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
alt=A river flows through a flat, grassy area with a small windmill|thumb|The Sammamish River near its source in [[Marymoor Park]]
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 an suburban residential and commercial area of Redmond, passing by a golf course.
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
alt=A river with many trees along it, and in the distance, a small bridge|thumb|The Sammamish River in Bothell, overlooking the Bothell Landing Bridge
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.
{| class="wikitable"
|+Major Sammamish River tributary streams
!Name
!Drainage area
!Length
!Percentage of total streamflow
!Confluence
|-
! scope="row" |Bear Creek
|
|
|19%
|
|-
! scope="row" |Little Bear Creek
|
|
|6%
|
|-
! scope="row" |North Creek
|
|
|14%
|
|-
! scope="row" |Swamp Creek
|
|
|9%
|
|-
! scope="row" |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 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.
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.
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.
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. The name of the river was derived from a Lushootseed word meaning "crooked". 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.
The Sammamish valley was first surveyed for Euro-American settlement in 1870. The clearing of forests for homesteading over the following decade triggered a logging boom in the area. The river was used as a means to transport logs to Kenmore and across Lake Washington. Heavy logging continued until the early 20th century, with a peak from around 1880 to 1900, destroying almost all old-growth forest. Small-scale farming efforts began soon after Euro-American settlement, but were impaired by the area's marshy terrain. Perennial floods which would often cover the entire floor of the river valley in of water. Following the authorization of drainage districts by the State Legislature in 1895, a drainage district was created around the mouth of the river in Kenmore.alt=A steamboat traveling along a wooded river|thumb|The [[Steamboat|steamer May Blossom on the Sammamish River near Bothell, ]]During the 19th century, the Sammamish was one of the main travel routes in the Eastside of King County, with pole-drawn barges serving as the main form of transportation. In 1884, the steam-powered scow Squak began operating as a ferry on the river. However, the Seattle, Lake Shore and Eastern Railway expanded to Issaquah by the end of the decade, rendering river travel on the Samammish less important. Shipping traffic to Bothell continued on the river until 1916.
River modification
In 1917, the Ballard Locks were opened. This led Lake Washington to no longer drain into the Duwamish River, but instead flow directly into Puget Sound through the Lake Washington Ship Canal, lowering water levels in the lake by about . Although the water level of Lake Samammish also decreased, the elevation difference between Lake Washington and Sammamish increased to about , increasing the river's flow rate. All of the river's present course in what is now Marymoor Park was likely an extension of Lake Sammamish prior to the opening of the ship canal.
At around this time, farmers in the region formed a drainage district to manage the river and drain its wetlands. The district took extensive channelization efforts, straightening the upper course of the river to roughly its present position and digging drainage ditches. Most of the river's wetlands and side channels were destroyed, as were many of the spring-fed creeks which drained into it. Beginning in 1928, hydroplane began to race on the river annually in the Sammamish Slough Race. This event was held until 1976, after a boat injured a spectator.
alt=A color photograph of a paved trail along a river, passing under a highway interchange|thumb|The Sammamish River crossing under [[Interstate 405 (Washington)|Interstate 405 near Woodinville, 1992]]
The Sammamish continued to flood annually throughout the early and mid-20th century, with dredging by the drainage district yielding only partial success in limiting floods. In 1961, federal funds were appropriated for the U.S. Army Corps of Engineers and King County to begin flood control efforts, clearing wetland and abandoned river channels along the river. They also removed riparian vegetation such as willows, leading to increased water temperatures. Beginning in 1962, the Corps dredged and channelized much of the river, hardening its banks and deepening it by about through its course. Around this time, the Corps upgraded and expanded a weir about downstream from the source of the river. A ramp downstream of the weir was added to form as a transition zone, aimed at limiting the river to a flow of per second, and ensuring a stable height for Lake Sammamish. A notch was added to the structure in 1998, to allow fish and small boats to pass.
Development and restoration
During the mid-20th century, the Sammamish River valley hosted various dairy, poultry, and berry farms. Land development spread through the area in the 1960s, with the food and beverage industry among the primary drivers. In 1985, the county designated as a protected agricultural zone in response to urban encroachment.
Rapid urban development began in and around Redmond during the 1980s and 1990s, boosted by the growth of technology businesses such as Microsoft and Amazon. Redmond's population grew from 11,000 in 1970 to over 65,000 by the 2020s, with almost twice as many present during the workday. A habitat restoration project in 1994 focused on improving fish and wildlife habitat across three stretches of the river and a tributary stream, while restoration efforts planted of riparian forest along the Sammamish from 2015 to 2025. During this time, a small area of forest was destroyed along the river during construction work on Interstate 405.