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Site history

Until the 20th century, the Duwamish River was a wide, meandering river with large areas of mudflats and marshes. The river is named for the Duwamish Tribe, who lived throughout the surrounding area.

The Duwamish begins as the Green River in the Cascade Mountains of Southeast King County. The Green becomes the Duwamish at its former confluence with the Black and White Rivers. The White River changed course after a flood in 1906 and now flows into the Puyallup River. The Black River dried up after the 1911 diversion of the Cedar River into Lake Washington and the 1916 opening of Seattle’s Ship Canal.

In the early 1900s, the lower (northern) section of the river was straightened and dredged for industrial development. By the 1940s, channelization had transformed a 9-mile estuary into the 5-mile Lower Duwamish Waterway (LDW). Some of the dredged material was used to create Harbor Island and some was used to fill in parts of the old channel. Since 1916, the LDW has been regularly dredged to support ship navigation.

The waterway serves as a major shipping route for containerized and bulk cargo. Common shoreline features include constructed bulkheads, piers, wharves, buildings extending over the waters, and steeply sloped banks armored with fill materials.

Although the LDW is now viewed primarily as an industrial corridor, two residential neighborhoods border the banks of the river: South Park and Georgetown. Both were independent towns in the early 1900s before being annexed by the city of Seattle.

Cleanup and restoration

The LDW Superfund Site is an approximately 5-mile section of the Duwamish River which flows northward into Elliott Bay in Puget Sound. It extends from the southern tip of Harbor Island to the end of the straightened waterway in Tukwila.

In 1998, the U.S. Environmental Protection Agency (EPA) investigated the sediments (mud at the river bottom) in the LDW and found many harmful chemical compounds and toxic metals.

In 2001, EPA listed the LDW sediments and its sources of contamination as a federal Superfund site. The EPA’s Superfund National Priorities List includes the nation’s most-contaminated sites, targeting them for investigation and cleanup.

Ecology and the EPA jointly entered into a legal agreement called an Administrative Order on Consent (AOC) with the Lower Duwamish Waterway Group, a partnership formed between the City of Seattle, King County, Port of Seattle, and the Boeing Company.

Under the AOC, Lower Duwamish Waterway Group was required to perform a remedial investigation and feasibility study for LDW sediments. The remedial investigation, completed in 2010, assessed potential risks to human health. The feasibility study, completed in 2012, evaluated cleanup alternatives.

EPA is the lead agency for investigation and cleanup within the waterway — about 450 acres of sediments. In 2014 the EPA released their cleanup plan, called the Record of Decision. Cleanup methods will include dredging, capping, and natural sedimentation.

We are the lead agency for source control — finding and reducing sources of contamination to LDW sediments, from more than 20,000 acres of land draining into the river. Our Toxic Cleanup and Water Quality programs collaborate with other regulatory agencies to implement source control. Visit our Source Control page for more information.

Sediment contamination

At least 41 different hazardous chemicals have been found in LDW sediments. The contaminants of greatest concern include:

  • Polychlorinated biphenyls (PCBs)
  • Carcinogenic polycyclic aromatic hydrocarbons (cPAHs)
  • Dioxins/furans
  • Arsenic

Sediment contamination in many areas exceeds the Washington Sediment Management Standards. Many sources have contributed to the contamination, including historical past practices regarding waste disposal, industrial use, combined sewer overflows, and more than 200 storm drain outfalls.

For decades, much of the land adjacent to the LDW has been industrialized. Current commercial and industrial operations include:

  • Cargo handling and storage
  • Automotive and chemical drum recycling
  • Marine construction
  • Boat manufacturing
  • Marina operations
  • Concrete manufacturing
  • Airplane parts manufacturing
  • Paper and metals fabrication
  • Food processing

Early action areas

The concentrations of sediment contamination in the LDW vary widely. As part of the first phase of the remedial investigation, EPA identified five priority areas of the waterway that had higher contamination levels that were identified for early cleanup. These early action areas included Duwamish/Diagonal Way, Slip 4, Boeing Plant 2/Jorgensen Forge, Terminal 117, and Norfolk CSO.

King County’s Sediment Management Program completed sediment removal at Norfolk CSO in 1999, before the Superfund work began in the waterway. They also completed sediment removal at Duwamish/Diagonal Way in 2005.

With EPA oversight, LDWG has completed sediment cleanup at three additional Early Action Areas: Slip 4, Boeing Plant 2/Jorgensen Forge, and Terminal 117.

Habitat restoration

The mouth of the Duwamish is an estuary, where saltwater from Puget Sound and freshwater from the river mix. Water levels fluctuate with the tide and amount of water in the river. Broad tideflats once provided rich habitat for clams and oysters. A mosaic of grassy islands and eelgrass beds provided ideal habitat for shorebirds, juvenile salmon, and spawning herring.

More than 97 percent of the wildlife habitat that existed in the Duwamish River was destroyed when the waterway was turned into a channel. There are still small patches of intertidal habitats where birds, fish, and marine invertebrates live. Kellogg Island is the largest continuous area of intertidal habitat remaining in the LDW.

Natural Resource Damage Assessment and Restoration (NRDAR) is a process for determining the damages after an oil spill, ship grounding, or hazardous release. Federal, state, and tribal trustee agencies identify negative impacts to natural resources resulting from the incident. The responsible parties must then complete restoration projects to make up for the injury caused to wildlife and habitat.

The Elliott Bay Trustee Council oversees the NRDA process for injury to natural resources caused by contaminated discharges to the LDW. A 1991 settlement with the City of Seattle and King County (Metro) resulted in 4 restoration projects on the LDW:

The Trustee Council finalized its Restoration Plan in 2013, and several more restoration projects have already been completed by The Boeing Company and Bluefield Holdings Inc. The Trustee Council continues to pursue settlements for additional projects for restoration of mudflats, marshes, and shoreline vegetation on the river.

The Port of Seattle has also completed several other restoration projects on their property, including Terminal 117.