Restoring Arboretum Creek!

Working together we can help recreate a thriving stream full native plants and lifeforms. Arboretum Creek can be a living legacy for our children, a shining example of humanity living in harmony with nature, the core of a functional ecosystem and an ongoing educational opportunity for generations of King County residents.

The History:

Challenges for Arboretum Creek began in the 19th Century. The Arboretum Creek watershed was extensively logged. The removal of trees and vegetation would have reduced water retention while increasing silt, sediment, and scouring. Without the native vegetation to help moderate seasonal rainfall, human intervention would ultimately be required to manage storm surges. In addition, a trestle was built across Madison Valley to provide streetcar access.

During the next century, the challenges accelerated. The trestle was filled in and the rail line was converted to a roadway e.g. Madison Avenue. In effect, the new road became a dam which diverted the majority of Arboretum Creek. Topographic maps show that the Arboretum Creek Watershed would have originally collected water from as far south as Garfield High School.

With the opening of Montlake Cut in 1917 Lake Washington dropped nearly ten feet exposing an extensive shore of mud around the mouth of Arboretum Creek. For decades the exposed mud around Union Bay was used as a place to dump city refuse.

After the neighborhoods expanded in the 1920’s much of the remaining stream water was diverted into underground piping and combined with the sewer. This Combined Sewer Overflow system still exists and on stormy days combines clean Alder Creek water with street runoff and our overflowing sewer system. The untreated water then flows directly into Montlake Cut.

When Lake Washington Blvd was created the resulting untreated road runoff was aimed directly at the stream and finally, even the small remaining balance of Arboretum Creek was diverted into an underground pipe. This diversion occurs below the Wilcox Bridge. This pipe routes the stream under the boulevard to minimize traffic disturbances. The outflow from this underground pipe is elevated above the final section of the stream and creates a barrier that fish are unable to cross.

The Result:

The small remaining water flow in the upper portion of the stream, starting just below the Japanese Garden ponds, comes to a complete stop during the warmer half of the year. During these months the creek becomes a series of stagnant pools, full of overheated water which nurtures an explosion of unwanted, and often invasive plants. Without water flow, there is insufficient oxygen to support native fish.Lately:

As part of the Arboretum Loop Trail, the upper portion of the Arboretum Creek streamed has been restructured including an initial planting of native vegetation along the shore. Additional growth and plantings would greatly benefit the stream habitat.

The Future:

When the 520 bridge is completed there are plans to remove the lower portion of the stream from the underground pipe below Lake Washington Boulevard.

There are also plans to someday create a large-scale retention facility to stop untreated Combined Sewer Overflow from reaching Montlake Cut.

The Opportunities:

During the last two years, Friends of Arboretum Creek have located and tested Alder and Alley Creek water which is currently being routed into the Combined Sewer Overflow (CSO). This clean water which is uphill from the Japanese Garden ponds historically flowed into Arboretum Creek. Re-uniting these small streams with Arboretum Creek will provide more than 40,000 gallons per day of year-round flow in Arboretum Creek.

Removing this water from the CSO will:

  • Revitalize Arboretum Creek,
  • Reduce pollution in Montlake Cut,
  • Increase capacity in the King County Sewer and
  • Save money for King County Ratepayers.

Multiple relevant stakeholder organizations have responded positively to this concept. Including:

  • Seattle Parks and Recreation
  • Arboretum Foundation
  • University of Washington Botanic Gardens
  • King County Wastewater Treatment Division
  • Seattle Public Utility
  • Seattle Department of Transportation
  • Seattle Parks Foundation

Funding has been approved via the 2019 King County Council-Approved Waterworks program for the next phase of our project. More information is available by Clicking Here.

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