resources

Identify wetland impacts with an impact assessment. Compare the surveyed wetland boundaries from the Wetland and stream assessment to proposed impact areas within the project footprint during environmental review.

For each wetland impact, include:

  • Area of wetland or other aquatic resources impacted
  • Type of impact
  • Expected duration of impact
  • Wetland category
  • Ecology wetland rating
  • Wetland functions assessment

Common transportation project activities that may impact wetlands or other waters include:

  • Filling wetlands
  • Draining wetlands
  • Altering natural drainage patterns
  • Increasing or decreasing water levels
  • Discharging sediment or toxicants in runoff
  • Mechanically removing wetland vegetation
  • Compacting wetland soils
  • Using wetlands as staging areas
  • Altering wetland or stream buffer areas
  • Shading impacts to wetlands
  • Converting aquatic resource

Impacts to wetlands and other aquatic resources include:

  • Permanent - when work results in the permanent loss of wetlands or other aquatic resources.
  • Long-term temporary - when work affects wetland functions, and the functions are restored in a year or more following impacts.
  • Short-term temporary - when work affects wetland functions, but functions are restored within one year or within one growing season following impacts.
  • Indirect - when work may affect the functions of wetlands or other aquatic resources.
  • Loss of a wetland - when the entire area of the wetland is permanently impacted or no longer provides any functional value.
  • Aquatic resource conversions - For example, converting a wetland to a stream channel during a fish passage barrier correction, or a forested or scrub-shrub wetland to an emergent wetland. 

Regulators may not require compensatory mitigation for unavoidable impacts for projects designed for aquatic habitat restoration or enhancement if they result in net increases in aquatic resource functions and values. Find more information on the Fish passage policies and procedures and Section 404 & 10 Nationwide Permits pages.

See Ecology’s Wetland Mitigation in Washington State page for detailed definitions of the different types of impacts and when regulators may require compensatory mitigation.