The Sammamish River runs for about through portions of King County, Washington, draining Lake Sammamish into Lake Washington. It flows out of Lake Sammamish at Marymoor Park in Redmond, before continuing through suburban residential areas in Redmond, Woodinville, Bothell, and Kenmore. About 32% of the river's riparian habitat is forested, with the invasive reed canarygrass found across its course. The river hosts fish such as salmon as they migrate between the lakes. It has four major tributary streams: Bear Creek, North Creek, Swamp Creek, and Little Bear Creek. The floodplain was carved out by a glacial meltwater channel during the Vashon Glaciation, about 15,000 years ago, resulting in a geology dominated by glacial deposits and alluvium.
The Marymoor Archaeological Site is located near the river's mouth, estimated to be around 2400 to 8000 years old. At the time of contact, the area was inhabited by the Sammamish people, a division of the broader Duwamish, a Coast Salish nation. European settlement of the valley began during the 1870s, and loggers cleared its forests over the following decades. The river was subject to heavy modifications and flood control measures. The opening of the Lake Washington Ship Canal in 1917 caused the level of Lake Washington to drop, and a local drainage district attempted to alleviate floods by constructing drainage ditches and straightening its upper course. The U.S Army Corps of Engineers oversaw measures to channelize and dredge the river during the 1960s, expanding a weir near the river's source. Redmond entered a period of extensive growth and urbanization in the late 20th century, while riparian habitat restoration efforts were taken across the river's course during the 2010s.
Course
The Sammamish River runs for about through Western Washington, draining Lake Sammamish into Lake Washington. It emerges from Lake Sammamish at Marymoor Park in Redmond, flowing northwest through the park and adjacent to a residential area. It receives two small, unnamed tributary streams before turning north, crossing a highway overpass for State Route 520, and receiving Bear Creek (also known as Big Bear Creek) as a major tributary on its right bank. For about downstream of the confluence, it flows north through a suburban residential and commercial area of Redmond, passing by a golf course. The Sammamish River Trail runs along the river between Marymoor Park and Bothell.
After this, the Sammamish exits the city limits of Redmond and flows north through land mainly used for agriculture and recreation. It then flows northwest through developed residential areas in the cities of Woodinville and Bothell, a region featuring the greatest amount of wetland in the river corridor. It receives two major tributaries on its right bank as it turns west through Bothell: Little Bear Creek, and then, about downstream after crossing under a highway interchange, North Creek. It flows west through several wetlands in downtown Bothell, briefly turning south within the city before curving back towards the west.
In the last mile of its course, the Sammamish flows west through residential areas as it exits Bothell and enters Kenmore. It receives Swamp Creek as a tributary from the north about from its mouth. The area around the Sammamish's outflow into Lake Washington is dominated by wetland, with a former industrial site to the north and a river island situated at the mouth. Lake Washington currently drains into Puget Sound through the Lake Washington Ship Canal. Prior to the canal's construction in 1917, the lake instead drained south through the Black River into the Duwamish River, which flowed north and emptied into the sound.
Hydrology
Most of the Sammamish watershed lies within King County, Washington, although a small portion is within Snohomish County to the north. The watershed primarily consists of suburban areas, although it contains portions of urban centers such as Bellevue, Kirkland, and Redmond, alongside relatively large undeveloped areas.
The Sammamish River's four largest tributaries, from largest to smallest in terms of drainage area, are Bear Creek, North Creek, Swamp Creek, and Little Bear Creek. The only other comparable tributary to these is Gold Creek, which the river receives alongside a number of much smaller minor tributaries. The Sammamish receives most of its streamflow from surface water such as these tributaries, although some groundwater drains into the river from aquifers. +Major Sammamish River tributary streamsNameDrainage areaLengthPercentage of total streamflowConfluence Bear Creek19%Little Bear Creek6%North Creek14%Swamp Creek9%Sammamish River The Sammamish River has a low elevation and a gentle gradient. Its source is at about above sea level, while its mouth is at about . The river's floodplain varies from almost in width in the upper two-thirds of its course, to a minimum of near Bothell.
The Sammamish is the second-largest tributary to Lake Washington, behind the Cedar River, and provides about 40% of the lake's water. Backflow from the lake impacts the lower of the Sammamish's course.
Streamflow and water quality Between 1965 and 2000, the Sammamish had an average flow of per second, measured from a station in Woodinville. The river reaches its lowest flows in August, with an average annual low of around per second. The 100-year flood and 100-year-drought (the maximum and minimum flows expected in a 100-year period) for the river are over and around per second.
A sampling study of groundwater in the Sammamish Valley in 2002–2020 found arsenic and sodium levels to exceed the maximum allowed by state drinking water safety standards. Levels of elements serving as secondary indicators, such as iron, manganese and aluminum were also found to exceed safety standards. The groundwater was found to have much higher levels of metals and organic carbons than the river's water, but substantially less dissolved oxygen. Average groundwater temperatures were also much lower than that of the river, ranging between .
Geology At least four major glaciations occurred in the Puget Sound region during the Pleistocene, of which the Vashon Glaciation was the most recent. Beginning about 15,000 years ago, the Vashon Glaciation saw the Puget Lobe of the Cordilleran ice sheet advance south and cover much of the region around Puget Sound, before retreating several thousand years later. The advance and retreat of the glacier deposited sediments such as glacial till (dense, silty sand) and glaciolacustrine deposits (a mix of silt and clay), with deposit layers thicker than in some areas. A major meltwater channel formed what is now Lake Sammamish and the Sammamish River valley. This broad and shallow channel allowed the nascent Sammamish River to deposit fine-grained alluvium across a broad floodplain, creating meanders and oxbow lakes. These alluvial deposits mainly consist of silt, sand, and gravel, with small amounts of clay.
Biology
As it connects Lake Washington and Lake Sammamish, the Sammamish River is used by a greater variety of fish species than many other streams in western Washington. The Sammamish allows Chinook and Coho salmon from the Issaquah Creek system and the Issaquah Salmon Hatchery to access Lake Washington. Other salmonids present in the river include Sockeye salmon, Kokanee salmon, and cutthroat trout. Small numbers of steelhead trout have been observed in the Sammamish's tributaries.
Other fish known to be present in the watershed include various sculpin species, longfin smelt, northern pikeminnow, peamouth chub, three-spined stickleback, largescale sucker, longnose dace, and brook lamprey. Non-native species such as yellow perch, largemouth bass, and brown bullhead are known to be present; predation by these introduced species may pose a risk to salmon fry.
The river has a limited diversity of benthic invertebrates. Fly larvae (especially chironomids) and oligochate worms are by far the most common invertebrates found in the river, alongside variable quantities of mayfly nymphs. The dominance of fly larvae in the river may serve as an indication of environmental stress. Fingernail clams are also frequently found across the Sammamish, but especially on its lower course, while mites, ramshorn snails, ostracods, amphipods and various insect larvae are more common upstream. Other common invertebrate species noted in surveys include Asiatic clams, bladder snails, nematodes, and the leech Helobdella stagnalis. The removal of the old-growth forests and wetlands around the Sammamish has pushed out many of the species which historically used the watershed. As of 2021, about 32% of the Sammamish River's riparian habitat is forested, totaling . These forests improve water quality and allow for greater food resources to fish such as salmon. Willows are the most common trees near the beginning and end of the Sammamish's course, while Populus trichocarpa (cottonwood) makes up much of the tree foliage around the river in Bothell. Conifers make up only a small portion of the trees in the riparian forests. Himalayan blackberry and reed canarygrass are common invasive plants across the riparian zone, with the latter originally introduced as an anti-erosion measure.
Mammals adapted to human-modified environments such as deer, river otter, beavers, opossums, and coyotes are known to reside in the watershed. The wetland and riparian forest habitats along the river have traditionally served as habitat for migratory birds. Birds such as red-tailed hawks, great blue herons, and harlequin ducks may be found in the area.
Human history Prior to human modification, the river was slow-moving and meandering, forming a large floodplain. The river had an estimated length of around , more than twice its current length, and regularly overflowed its banks. The river valley was densely forested, dominated by western redcedar, hemlock, and Douglas fir trees, with willows and deciduous vegetation most common directly along the riverbanks. Early Euro-American settlers reported plants such as Malus fusca (wild crab-apple), Corylus cornuta (beaked hazelnut), Urtica dioica (nettle), and Typha latifolia (cattail) around the river. About half of Lake Washington's salmon and trout populations traveled through the river to spawn.
The Marymoor Archaeological Site, among the most influential archaeological sites in the Puget Sound region, is located on the Sammamish floodplain near its source. Dating estimates for the site range from 2400 years before present to over 8000. At the time of Euro-American contact, the Sammamish basin was inhabited by the eponymous Sammamish people, an autonomous division of the broader Duwamish people. They were closely connected to other nearby Coast Salish nations, and spoke a similar dialect of Lushootseed to the rest of the Duwamish. The village of ('a place where something grows') was located near the mouth of the river on Lake Washington.
After colonization Early Euro-Americans in the region dubbed the Sammamish people the "Squak" tribe. Due to this, the Sammamish River was originally referred to as the Squak Slough. The Sammamish river valley was among the territories ceded under the 1855 Treaty of Point Elliott in exchange for federal recognition of hunting and fishing rights. The treaty rights for the Sammamish area are currently held by the Muckleshoot Tribe.