SR 520 Bridge Replacement and HOV Program - Union Bay Natural Area mitigation

Updated: May 2018


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.

overview map of Union Bay Natural Area

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 across Union Bay. The UBNA Mitigation Project, completed in early 2018, added and enhanced approximately 22 acres of wetlands and buffer areas at the UBNA site.

photo of wetland in Union Bay Natural Area



The UBNA mitigation project includes:

  • Enhancement of existing wetlands.
  • Creation of new wetlands.
  • Removal of invasive and non-native plants -- including purple loosestrife, blackberry, yellow garden loosestrife, and reed canary grass -- on 19 acres of the site.
  • Planting of more than 70,000 native plants in the wetlands and buffer areas, and more than 2,700 trees.
  • Protection of wetlands by improving buffer  areas.



View a UBNA project map (pdf 845 kb) of the various wetland areas within the Union Bay Natural Area

Project timeline

  • Spring 2016: Begin construction.
  • Early 2018: Construction completed.
  • Approximately 10 years following completion: Monitoring and maintenance period to verify mitigation goals have been met