A rich history, a cleaner future

After more than 13 years of scientific studies and early cleanup efforts, the U.S. Environmental Protection Agency (EPA) released its Record of Decision (ROD) in December 2014. This is an important and welcome milestone for the Lower Duwamish Waterway cleanup.

A legacy of chemical pollution is present in the Lower Duwamish Waterway sediment (mud at the river’s bottom). This pollution came from many sources, including industries along the waterway and stormwater runoff from upland activities, streets, and roads. Most of the pollution is historical, from times before modern pollution controls were in place.

Neighborhoods, tribes, waterfront industries, and recreational users of the Lower Duwamish all stand to benefit from a timely and protective cleanup. The Duwamish corridor helps drive the regional economy, supporting 100,000 maritime and industrial jobs and 25 percent of all manufacturing businesses in King County.

The Lower Duwamish Waterway Group (LDWG, which is made up of King County, the Port of Seattle, the City of Seattle, and The Boeing Company) has been actively cleaning up the pollution for decades. LDWG’s investment of nearly $200 million both on studies to-date and cleaning up “early action” areas will reduce Polychlorinated biphenyl (PCB) contamination found in the waterway sediment by 50 percent. These projects have already made the Lower Duwamish communities a healthier place to live, work, and play.

EPA’s Record of Decision addresses the remaining sediment contamination, directing cleanup actions and offering plans for source control and long-term monitoring in the waterway. The Washington Department of Ecology is working to finalize their strategy to control ongoing upland sources of pollution so the river can be cleaned up and stay clean.

The LDW then and now

At one time, the Duwamish River meandered across lush tidal marshes and mudflats to meet Elliott Bay, providing ideal habitat for fish, birds, and wildlife. A century of development put ever-increasing stresses on the natural resources of the river, particularly in its lower reaches.

In the late 1960s, the tide began to turn, as public concern led to many initiatives to mitigate these stresses. While the Lower Duwamish Waterway (LDW) may never return to its pre-industrialized state, much progress has been made. The following chronology traces the major events that have left their mark on the Duwamish River in the past 120 years.

Duwamish River Chronology


  • 1890: The City of Seattle began construction of a sewer system under a voter-approved bond. Sewage and stormwater was transported via tunnels to discharges into Lake Washington, Duwamish River, Elliott Bay, and Puget Sound.
  • 1906: White River diverted from the Green River into the Puyallup River for flood control after major flood.
  • 1905: Dredging of East and West Waterways began in south Elliott Bay. Dredging of intertidal area created deep draft navigational access and dredged sediments were used as fill for creation of Harbor Island and adjacent Duwamish industrial area.
  • 1909: The King County Commercial Waterway District #1 (KCCWD#1) was formed to replace the lower Duwamish River with a dredged deep draft navigation channel and to fill adjacent aquatic area for subsequent industrial development (e.g., boat building, timber and pulp mills, naval stores, heavy industry, coal, oil, shipping, etc.).
  • 1910: Work by the KCCWD#1 extended to connect the LDW with the East and West Waterways in south Elliott Bay.
  • 1911: Port of Seattle (Port) created by Washington State Legislature.
  • 1913: Tacoma Headworks Dam constructed on the Green River to divert additional water for Tacoma municipal water supply.
  • 1916: Flows from the Cedar River, Lake Sammamish, and Lake Washington tributaries to the Duwamish River (via the Black River) were diverted to the Lake Washington Ship Canal and Chittenden Locks, by US Army Corps of Engineers (USACE).
  • 1915-1920s: Waterway dredging and aquatic area fill continued. The USACE was authorized to maintain the completed Lower Duwamish Waterway and East and West Waterways as Federal navigation channels.
  • 1930s: Fill and industrial development of south Elliott Bay and Duwamish River flood plain continued.
  • 1938: The Diagonal Way Treatment Plant was opened to treat combined sewage from the eastern shoreline of the LDW and the Rainier Beach area.
  • 1945: The Washington State legislature created the Washington Pollution Control Commission, established water quality standards, and created a control board to enforce them. In 1955, the Washington legislature strengthened control standards for industry.
  • 1945: Washington Pollution Control Board issued a report on Sources of Pollution in the Duwamish-Green River Drainage Area, by R.F. Foster. The report noted that a large amount of industrial wastes from metal plating, slaughter houses, packing plants, carbide sludge, acid cleaning, caustic cleaning, spilled oil, etc. were discharged directly into the river along with raw sewage from many municipalities and some treated sewage. The report noted the low dissolved oxygen in the subsurface waters of the upstream portion of the Lower Duwamish Waterway.

The 1950s and 1960s

  • 1955: Washington State Pollution Control Commission report (An Investigation of Pollution in the Green/Duwamish River [Tech. Bulletin # 20]) noted that although much of the industrial wastes and sewage being dumped into the Green/Duwamish River and the Lower Duwamish Waterway was getting some level of treatment, there were still major pollution problems along the river from toxic industrial discharges, inadequate sewage treatment and significant quantities of raw sewage. The report detailed the dissolved oxygen problem, the bacterial problem, and other studies.
  • 1957: Washington Pollution Control Board required industrial discharges to the Green/Duwamish River and the LDW to be sent by sewer lines to treatment plants wherever possible.
  • 1958: Metro (now part of King County) formed to clean up local water bodies and establish a regional sewerage system. The new agency began multiple studies in a number of local water bodies, including the Green/Duwamish River.
  • 1961: The USACE finished construction of the Howard Hanson Dam to control flooding in the winter and fish enhancement in the summer. Metro began clean-up efforts in the Green/Duwamish River.
  • 1963: Summer levels of dissolved oxygen in the Green/Duwamish River were found to be far below minimum requirements for fish because of discharges of untreated wastewater and stormwater over the previous 50 years.
  • 1964: Metro installed automatic monitors in the Green/Duwamish River to begin 17 years of continuous data collection on water quality.
  • 1965: Metro completed its secondary wastewater treatment plant in Renton, discharging the treated wastewater into the Green River. Untreated sewage and stormwater from cities along the Green/Duwamish River was sent to the new treatment plant.
  • 1967: Metro completed the East and West Marginal Way Interceptor Sewers, transferring industrial effluent to the West Point Treatment Plant.
  • 1968: Voters approved the Forward Thrust, authorizing bonds for $138 million dollars of sewer improvements. Approximately $1.4 million was spent in the LDW to separate sanitary and storm sewers and improve drainage.
  • 1969: Metro launched the Industrial Waste Pretreatment Program to remove toxicants before wastewater is discharged into the sewerage system.
  • 1969: The Diagonal Way Treatment Plant was closed, eliminating discharge to the LDW.

The 1970s

  • 1970: Metro began ongoing monitoring in the Green/Duwamish River to chart trends and to detect emerging problems in water quality.
  • 1972: The federal Clean Water Act became law.
  • 1973: The Kent lagoons were closed and wastewater was transferred to the South Treatment Plant in Renton.
  • 1974: Municipalities began to form stormwater utilities.
  • 1974: On September 13th, a PCB transformer was dropped during handling at the General Services Administration dock on Slip 1. Cracked transformer case released 255 gallons of 100% PCB transformer oil to the LDW. EPA assumed cleanup responsibility as Federal Onsite Coordinator. Initial removal of approximately 80 gallons of oil from the river bottom was performed in October 1974. Final cleanup of sediments in March 1976 achieved removal of approximately 92% of PCBs. This action focused need for cleanup activities on the LDW.
  • 1976: Dissolved oxygen concentrations in the Green/Duwamish River improved, increasing by a factor of 4 since the 1960s (1 mg/L to 4 mg/L).
  • 1977: The Auburn lagoons were closed and wastewater was transferred to the South Treatment Plant in Renton.
  • 1978: Metro issued the first comprehensive area-wide water quality plan for the Cedar and Green River basins.

The 1980s

  • 1980: Metro and area jurisdictions began planning the Household Hazardous Waste Disposal Program, the first in the country to target consumer products as a source of toxic chemicals.
  • 1980: Dissolved oxygen concentrations in the Duwamish River continued to improve (increasing from 4 mg/L to 7 mg/L).
  • 1981: The U.S. Environmental Protection Agency and Washington State Department of Ecology (Ecology) classified the Duwamish River estuary as a high-priority study area.
  • 1983: The Port agreed to set aside Kellogg Island, the largest remaining intertidal area in the LDW, as a habitat reserve.
  • 1983: Metro issued the Duwamish Clean Water Plan, addressing pollution problems in the lower river.
  • 1985: EPA and Ecology, working with City of Seattle, Metro, the Port of Seattle, and others began the Elliott Bay Action Program, a large multi-year comprehensive program focusing on toxic sediment contamination, its sources, and solutions for cleanup in Elliott Bay and the LDW. This program was part of the Urban Bay Action Program of the multi-agency Puget Sound Estuary Program.
  • 1985: The Port agreed to work with the Muckleshoot Tribe to develop and test a method for improving riprap slopes with sand substrate intertidal benches. The initial project was constructed at T-106 in 1987.
  • 1985: The Elliott Bay Action Program published the summary of existing information on Elliott Bay and Duwamish sediments and sources, and began work on additional studies needed.1986: Metro began work on a new outfall for the South Treatment Plant in Renton so that treated wastewater discharges could be diverted from the Green River into Elliott Bay.
  • 1986: The Metro Council amended the Comprehensive Sewerage Plan by adopting a $1.1-billion secondary treatment/combined sewer overflow control plan to help reduce the impacts of a growing population in the central Puget Sound area.
  • 1987: Metro and the City of Seattle completed the Hanford separation project, reducing combined sewage/stormwater flows to the Diagonal Way storm drain by two-thirds.
  • 1988: Port excavated street end trash dump and restored intertidal habitat at the Duwamish/Diagonal mitigation site as compensation for marine terminal development in East Waterway, combining habitat restoration with additional public shoreline access.
  • 1988: The Elliott Bay Action Program published the Elliott Bay Action Plan, which ranked problem areas and provided a blueprint for moving forward with corrective actions on Elliott Bay and Duwamish sediments and sources.

The 1990s

  • 1990: The Local Hazardous Waste Management Program began targeting the small quantities of hazardous wastes generated by households and businesses.
  • 1990: Dissolved oxygen concentrations (10 mg/L) were 10 times the levels in the 1960s (1 mg/L).
  • 1991: A federal consent decree established the Elliott Bay/Duwamish Restoration Program (EBDRP). The City of Seattle and King County were required to spend approximately $24 million on habitat restoration, sediment cleanup, and source control projects.
  • 1992: Metro completed the Lander sewer separation project, which added 1.4 million gallons of storage in the combined system, thereby reducing overflows to the Duwamish basin.
  • 1993: Boeing began Facilities Investigations and a series of interim measures to address historic contamination of soil, groundwater, and sediments at its Plant 2 facility. The program was authorized under the Resource Conservation and Recovery Act.
  • 1995: The southern transfer project significantly reduced the volume of overflows from the Norfolk combined sewer overflow.
  • 1996: The Port removed sunken barges, excavated an existing upland area, and restored intertidal marsh and riparian areas at the Upper Turning Basin in the LDW as a demonstration project in partnership with USFWS, EPA, and Army Corps of Engineers.
  • 1997: The Port excavated an additional intertidal channel in an industrial area with fill as an additional demonstration of a aquatic habitat restoration project in coordination with federal and local agencies, establishing intertidal marsh, native riparian vegetation, and public shoreline access.
  • 1998: The Port constructed an intertidal and marsh restoration site at the Upper Turning Basin, including removal of a derelict ferry hull as a fish and wildlife habitat compensation action.
  • 1999: The Regional Wastewater Services Plan was approved, which identified future combined sewer overflow projects to control the remaining combined sewer overflows in the LDW (five projects at a cost of more than $75 million, beginning in 2012).
  • 1999: King County completed sediment clean up at the Norfolk combined sewer overflow, removing 5,190 cubic yards of contaminated sediments.

2000 - the present

  • 2000: The Elliott Bay/Duwamish Restoration Program created aquatic habitat and riparian stream restoration at South Hamm Creek, and rerouted discharge.
  • 2000: The Elliott Bay/Duwamish Restoration Program completed habitat restoration at Herring’s House, including the removal of fill at a former industrial site.
  • 2000: The Elliott Bay/Duwamish Restoration Program, working with the Muckleshoot Tribe, purchased the Kenco Marine property to complete intertidal habitat restoration on the west side of the Upper Turning Basin.
  • 2000: The Boeing Company, the City of Seattle, King County, and the Port (the Lower Duwamish Waterway Group or LDWG) completed a voluntary agreement to begin investigation of the LDW sediments, toward an ultimate waterway cleanup plan.
  • 2001: The Elliott Bay/Duwamish Restoration Program completed aquatic habitat restoration at North Wind’s Weir and Cecil B. Moses Park.
  • 2001: The Port excavated previously placed fill to construct intertidal marsh at the Terminal 107 public shoreline access site at the mouth of Puget Creek on Port of Seattle T-107.
  • 2001: EPA placed the LDW on its National Priorities List (NPL or “Superfund” list).
  • 2002: Ecology placed the LDW on its Hazardous Sites List.
  • 2003: The LDW Group proposed seven sites in the LDW for “early action” sediment cleanups.
  • 2003: The Boeing Company conducted cleanup of river sediments offshore of its south storm drain at the Boeing Developmental Center. Using a specialized vacuum excavator, approximately 60 cubic yards of sediment were removed and the excavation was then backfilled with clean material. Annual monitoring of this site has continued to the present.
  • 2003: The Port and the City of Seattle agreed to implement cleanup at the Terminal-117/Malarkey early action area, including substantial additional data collection and evaluation.
  • 2003: The City of Seattle and King County agreed to take on the Slip 4 early action area.
  • 2004: The City of Seattle and King County Cleaned the Diagonal storm drain line and removed more than 600 tons of inline sediments contaminated by historical releases.
  • 2004: King County removed 66,000 cubic yards of contaminated sediment at the Duwamish combined sewer overflow and Diagonal storm drain.
  • 2004: The Port and the City of Seattle submitted the Preliminary Boundary Memo, proposing the extent of the remediation area for the Terminal-117/Malarkey early action area.
  • 2005: King County placed a thin sand layer over four acres adjacent to the Duwamish combined sewer overflow and Diagonal storm drain cleanup site to control residuals from the dredging and to compare enhanced natural recovery to monitored natural recovery in that portion of the LDW.
  • 2005: The City of Seattle conducted an independent cleanup of contaminated soils at Dallas Avenue in city streets near the Terminal-117 early action area.
  • 2006: The Port got approval of a Time-Critical Removal Action of PCB-contaminated soils at the Terminal-117 early action area.
  • 2006: The Elliott Bay/Duwamish Restoration Program, working with the Muckleshoot Tribe, completed the Kenco Marine intertidal habitat restoration on the west side of the Upper Turning Basin.
  • 2006: The City of Seattle and King County completed the design and got approval for the Slip 4 cleanup action that included habitat restoration. Source control work in the basin identified ongoing sources that need to be controlled before cleanup can begin.
  • 2006: The Brandon Street Regulator Station was upgraded to reduce overflows from the combined sewer overflow. Project for full control will begin in 2013.
  • 2007: King County reduced LDW combined sewer overflows from 834 million gallons per year in 1990 to about 78 million gallons in 2007.
  • 2007: EPA approved the City of Seattle’s and King County’s 100% design plans for the cleanup of Slip 4 sediments.
  • 2007: Draft Remedial Investigation submitted to EPA and Ecology and released for public review. The final Remedial Investigation is anticipated to be approved in 2009.
  • 2007: Baseline ecological and human health risk assessments were finalized for the LDW.
  • 2007: The City of Seattle completed habitat restoration south of the Duwamish substation on the west side of the Upper Turning Basin.
  • 2008: The City of Seattle, The Boeing Company, and King County sign an agreed order with Ecology to conduct a Remedial Investigation and a Feasibility Study for North Boeing Field, which drains to Slip 4.
  • 2009: Draft Feasibility Study submitted to EPA and Ecology and released for public review.
  • 2009: Construction began by the City of Seattle to clean and replace the Georgetown Steam Plant flume that leads to Slip 4.
  • 2010: Feasibility Study released to the public. Comment period from Oct. 18 to Dec. 23.
  • 2011: The City of Seattle completed the Slip 4 cleanup and habitat restoration.
  • 2011: King County begins the $19M Hanford #1 CSO control project to control overflows to the Diagonal/Duwamish outfall. The project will be completed in 2018.
  • 2012: King County updates the Combined Sewer Overflow Control Plan, which refines the CSO projects to control the remaining CSOs in the LDW (two projects at a cost of more than $158 million, beginning in 2013; and two projects in the East and West Waterways at a cost of $332 million, beginning in 2016).
  • 2013: The U.S. Environmental Protection Agency (EPA) announced its proposed cleanup plan for the Lower Duwamish Superfund site in February 2013. The 90-day public comment period closed on June 13, 2013, and EPA is now working on the Record of Decision (ROD), which will direct cleanup actions and offer plans for source control and long-term monitoring in the Lower Duwamish Waterway.The Lower Duwamish Waterway Group (LDWG) submitted a detailed set of comments reflecting the group's key goals for the cleanup.
View of the Duwamish from E. Marginal Way, 1916
View of the Duwamish from East Marginal Way, 1916
View of the Duwamish looking west
View of the Duwamish, looking west
Looking south along the Lower Duwamish Waterway
Looking south along the Lower Duwamish Waterway