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Wetland Mitigation

Weeping Willow at SR 16 Wetland Site

Mitigation Options
Advance Mitigation
Mitigation Banks
In Lieu Fee Programs
Permittee-Responsible Mitigation
Mitigation Site Buffers

Wetland mitigation compensates for adverse effects transportation projects have on wetlands, streams, lakes, and other aquatic resources. The mitigation process follows a specific sequence of actions required to comply with WSDOT's policy for no net loss and protecting wetlands (pdf 58 KB) and state and federal regulations. 

The required sequence of mitigation is:

  • avoid impacts,
  • minimize impacts,
  • rectify impacts,
  • reduce the impact over time,
  • compensate for impacts, and
  • monitor the impact.

This means that if, after avoiding and minimizing impacts by alignment and design, the project still has adverse effects on wetland resources, then WSDOT is required to monitor the construction project to be sure the impacts were as permitted, and provide compensatory mitigation to offset those impacts.

WSDOT project teams begin evaluating available compensatory mitigation options during the Planning phase of the Transportation Decision-Making Process. This evaluation continues in the Scoping phase, and in coordination with permitting agencies, project teams make a firm decision on which mitigation option to use in the Environmental Review phase.

Evaluating Mitigation Options

The 2008 Final Rule on Compensatory Mitigation for Losses of Aquatic Resources (pdf 568 kb) discusses mitigation options. The list below follows the generally preferred order of consideration.

  • Mitigation Banks - the bank sponsor develops a mitigation site, and after it is certified, can sell or use credits to compensate for later project impacts.
  • In-Lieu Fee - the sponsor develops a plan for a mitigation site, and after the plan is certified, sells credits to compensate for impacts. The In-Lieu Fee (ILF) sponsor uses the income to develop and monitor the mitigation site. 
  • Permittee-Responsible Mitigation – the permittee plans, designs, permits, constructs and monitors a new mitigation site to compensate for project-specific impacts.

For additional information, see the sections below and the Washington State Department of Ecology “Mitigation that Works” web page.

WSDOT uses project impact assessments to determine if a mitigation option can replace lost functions and provide appropriate compensation. When making decisions about compensatory mitigation options, the project team selects the most cost-effective option that yields the greatest environmental benefit. WSDOT coordinates with regulatory agencies early in the Environmental Review phase to ensure concurrence on the choice of mitigation options prior to submitting a permit.

WSDOT typically evaluates mitigation options in the following order of preference:

  1. Using Existing WSDOT Mitigation Value

  2. WSDOT determines if existing investments are available and appropriate to compensate for the proposed impact. Using existing value often results in more favorable mitigation ratios, and may be available to WSDOT projects at no cost.

    • Excess credit at a constructed advance mitigation site (this is a form of Permittee-Responsible Mitigation) 
    • Credit at a WSDOT Mitigation Bank 
  3. Purchasing Non-WSDOT Mitigation Bank or ILF credit

    Purchasing mitigation credit from a certified non-WSDOT mitigation bank or In-Lieu Fee program may be a cost-effective option. A benefit of these options is the sponsor is responsible for all aspects of the mitigation site. The project impact needs to be within a service area, and the available mitigation needs to compensate for the impact. It is important to contact the sponsor directly to ensure the availability of appropriate credit. WSDOT’s process to procure credit (pdf 46 kb) from non-WSDOT mitigation banks ensures applicable procurement laws are followed.

  4. Developing New WSDOT Mitigation (Permittee-Responsible)

  5. If other options are not feasible or appropriate as compensation, WSDOT develops new project-specific compensatory mitigation. WSDOT uses an impact assessment to be sure developed mitigation plans provide adequate compensation. Developing a new mitigation site includes design, permitting, construction, monitoring and long-term management.

    • Advance mitigation - Advance mitigation is a form of permittee-responsible mitigation planned, designed, permitted, constructed and monitored for at least 2 years before projects obtain permits for an impact. 
    • Concurrent Mitigation - WSDOT plans and designs mitigation, permits impacts and constructs mitigation sites concurrent with project impacts, and later monitors the mitigation site.

Chapter 431 04(2) of the Environmental Manual provides additional information on WSDOT’s policy on mitigation options.

Additional information on advance mitigation, WSDOT’s mitigation banks, In Lieu Fee programs, and WSDOT project-specific (permittee-responsible) mitigation follows below.

Advance Mitigation

Advance mitigation can be a cost effective and ecologically beneficial mitigation option. There is risk that the mitigation site will not be used if projects it is constructed for are not funded. Advantages of advance mitigation from the regulatory perspective are the greater certainty of mitigation success and reduced temporal loss of wetland function. The benefits of advance mitigation can include:

  • If the mitigation site is already developed, permitting time may be reduced for future projects 
  • More value per unit area than concurrent mitigation 
  • Sites can be larger to provide compensation for several projects 
  • Larger sites provide more ecological function and reduced monitoring and management costs compared to several individual sites

How is Advance Mitigation different?

In contrast to Non-WSDOT mitigation banking, and In-Lieu Fee Programs, WSDOT remains responsible for mitigation success and management in perpetuity. Advance mitigation value cannot be sold to a party that was not part of the mitigation site's development.

Early Planning and Design

WSDOT considers advance mitigation within the context of other feasible mitigation options during the project planning phase. Planning for an advance mitigation site requires an early assessment of wetlands, and knowledge that future projects in the vicinity will have unavoidable wetland impacts.

Design of an advance mitigation site begins in the Scoping phase of the first project to use the mitigation credit. Mitigation planning, coordination, design, construction, and the required 2-year establishment period may require a 3 to 4 year lead time before credit is available for construction impacts. Designing an advance mitigation site is similar to designing a concurrent mitigation site. The mitigation plan identifies how credit value will be created and how its use will be tracked.


Advance mitigation projects are permitted during the Scoping phase of the first project, before construction. Separate permits are required for transportation project impacts during the Permitting phase. The ratio of mitigation to impact area depends on the condition and function of the advance mitigation site at the time credits are used. The process for using advance credits includes evaluating the suitability of mitigation value to compensate for project impacts and maintaining a ledger of available area.

The Washington State Departments of Transportation, Ecology and Fish and Wildlife and the US Army Corps of Engineers have developed guidance on advance mitigation available on Ecology’s “Mitigation That Works” web page.

WSDOT Mitigation Banks

As a bank owner, WSDOT created, restored, enhanced and preserved functioning wetlands prior to environmental impacts. These acres were converted to “bank credits” that are available for use as compensation for unavoidable wetland impacts within the bank's specified service area. Because mitigation banks generate ecological functions before wetland impacts, the value of the mitigation increases as the site matures prior to its use as mitigation.

WSDOT has three certified mitigation banks

 Map of Bank Sites
Additional information about WSDOT's 3 banks is available on the following fact sheets.


WSDOT also uses private mitigation banks

WSDOT has a process to procure credit (pdf 46 kb) from private mitigation banks.

In-Lieu Fee Mitigation

In-Lieu Fee (ILF) mitigation is an option to pay another party to provide mitigation instead of building a project-specific mitigation site. This option has the benefit of transferring the responsibility for developing, monitoring and long-term management to the ILF sponsor. ILF sponsors are government agencies or non-profit natural resource management entities. Early coordination with regulatory agencies streamlines the permitting process, and is especially important for complex projects.

The process for procuring credit from an In-Lieu Fee program is similar to that developed for procuring mitigation bank credits (pdf 46 kb).

For additional information, see the Washington State Department of Ecology “In-Lieu Fee Mitigation” web page.

Project-specific Mitigation (Permittee-Responsible)

WSDOT provides new, project-specific mitigation if mitigation banks, in-lieu fee programs, or advance mitigation options are not appropriate or available for project impacts. With this option, WSDOT typically purchases land, permits the project, constructs the mitigation site, monitors it for 10 years, and provides stewardship in perpetuity. WSDOT has expert mitigation designers, an independent monitoring staff, and a history of providing successful sites. Project-specific mitigation is constructed concurrently with project impacts, so permitting agencies may require more mitigation area than other options.

Mitigation Site Buffers

WSDOT provides banks, project-specific mitigation sites, and advance mitigation sites with buffers, as required by the local Critical Area Ordinance. City and County Critical Areas Ordinances contain buffer width recommendations.