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SR 520 – Union Bay Natural Area Mitigation

About > Environmental Stewardship > Union Bay Natural Area

Updated: May 1, 2017


The Union Bay Natural Area (UBNA) is an undeveloped, 74-acre nature reserve and outdoor research laboratory on the north end of Lake Washington’s Union Bay, less than a mile north of SR 520. For many years, the land served as Seattle’s largest garbage dump. After the landfill was closed in 1966, work began to restore the site to a more natural environment. The land, just east of the University of Washington (UW) campus, is now owned by the university and managed by the College of the Environment.

The UBNA Mitigation Project is a partnership between the UW and the Washington State Department of Transportation (WSDOT) to create and enhance existing wetlands and replace invasive and non-native plants with native wetland plants. The mitigation supports and continues work previously begun by the UW as part of the 2010 UBNA Shoreline Guidelines, and mitigates for adverse effects from the SR 520 West Approach Bridge North project, currently under construction in the Montlake area. When complete, the UBNA Mitigation Project will have added and enhanced approximately 22 acres of wetlands and buffer areas at the UBNA site.

What's new on the UBNA mitigation site?

Short break in Union Bay Natural Area (UBNA) wetland mitigation work due to weather until August

Crews will take a temporary break on the Union Bay Natural Area wetland mitigation project prior to finishing the remaining planting activities in the fall. The mitigation project is nearly complete but record rainfall this spring resulted in some areas of the site being too saturated for planting. Crews will be back onsite to complete the remainder of the planting and trail restoration in early August and expect to complete the project in late August or early September.

During this temporary break you may continue to see small groups of crews onsite working to remove invasive species. Recently, crews have also noticed plant damage due to herbivores. In order to prevent these newly planted species from being eaten, crews will be spraying an organic non-toxic repellent called Plantskydd on some of the plants.


The UBNA mitigation project includes:

  • Enhancing existing wetlands.
  • Creating new wetlands.
  • Removing invasive and non-native plants.
  • Planting native wetland plants.
  • Protecting wetlands by improving the areas that buffer them from adverse effects.

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What to expect during construction

WSDOT and the UW are committed to construction management practices that avoid, minimize and mitigate the effects of WSDOT construction activities on neighbors, park users, and the traveling public. As mitigation work occurs, you can expect:

  • Up to 50 days of trail closures: In order to maintain safety for trail users, the contractor will need to intermittently close the trail along Wahkiakum Lane and the Loop Trail. The contractor is permitted to close the trails for up to 50 days but is required to keep the trails open over the weekends.
  • Restricted construction areas: The contractor will install orange fencing and stakes around project boundaries, research plots and key areas that need additional protection during construction.
  • Staging areas: In order to minimize impacts to the traveling public, the contractor plans to stage construction equipment and offices in the E4 parking lot and near Douglas Road Northeast.
  • Vegetation removal: In order to provide access for construction equipment to the work areas within the UBNA, the contractor will prune select trees and vegetation. The contractor will also remove selected non-native plants and replant with native wetland plants.
  • Restricted construction during bird nesting season: In order to protect nesting birds, the contractor will not be permitted to conduct certain construction activities between March 1 and July 31, such as vegetation removal and excavation. Some activities such as materials stockpiling and other staking activities will be permitted as long as the work does not disturb nesting sites. All construction activities will be monitored by WSDOT and the contractor to ensure that the construction activities do not disturb nesting sites.

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Project timeline

  • Spring 2016: Begin construction.
  • Summer 2017: Construction scheduled to be complete.
  • Approximately 10 years following completion: Monitoring and maintenance period to verify mitigation goals have been met