Duwamish River
The Duwamish River is the lower segment of Washington state's Green-Duwamish River system, spanning approximately 12 miles from its origin near Tukwila in King County—effectively at the point where the Green River is joined by flows historically including the Cedar River following early 20th-century diversions—to its mouth at Elliott Bay in Puget Sound, passing through densely industrialized areas of Seattle.[1][2] Once a meandering waterway with extensive tidal flats and distributaries supporting diverse ecosystems, the river was dramatically altered beginning in 1913 when the U.S. Army Corps of Engineers straightened and deepened its course, shortening the estuary from about 9 miles of bends and wetlands to a 5-mile armored navigation channel to enable port development and flood control.[3][4] This engineering transformed the Duwamish into a heavily modified industrial waterway, lined by manufacturing facilities, shipping terminals, and wastewater outfalls that discharged pollutants including heavy metals, PCBs, and PAHs into its sediments over decades.[5] The lower 5-mile portion, designated the Lower Duwamish Waterway, was listed as a Superfund National Priorities List site by the U.S. Environmental Protection Agency in 2001 due to pervasive contamination threatening human health and aquatic life, prompting extensive investigations, early action cleanups, and a record of decision outlining remedies like sediment capping and dredging.[6][7] Ongoing restoration initiatives, including habitat reconnection, riparian planting, and off-channel marsh creation, seek to mitigate ecological losses and support recovering salmon populations, though the river's impaired water quality and reduced natural flow dynamics continue to limit full recovery.[8][9]Geography and Physical Characteristics
Course and Hydrology
The Duwamish River constitutes the estuarine lower reach of the Green River, commencing near Tukwila, Washington, where tidal effects predominate, and extending approximately 12 miles northward through the industrialized South Seattle and West Seattle areas before discharging into Elliott Bay of Puget Sound.[10] This course follows a largely straightened channel resulting from early 20th-century engineering efforts to mitigate flooding and facilitate navigation, directing flow primarily northwest initially then curving north.[11] The hydrology of the Duwamish River is governed by the broader Green-Duwamish watershed, spanning 484 square miles from headwaters in the Cascade Mountains to the Puget Sound lowlands, encompassing urban, agricultural, and forested lands primarily in southern King County.[11] Mean annual discharge at the upper Duwamish near Tukwila averages 1,350 cubic feet per second (38 cubic meters per second), derived from precipitation-dominated runoff with contributions from snowmelt in upper reaches.[12] Flows exhibit pronounced seasonal variation, typically dropping to 200–300 cubic feet per second during summer low-flow periods and surging during winter storms, though regulated by the Howard A. Hanson Dam on the Green River since 1962 to attenuate floods.[13][14] Tidal influences extend upstream into the Duwamish, creating a brackish estuary where freshwater inflow mixes with saltwater, affecting salinity gradients and sediment transport; maximum tidal excursion reaches several miles inland under low river flow conditions.[15] Groundwater interactions with the river are significant in the lower valley, with aquifers recharging from and discharging to the waterway, influencing baseflow contributions.[16]Name and Etymology
The Duwamish River is named for the Duwamish people, a Lushootseed-speaking Coast Salish tribe historically inhabiting the Puget Sound lowlands, particularly the river valley and adjacent Elliott Bay shoreline.[17][18] The English name "Duwamish" represents an anglicized adaptation of the tribe's autonym, dxʷdəwʔabš (phonetically approximated as "dduh-duh-wubsh"), which in Lushootseed means "people of the inside" or "people from the interior."[17][19] This designation distinguished them from coastal Salish groups, emphasizing their settlements in sheltered inland waterways and tidelands east of the bay, where the river's delta provided resources like salmon runs and cedar forests central to their sustenance and culture.[20] Lushootseed, the southern dialect continuum of the Salishan language family spoken across central Puget Sound, encodes such locative descriptors in tribal ethnonyms, reflecting ecological and territorial realities rather than abstract identities.[21] European explorers and settlers, arriving in the mid-19th century, adopted and simplified the term for the waterway during early mapping and navigation efforts, with records from the 1840s onward applying "Duwamish" to the estuary and lower channel amid initial hydrographic surveys by figures like the Wilkes Expedition.[22] The name has persisted without significant alteration, though the upstream watershed is formally the Green-Duwamish River to denote its confluence origins in the Cascade foothills.[23]History
Indigenous and Pre-Contact Era
The lower Duwamish River valley and estuary were central to the territory of the xʷdəwʔabš (Duwamish), a Southern Coast Salish group speaking the Lushootseed language, who maintained seasonal and permanent villages along its banks for millennia prior to European contact in the 19th century.[24] Archaeological evidence from the broader Puget Sound region, including sites proximate to the Duwamish watershed, confirms human occupation dating to at least 10,000 years ago, shortly after the Pleistocene glaciation's retreat, with middens and artifacts indicating reliance on riverine and estuarine resources.[21][25] Precontact Duwamish settlements concentrated at resource-rich confluences, such as the junction of the Duwamish, Black, and Cedar Rivers, where winter villages supported populations through salmon fishing during annual runs of species like chinook and coho, supplemented by weirs, traps, and drying techniques for storage and trade.[26] These villages featured plank longhouses housing extended families, with sites like Hahapoos near the estuary's mouth containing multiple such structures and serving as hubs for processing fish, shellfish from tidal flats, and plants like camas from adjacent wetlands.[27] Excavations in the Duwamish Valley have uncovered precontact artifacts, including lithic tools and faunal remains, evidencing stratified social organization with emerging hierarchies and status markers by the late Holocene.[28][29] The river's dynamic hydrology—featuring braided channels, extensive marshes, and seasonal flooding—shaped Duwamish land use, enabling diverse foraging strategies while necessitating adaptive engineering like fish traps and canoe navigation, as inferred from ethnographic analogies corroborated by archaeological patterns in the watershed.[30] Allied Salish groups shared access to these resources, fostering inter-village networks for exchange, though core Duwamish identity tied to the waterway's productivity sustained semi-sedentary communities estimated at several thousand individuals regionally before contact disruptions.[31] Sites yielding eligible cultural materials underscore the valley's longstanding role in sustaining these societies without evidence of large-scale conflict or abandonment prior to external influences.[29]Settlement and Early European Influence
 to severe (Phase 4) based on river gauges.[53]Environmental Status
Sources of Pollution and Contaminant Profiles
The Lower Duwamish Waterway's sediments contain elevated levels of persistent organic pollutants and heavy metals, resulting from more than a century of industrial operations, urban development, and inadequate waste management. Historical pollution arose from direct discharges by manufacturing facilities, shipyards, and food processing plants established along the river since the early 1900s, which released untreated wastewater and solid wastes into the waterway.[54] Ongoing sources include stormwater runoff from over 20,000 acres of surrounding urban and industrial land, which transports contaminants from impervious surfaces, soil, and groundwater into the river; combined sewer overflows during heavy rain events; and leaching from upland sites such as former industrial properties.[55][56] Upstream sediment inputs from the Green River, carrying suspended particles laden with pollutants during high-flow episodes, contribute significantly to deposition, accounting for up to 76% of the annual sediment load estimated at 74,800 metric tons between 1960 and 1989.[57] Key contaminants exhibit profiles characterized by high persistence, bioaccumulation potential, and toxicity to aquatic organisms and human consumers via fish and shellfish. Polychlorinated biphenyls (PCBs), synthetic compounds once used in electrical equipment and paints, dominate human health risks and stem primarily from legacy industrial releases and urban runoff, with daily stormwater loads ranging from 0.06 to 1.21 grams and annual estimates of 155 to 2,270 grams based on 1960–1989 data.[54][57] Carcinogenic polycyclic aromatic hydrocarbons (cPAHs), byproducts of fossil fuel combustion and industrial processes, enter via stormwater and atmospheric deposition, showing daily loads of 0.30 to 12 grams.[57] Heavy metals like arsenic, derived from both natural watershed erosion and anthropogenic inputs, have sediment concentrations of 9.2 to 24.3 mg/kg dry weight and daily loads up to 7.28 kg.[57] Dioxins and furans, chlorinated organics from waste incineration and chemical manufacturing, persist at 0.83 to 16.2 ng TEQ/kg, contributing to sediment toxicity.[57] Additional metals (e.g., lead, copper, zinc) and phthalates from plastics and consumer products complete the profile, often exceeding regulatory cleanup levels in hotspots.[56]| Contaminant | Primary Sources | Sediment Concentration Range (dry weight) | Daily Load Range (from stormwater) |
|---|---|---|---|
| PCBs | Industrial legacy, urban runoff | 1.2–62.1 μg/kg | 0.06–1.21 g |
| cPAHs | Combustion byproducts, stormwater | 13.1–588 μg TEQ/kg | 0.30–12 g |
| Arsenic | Watershed erosion, industrial inputs | 9.2–24.3 mg/kg | 0.13–7.28 kg |
| Dioxins/Furans | Incineration, chemical processes | 0.83–16.2 ng TEQ/kg | 0.03–0.73 mg TEQ |
Regulatory Designation and Cleanup Initiatives
The Lower Duwamish Waterway, comprising the final five-mile industrialized segment of the Duwamish River from the Turning Basin to Elliott Bay, was placed on the National Priorities List as a Superfund site by the United States Environmental Protection Agency (EPA) on September 13, 2001, under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).[54] This federal designation targeted persistent sediment contamination from polychlorinated biphenyls (PCBs), dioxins, furans, and heavy metals such as arsenic and lead, stemming primarily from historical industrial discharges, stormwater runoff, and atmospheric deposition rather than acute spills.[54][7] The site's inclusion reflected empirical sediment sampling data indicating exceedances of EPA risk thresholds for human health and ecological receptors, including bioaccumulation in fish tissues prompting fish consumption advisories.[58] Remedial investigations from 2002 to 2013 quantified over 120 acres of contaminated sediments requiring intervention, leading to the EPA's Record of Decision (ROD) for the final cleanup plan on December 2, 2014.[59] The ROD prescribes a combination of dredging, thin-layer capping, and monitored natural recovery to achieve at least a 90% reduction in principal contaminants like PCBs and mercury across divided waterway reaches, with projected completion by 2037 contingent on adaptive management based on post-remediation monitoring.[54][60] Source control measures, including upgrades to combined sewer overflows and upland remediation, were integrated to prevent recontamination, as causal analysis linked ongoing inputs to legacy sediment persistence.[61] Prior to the ROD, early action cleanups addressed high-priority hotspots, such as the Norfolk Sound Combined Sewer Overflow, where dredging and capping removed an estimated 50% of total PCB mass from sediments by December 2015.[54] Construction in the upper reach began in late 2024, focusing on 28 acres of sediment removal via hydraulic dredging and backfilling with clean material; the 2024-2025 in-water work season concluded in early 2025, with resumption scheduled for October 2025 and phase completion targeted for February 2027.[62][61] The Lower Duwamish Waterway Group—comprising liable parties like the Port of Seattle, City of Seattle, and King County—funds these efforts under EPA oversight, having expended over $80 million on sediment and upland projects as of 2023.[63] The Washington State Department of Ecology co-regulates via the Model Toxics Control Act, enforcing state sediment quality standards that align with but sometimes exceed federal criteria, including five-year reviews to verify remedial effectiveness against bioavailability metrics.[7] Long-term monitoring emphasizes empirical endpoints like porewater contaminant concentrations and benthic invertebrate health, rather than proxy assumptions, to guide adjustments amid tidal influences and hydrodynamic sediment transport.[54]Debates and Controversies in Remediation Efforts
Remediation efforts for the Lower Duwamish Waterway Superfund site, designated by the EPA in 2001, have been marred by disputes over cost allocation among responsible parties, including Boeing, the Port of Seattle, the City of Seattle, and King County.[64] Negotiations under Superfund law remain confidential, with the Port of Seattle estimating total costs exceeding $1 billion in a 2022 lawsuit against Boeing, accusing the company of attempting to shift its financial burden onto taxpayers and other entities, potentially by tens of millions of dollars.[64] Boeing has countered that the Port refuses to pay its fair share and anticipates spending hundreds of millions more itself, while over $200 million has already been expended on early action cleanups since 2001; the lawsuit was withdrawn in June 2023 amid ongoing talks, delaying full-scale work until 2024.[64] Critics have questioned the long-term effectiveness of sediment remediation, arguing that insufficient control of upland pollution sources risks recontamination of dredged areas.[65] The Washington Department of Ecology's source control program, initiated in the early 2000s across over 20,000 acres of the watershed, targets contaminants like PCBs and arsenic through stormwater management and site cleanups but faces challenges including data gaps, exclusion of pathways such as regional air deposition, and variable compliance across facilities.[65] A specific flashpoint involves Boeing's Military Delivery Center in Tukwila, where stormwater discharges of PCBs were detected at levels thousands of times above legal limits—such as approximately 1 part per trillion in January samples—prompting a 2022 Ecology order for enhanced treatment, which Boeing appealed; environmental groups like Puget Soundkeeper have threatened lawsuits, warning that such ongoing inputs undermine the projected $350 million sediment cleanup.[66] Tribal representation has fueled additional controversies, particularly regarding the non-federally recognized Duwamish Tribe's push for involvement in decision-making and benefits from remediation.[67] The Duwamish filed a 2022 federal lawsuit against the Department of the Interior seeking recognition to secure a formal role, amid opposition from recognized tribes like the Muckleshoot, who credit themselves with advancing Superfund status but argue against Duwamish claims that could dilute resources or authority over the river's cultural and ecological restoration.[67] This tension highlights broader debates on equitable indigenous participation in addressing historical and ongoing environmental harms tied to the waterway's industrialization.[67]Ecology and Biological Resources
Pre-Modification Ecosystem and Biodiversity
Prior to hydrological modifications in the early 20th century, the Duwamish River constituted a wide, meandering waterway forming an extensive estuary with large areas of tidal mudflats, marshes, and swamps covering approximately 5,200 acres in the lower valley.[68] [32] Unregulated freshwater inflows from upstream sources sustained a dynamic estuarine system, where tidal fluctuations interacted with fluvial sediment deposition to maintain diverse shallow-water habitats.[5] This pre-modification landscape provided critical rearing and foraging grounds for juvenile salmon, particularly Chinook, which relied on the mudflats and emergent vegetation for protection, nutrient accumulation, and physiological adaptation to marine conditions.[69] The estuary supported robust runs of anadromous fish, including Chinook, chum, and historically pink salmon, with off-channel sloughs and floodplain connectivity enhancing juvenile survival through diverse prey resources and refuge from predators.[13] Shorebirds and waterfowl inhabited the tidal zones, exploiting the productive invertebrate communities in the sediments.[69] The overall ecosystem exhibited high productivity driven by natural cycles of flooding, tidal exchange, and organic matter cycling, fostering interconnected food webs that sustained terrestrial mammals, amphibians, and riparian flora adapted to periodic inundation.[70] These conditions yielded abundant biomass, as evidenced by historical indigenous reliance on the river's fisheries and the subsequent drastic habitat losses—exceeding 97% of original estuarine features—following channelization efforts commencing in 1913.[71] [42]Current Wildlife Populations and Health Impacts
The Lower Duwamish River hosts migratory salmon populations, including Chinook, coho, and chum, which pass through the waterway en route to upstream spawning grounds in the Green River system, with thousands returning annually.[72] Recent monitoring at restored habitats, such as the Duwamish River People's Park completed in 2023, recorded over 2,300 juvenile chum salmon and nearly 240 natural-origin juvenile Chinook salmon, alongside more than 900 non-salmonid fish, indicating improved juvenile utilization of enhanced nearshore areas.[73] Resident fish species, such as English sole and staghorn sculpin, persist in the river but exhibit elevated tissue concentrations of polychlorinated biphenyls (PCBs) and other persistent pollutants due to sediment contamination.[74] Bird populations in the Duwamish include over 87 species documented across nine seasons of monitoring through 2025, with peak diversity observed in spring, encompassing waterfowl, shorebirds, and raptors that forage and nest along restored shorelines.[75] Mammalian wildlife features river otters (Lontra canadensis), which have recolonized the area despite pollution, alongside beavers, muskrats, and raccoons; otters serve as apex predators bioaccumulating contaminants through their fish-heavy diet.[76] [77] Health impacts on wildlife stem primarily from bioaccumulation of legacy pollutants like PCBs, polyaromatic hydrocarbons (PAHs), and heavy metals, which concentrate in fatty tissues of higher trophic levels. River otter scat from the Duwamish in 2023 contained nearly 26 times higher PCB levels and 10 times higher PAH levels compared to otters in cleaner Puget Sound watersheds, marking among the highest exposures recorded in wild river otters and posing risks of carcinogenic and endocrine-disrupting effects.[76] [78] Resident fish tissues similarly show PCB concentrations exceeding protective benchmarks, potentially impairing growth, reproduction, and immune function, though assessments indicate no widespread acute mortality in juvenile salmon.[74] [79] Restoration efforts have bolstered habitat resilience, enabling species persistence amid these stressors, but full ecological recovery awaits sediment remediation completion projected for 2037.[60]Restoration Projects and Habitat Recovery
Restoration efforts for the Duwamish River have focused on recreating estuarine, riparian, and marsh habitats to support salmonid species, birds, and other wildlife diminished by historical channelization and pollution.[63] These initiatives, often integrated with Superfund remediation, emphasize native vegetation planting, invasive species removal, and shoreline reconfiguration to enhance ecological functions such as fish rearing and sediment stabilization.[80] The Port of Seattle completed the Duwamish River People's Park in 2022, a 14-acre site representing the largest habitat restoration project on the river in decades, featuring estuarine environments and sustainable shorelines designed for threatened Chinook salmon.[63] This project increased salt-marsh habitat availability by 40% in the local area, providing off-channel refugia and improved water quality through biobarges and vegetated buffers.[63] King County has restored over 25 acres across multiple sites in the Lower Duwamish, including the Hamm Creek project with 2,300 feet of new riparian stream bed and 1 acre of estuarine marsh developed in partnership with the U.S. Army Corps of Engineers.[8] Other efforts encompass 1.8 acres of intertidal bay and riparian buffer at Herring's House Park, 1 acre of marsh at Cecil B. Moses Memorial Park, 2 acres of wetlands at North Wind's Weir, and shoreline vegetation at Turning Basin No. 3 and the South Park Bridge site, all aimed at salmon transition zones and wildlife corridors.[8] The U.S. Army Corps of Engineers' Green/Duwamish River Ecosystem Restoration program includes 45 authorized elements targeting critical habitats throughout the watershed, prioritizing natural refuge areas for fish and wildlife species.[9] Complementing these, the Green Duwamish Restoration collaborative has rehabilitated 383 acres and 51,724 linear feet of shoreline since 2015 through invasive removal and native plantings.[80] In September 2025, partners achieved a milestone with nearly 3 acres of new off-channel marsh, intertidal, and riparian habitat construction.[81] Tribal and community-led projects, such as the Duwamish Tribe's ongoing Upland Reforestation of 5,800 square feet at the Longhouse and Cultural Center, further bolster riparian recovery by protecting and replanting native species.[82] Collectively, these restorations have fostered habitats for fish, invertebrates, and birds in an industrialized setting, though long-term monitoring is required to verify sustained biodiversity gains amid ongoing urban pressures.[75]Economic and Industrial Role
Port Operations and Maritime Commerce
The Lower Duwamish Waterway, the navigable channelized portion of the Duwamish River, supports maritime commerce through specialized terminals and barge operations focused on breakbulk, bulk, and regional containerized cargo rather than large-scale deep-water container transshipment. Terminal 115, located along the waterway, handles breakbulk and container loads, including refrigerated cargo with 780 reefer plugs, primarily serving Alaska Marine Lines and other carriers for routes to Alaska via barge and short-sea vessels.[83] Cargo types processed there encompass groceries, automobiles, oversized machinery, project supplies, and temperature-controlled products, facilitating domestic and regional trade.[84] Barge traffic constitutes a core element of operations, with facilities like SeaTac Marine's terminal on the Duwamish specializing in breakbulk cargoes such as lumber, heavy-lift equipment, construction materials, and bulk commodities like cement, using steel-deck barges for efficient regional movement.[85] The waterway's channels, maintained to depths accommodating these vessels, enable frequent intra-Puget Sound and coastal transits, though vessel traffic data indicate peaks of around 160 bridge openings per month historically, reflecting moderate but consistent activity influenced by industrial demands.[86] Adjacent industrial sites support ancillary cargo handling, storage, and transloading for dry bulk, agricultural goods, and high-value items.[87] While the Duwamish contributes to Seattle's maritime ecosystem by linking upland industrial zones to Puget Sound, its role complements rather than competes with Elliott Bay's container terminals, emphasizing short-haul efficiency over global volumes; for instance, upland Duwamish-area facilities processed over 54 million tons of cargo annually as of early 2010s assessments, underscoring its bulk and support freight niche.[88] Ongoing dredging and maintenance ensure navigability amid sedimentation from river flows and vessel wakes, sustaining commerce amid environmental remediation constraints.[32]Industrial Facilities and Job Creation
The Lower Duwamish River corridor features a concentration of heavy industrial facilities, including aerospace manufacturing operations historically centered at Boeing Plant 2 on the east bank, which produced aircraft components and supported thousands of manufacturing jobs until its demolition in December 2011.[89][54] Other key sites encompass shipbuilding and repair yards, steel mills, cement production plants, forging operations like Jorgensen Forge, and container handling services such as Industrial Container Services.[90][3][7] Port of Seattle terminals, including Terminal 115 and North Terminal 115, facilitate cargo handling, marine construction, and logistics adjacent to the waterway.[7] These facilities contribute to substantial job creation in the region, with the Lower Duwamish industrial area sustaining more than 100,000 direct and indirect positions across manufacturing, transportation, warehousing, and related sectors as of recent assessments.[91] The Duwamish Manufacturing Industrial Center (MIC), encompassing 5,062 acres of predominantly industrial land use (85%), supports 58,771 jobs within its boundaries, functioning as a critical node for trade and logistics.[92] Employment in these industries generates significant multiplier effects, including secondary and induced economic activity from supply chains and worker spending, bolstering regional output valued at $13.5 billion annually.[93][91] Despite environmental remediation demands, the corridor's industrial base remains a foundational economic driver, with sectors like aerospace and maritime operations providing stable, high-wage opportunities.[63]Balancing Development with Environmental Claims
The Duwamish River serves as a critical artery for Seattle's industrial economy, with Port of Seattle operations along its waterway supporting over 100,000 jobs in maritime and related sectors, while contributing to approximately 25% of King County's manufacturing output.[63] These activities, including cargo handling and shipbuilding, generate substantial economic value, with the Port's broader impacts estimated in the billions annually, underscoring the river's role in regional trade and logistics.[94] However, historical industrial discharges have contaminated sediments with persistent toxins like PCBs and heavy metals, prompting environmental claims centered on human health risks, ecosystem degradation, and disproportionate burdens on nearby low-income communities.[54] [95] Cleanup efforts under the Superfund program, designated in 2001, have escalated costs, with EPA projections at $342 million for in-water remediation alone, though total liabilities may exceed $1 billion when including upland sites and responsible party allocations among entities like the Port, Boeing, City of Seattle, and King County.[54] [64] The Port has expended over $80 million to date on sediment removal, soil remediation, and habitat projects, arguing that stringent standards must account for ongoing industrial viability to prevent job losses and supply chain disruptions.[63] Environmental advocates, including the Duwamish River Cleanup Coalition, have contested proposed adjustments to sediment pollutant thresholds, warning that dilutions could perpetuate exposure risks despite empirical evidence of contamination persistence.[96] [97] A 2010 health impact assessment highlighted potential adverse effects on local industries from remediation pressures, yet affirmed that source controls have curtailed new inputs, allowing phased cleanup without immediate economic collapse.[98] Balancing persists through integrated initiatives like the Duwamish Valley Program, which couples remediation with community wealth-building, climate adaptation, and equitable access to industrial benefits, including green job training and public parks such as the Duwamish River People’s Park.[99] [63] The Port's Duwamish Valley Community Benefits Commitment addresses disparities by funding local hiring and pollution mitigation, while pursuing net-zero emissions by 2040 to align development with sustainability mandates.[100] These measures reflect causal trade-offs: unchecked development historically amplified pollution, but modern regulatory frameworks enable remediation alongside economic continuity, with data indicating improved sediment conditions post-dredging without derailing port throughput.[95] Ongoing negotiations over cost-sharing, however, reveal tensions, as stakeholders like Boeing and public taxpayers debate liability for legacy contaminants versus incentives for future industrial investment.[64]Infrastructure and Connectivity
Bridges, Roads, and Crossings
The Duwamish River, following its straightening and dredging in the early 20th century, necessitated multiple crossings to support industrial and urban connectivity in South Seattle.[101] The first drawbridge over the Duwamish Waterway was constructed by King County at Spokane Street around 1902-1903, enabling basic vehicular and pedestrian access shortly after initial channel improvements.[101] Major modern bridges include the West Seattle Bridge, a high-level concrete structure completed in 1984, which spans the waterway to link West Seattle with the Duwamish Industrial District and carries State Route 599 traffic.[102] This bridge replaced earlier low-level bascule designs, accommodating larger vessel passage without frequent openings.[38] The First Avenue South Bridge consists of paired double-leaf bascule bridges carrying State Route 99 over the river, with the northbound span built in 1956 and the southbound in 1998, providing drawbridge functionality for maritime navigation. Further south, the South Park Bridge, a bascule drawbridge opened in 2014, replaced a 1931 structure that closed in 2010 due to structural deficiencies; it connects the South Park neighborhood to industrial areas across the waterway.[35] Interstate 5 also crosses the Duwamish River via a dedicated bridge segment north of the South Park crossing, integrated into the highway's alignment through the valley, supporting high-volume regional traffic.[103] Railroad bridges, such as the Northern Pacific (now BNSF) bascule spans dating to 1911 and 1928, facilitate freight movement parallel to road infrastructure.[104]| Bridge Name | Type | Construction/Opening Year | Carries |
|---|---|---|---|
| West Seattle Bridge | High-level concrete | 1984 | SR 599 |
| First Avenue South Bridge (northbound) | Double-leaf bascule | 1956 | SR 99 |
| First Avenue South Bridge (southbound) | Double-leaf bascule | 1998 | SR 99 |
| South Park Bridge | Bascule drawbridge | 2014 | Local roads (14th Ave S) |
| I-5 Duwamish River Bridge | Fixed highway | Mid-20th century (part of I-5 buildout) | I-5 |