The Duwamish River runs north for about through the cities of Tukwila and Seattle in the US state of Washington before emptying into Elliott Bay on the Puget Sound. Historically, the river formed from the confluence of the White and Black rivers in what is now Renton, but human modifications in the early 20th century led it to become the lower portion of the Green River. During the Last Glacial Period, the Duwamish Valley was carved out by glacial meltwater from the advancing Cordilleran ice sheet, forming what was initially an inlet of Puget Sound, which was later filled by the eruption of Mount Rainier and the Osceola Mudflow around 5,600 years ago.
Lushootseed-speaking Coast Salish peoples, including the eponymous Duwamish people, occupied the region for thousands of years prior to colonization. Euro-American settlement began in the 1850s, and many Duwamish were forced to leave the region after the 1855 Treaty of Point Elliott. Industrial development in the growing city of Seattle led to various efforts to reclaim land in the river's broad mudflats, culminating in the construction of Harbor Island and the channelization of the river into the Duwamish Waterway during the early 20th century. During this period, the White River was diverted away from the Duwamish, and the construction of the Lake Washington Ship Canal cut off the river from the Lake Washington watershed, greatly lessening its flow. The area around the Duwamish Waterway was extensively industrialized during the 20th century, supporting industries such as ship and airplane manufacture. Extensive shoreline modifications such as bulkheads, piers, wharves and sheet pile retaining walls were installed across the river, resulting in limited natural habitat.
The Duwamish has been subject to restoration and cleanup efforts since the late 20th century, and was declared a federal Superfund site in 2001. The river continues to serve as a habitat for various fish (including several species of salmon), birds, and invertebrates. Although much of the watershed consists of commercial and industrial property, it contains some residential neighborhoods and various parks, allowing for recreational use of the river.
Name
The Lushootseed term , refers to the inland region centered on the confluence of the Duwamish, Black, Cedar and Green rivers. was a major center of settlement, with multiple towns. may have initially been the name of a single village in the region. The Duwamish people () take their name from this region. Euro-American settlers named the river after the Duwamish in the early 19th century. Early variant spellings for the river included Dewampsh, D'Wamish, and Dwamish.
Course
Historic (pre-1906) Prior to the rerouting efforts of the early 20th century, the Duwamish River formed from the confluence of the White and Black rivers. The Black River drained Lake Washington into the Duwamish. During this period, the Duwamish and its tributaries had a drainage basin covering .
Small tributary creeks flowed into the lower Duwamish as it proceeded through various meanders down the Duwamish Valley, before opening into salt marshes near the delta. The delta's mudflats stretched into Elliott Bay, encompassing most of the modern reclaimed land around the bay.
Present
In its present course, the Duwamish is the tidally-influenced lower of the Green River. The Green River becomes the Duwamish at the confluence with the remnants of the Black River in Fort Dent Park, Tukwila. It flows northwest through Tukwila, before broadening into the Duwamish Waterway shortly before entering Seattle city limits adjacent to Boeing Field. Just south of the city limits, Hamm Creek drains into the waterway from the west. The waterway splits into east and west channels on either side of the artificial Harbor Island before reaching Elliott Bay. A small stream, Longfellow Creek, drains into the west channel.
Bridges Going upstream from the river's source, a small pedestrian bridge crosses the river at the Foster Golf Course. 56th Avenue S, Interstate 5, 42nd Ave S, East Marginal Way, and Tukwila International Boulevard cross the river in Tukwila, alongside a pedestrian bridge at S 119th St. Straddling the city limits of Seattle is the South Park Bridge. Within the city itself are the First Avenue South Bridge and West Seattle Bridge. The Spokane Street Swing Bridge spans the western channel of the Duwamish Waterway, connecting Harbor Island to West Seattle.
Watershed
The Green and Duwamish Rivers have a combined drainage basin covering , including portions of Seattle, Tukwila, SeaTac, Renton, Federal Way, Auburn, Black Diamond, and Enumclaw, as well as various forested unincorporated areas across south-central King County.
The Puget Sound region has a marine climate, with cool moist air from the Pacific allowing for moderate temperatures during the summer and winter. Monthly temperatures at nearby Seattle–Tacoma International Airport average during the winter and during the summer. The majority of precipitation falls between October and January, with a total annual average of about .
Hydrology Prior to large-scale modification, the flow rate of the Duwamish averaged about per second throughout the year. During floods, this rate could increase to per second. The Howard A. Hanson Dam was constructed on the upper Green River in 1961 to control floods and ensure a more consistent water level in the river, decreasing the peak flow rates of the Duwamish. However, the periodic release of water from the dam has increased moderate flow rates to about per second.The riverbanks of the upper Duwamish are generally higher in elevation than the portions of the floodplain closest to the valley escarpments. The banks of the lower Duwamish descend from the rest of the valley, forming a terrace level. The lower waterway is influenced by both freshwater from the Duwamish-Green system and tidal flows of saltwater from Elliott Bay. A saltwater intrusion can extend up to upstream from the mouth, increasing the river's turbulence and resuspending sediments in the water.
About of tidal marshland once covered the area between Kellogg Island and the mudflats on Elliott Bay. Constrained by the terraces along the floodplain, the tidal marsh was about wide at its narrowest. The marshland was mainly spread across two main river islands, alongside various smaller islands and a small region on the northern coast of the mudflats. Kellogg is a remnant of the largest of these islands. Two lobes of primarily freshwater marshland (periodically inundated by the tides) stretched inland along the river, covering roughly . An additional of freshwater marshland was present in depressions along the floodplain.
River modification and surrounding construction has erased most of the Duwamish's former mudflats and estuary. The United States Army Corps of Engineers maintains a federal navigation channel along the lower portion of the river, dredging up to around from the mouth to ensure the waterway is deep enough for ships to navigate. The majority of the shoreline along the lower Duwamish is used for industrial and commercial purposes, with extensive shoreline modifications such as bulkheads, piers, wharves, sheet pile retaining walls and banks armored with riprap. Only pockets of natural habitat remain in the waterway, mostly limited to small, shallow intertidal benches.
Water quality Storm drains and sanitary systems across much of southern Seattle and adjacent municipalities feed into the lower Duwamish. Pollutants in these sources include metal, polycyclic aromatic hydrocarbons, polychlorinated biphenyl, dioxins, phthalates, and various other organic compounds, posing a risk to humans and wildlife. These sources account for only about 1% of the sediment entering the waterway, but have a far higher concentration of contaminants than sources further upstream.
Biology A 1995 survey found 33 resident and seasonal fish species in the lower Duwamish waterway. Salmon runs of various species traverse the Duwamish to access Green River spawning grounds. Chinook, chum, and coho salmon are the most common, although pink and sockeye salmon are rarely found. Steelhead and cutthroat trout are found in the river, with occasional instances of bull trout. Chinook and chum juveniles often reside in the Duwamish estuary, although they may be threatened by river lamprey. Spring chinook and sockeye salmon runs in the Duwamish greatly declined over the twentieth century, likely due to the river's rerouting.
Shiner perch, staghorn sculpin, snake prickleback, starry flounder, and Pacific sand lance are commonly found in the river. Sculpins and flounder frequently travel a significant distance upriver, able to tolerate low salinity environments. Freshwater species in the river include redside shiner, mountain whitefish, and largescale suckers.
Benthic invertebrates such as burrowing polychaete worms and bivalves are common in the sediments of the Duwamish, while large molluscs and crustaceans such as slender crabs, Crangon shrimp, and Pandalus shrimp are commonly found on the river bottom. In the higher salinity areas closest to the mouth, dungeness crabs are found.
Birds such as the great blue herons and cormorants prey on juvenile salmon in the Duwamish estuary. Birds under federal or state protections noted in the estuary include bald eagles, common loons, common murres, merlins, peregrine falcons, and western grebes.
Geology The oldest rocks in the Duwamish valley are Eocene sedimentary rocks of the Puget Group, subdivided into the Renton and Tukwila Formations, the latter interrupted by isolated andesite sills. A separate formation of marine sedimentary rocks contains shallow-water marine fossils mixed with volcaniclastic rocks. Additional sedimentary rocks date from Oligocene in the Blakeley Formation. The Pacific coastline was once the edge of a large flat plain stretching to the Rockies, akin to the Mississippi basin. The subduction of the Juan de Fuca Plate under the North American Plate formed the Cascade Range along the coast in the late Eocene, around 36 mya (million years ago). In the late Oligocene, around 25 mya, the mountains were subject to another round of uplifting. Receiving heavy rainfall, the earliest iterations of the Cascade rivers had formed by this period, although flowed in a more northwesterly direction. The Green River likely flowed through much of the Cedar's present course during this period.
The Puget Lowland was scoured by the expansion of the Cordilleran ice sheet during a series of six glaciations over the course of the Quaternary period. Each expansion led to the Puget Lobe, a large tongue of ice, filling the lowland region between the Cascade and Olympic ranges. The Vashon Stade, the most recent of these periods of glacial advance, lasted from , forming the majority of the topography of the Puget Sound. During this period, Seattle and the future Duwamish river were buried under at least of ice. The present landscape of the region sits atop several hundred meters of glacial sediment. The advancing lobe deposited a layer of glacial outwash consisting of sand and gravel. Overlying this is a dense and relatively thin layer (ranging from thick of glacial till, consisting of a poorly-sorted mixture of sand, silt, and gravel.