Hydrology

Sammamish Watershed

The information presented here is excerpted from the 2007 Shoreline Master Program Update prepared by the City of Sammamish.

Definition

The Sammamish watershed includes portions of the cities of Sammamish, Everett, Lynnwood, Kenmore, Brier, Mill Creek, Bothell, Woodinville, Redmond, Bellevue, and Issaquah as well as unincorporated areas of King and Snohomish Counties. The watershed is part of the Cedar – Sammamish River Water Resource Inventory Area (WRIA) known as WRIA 8, which includes two major river systems, the Cedar and Sammamish Rivers, as well as Lake Sammamish, Lake Washington, Lake Union, and numerous tributaries to each.

WRIA 8 is located predominantly within the borders of King County, with the northwest portion extending into Snohomish County. The boundaries of WRIA 8 follow the topographic features that define the drainage divide between the Snohomish WRIA (WRIA 7) to the east, and the Green/Duwamish WRIA (WRIA 9) and Puget Sound to the west. The majority (approximately 86 percent) of WRIA 8 is in the Puget Lowlands physiographic region, while the upper (eastern) portion of the WRIA is in the Cascade foothills.

WRIA 8 covers a land area of approximately 692 square miles and is the most populated WRIA in the state with roughly 1.4 million residents. The City and its PAA occupy approximately 21 square miles or about 3 percent of the WRIA 8 land area.

The majority of the City drains to the Sammamish watershed portion of WRIA 8, via the East Lake Sammamish (ELS) and Evans Creek basins. The far eastern edge of the City drains to the Snoqualmie watershed portion of WRIA 7 via the Patterson Creek basin.

The ELS basin encompasses most of the City of Sammamish including the City’s three SMA-regulated lakes, as well as areas to the west and south of the City. The Evans Creek basin includes a small area of northeastern Sammamish and unincorporated areas northeast of the Sammamish City limits.

History

The Sammamish watershed has changed dramatically since the arrival of white settlers, and intensively during the last few decades. During the first part of the 20th century, forests in the Sammamish area were largely harvested for lumber and many timber mills were located in present day Sammamish. After the area was cleared of timber it was used for dairy farming and other forms of agriculture for several decades.

Significant watershed changes occurred in 1917 with the construction of the Hiram Chittenden Locks, which were built to connect the Lake Washington system with Puget Sound. The navigational project lowered Lake Washington by 10 feet and Lake Sammamish by 6 feet, draining many of the associated wetlands, eliminating the majority of riverine and off-channel rearing-habitats for juvenile salmon, and ultimately reducing the Sammamish River gradient and flow patterns (the Sammamish River now represents a substantial thermal migration-barrier to adult salmon returning to their spawning grounds) (WRIA 8, 2005).

Other major changes to the Sammamish system involved channelizing the Sammamish River for flood control purposes (which eliminated 12 miles of river channel), diverting and straightening tributary channels, withdrawing water from streams and aquifers, decreasing floodplain connectivity, filling and draining wetlands, and development of urban infrastructure (WRIA 8, 2005; Kerwin, 2001; Kahler, 2000). The construction of the Locks and development of the Seattle Ship Canal connection to the Puget Sound also caused the historical outflow of Lake Washington / Lake Sammamish system, the Black River, to go dry. Additionally, the Cedar River was rerouted to flow into the south end of Lake Washington, forming WRIA 8 as it is described today.

Beginning in the 1970s rural farms were subdivided and platted for residential and commercial development. Sammamish was part of unincorporated King County until it incorporated as a city in August 1999. Since that time urban development and services have increased, and growth has continued to transform and alter the ecology of the watershed.

Habitats

Despite the alterations that have occurred over time, the Sammamish watershed contains important aquatic and terrestrial habitats that are of significant value to the fish, wildlife, and human occupants of the area. As an example, the watershed provides habitat for numerous anadromous fish species including Chinook (Oncorhynchus tshawytscha), coho (O. kisutch), sockeye/kokanee salmon (O. nerka); coastal cutthroat (O. clarki clarki) and steelhead trout (O. mykiss), and various other species. These species use Lake Sammamish and major tributaries such as Swamp Creek, North Creek, Bear Creek and Little Bear Creek, Cottage Lake Creek, Evans Creek, Issaquah Creek, Tibbetts Creek, and the Sammamish River for spawning, rearing, refuge, migration, and/or foraging (Table 2). Of these, Bear and Issaquah Creeks support the most significant salmonid populations. Additionally, numerous small streams along the Lake Sammamish shoreline including Ebright Creek provide spawning and rearing habitat for salmonids, such as late-run kokanee, coho and sockeye salmon and cutthroat trout.

Kokanee using smaller Lake Sammamish tributaries, such as Laughing Jacob’s and Lewis Creeks, are genetically distinct from other populations in the Lake Washington/Sammamish Basin and are also believed to be of native origin. They are unique in that they spawn later than other stocks (November through January), and they are larger in size (approximately 17.7 inches). An early-run population of kokanee, which once resided in Issaquah Creek, is now believed to be functionally extinct. Middle-run populations, which spawn from late September through November, currently use larger tributaries of the Sammamish River such as Bear Creek. Although recent escapement of middle and late-run kokanee populations have been somewhat stable, their sustainability may be at risk if development impacts continue to occur throughout the basin.

Other native fish species in the watershed are western brook lamprey, river lamprey, peamouth chub, largescale sucker, mountain whitefish, and one or more species of sculpin. Numerous (24) species of nonnative fish also occur in the watershed including brown bullhead, black crappie, pumpkinseed sunfish, and largemouth and smallmouth bass, which can be significant predators of juvenile salmonids.

Salmonid Species

Stream Chinook Sockeye Kokanee Coho Steelhead Cutthroat Trout
Swamp Creek

North Creek

Bear Creek

Little Bear Creek

Cottage Lake Creek

Evans Creek

Issaquah Creek

Tibbetts Creek

Sammamish River

Lake Sammamish

Lake Sammamish is a primary feature of the Sammamish watershed. Approximately 44 percent of the total area of Lake Sammamish is in the City’s jurisdiction. The two major tributaries to the lake lie mainly outside the Sammamish City limits. Issaquah Creek11, which enters at the south end of the lake, contributes approximately 70 percent of the surface flow. Tibbetts Creek, which also enters the south end of the lake west of the Issaquah Creek mouth, is the second largest tributary, contributing approximately 6 percent of surface flow to the lake. The third major tributary is Pine Lake Creek, which is located entirely within Sammamish and contributes about 3 percent flow. Surface water discharges from Lake Sammamish through the Sammamish River at the north end of the lake, where a flow control weir at Marymoor Park controls the discharge volume and rate.

Source: City of Sammamish Shoreline Master Program Update