Sammamish River
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The Sammamish River (referred to as Sammamish Slough by long-time local residents) flows through north King County, Washington, draining Lake Sammamish into Lake Washington. Although a relatively large portion of the river drainage is in unincorporated King County, towns along its banks include Redmond, Sammamish, Woodinville, Bothell, and Kenmore. The Sammamish River Trail is a paved bicycle and walking trail that runs along the river, from Marymoor Park in Redmond, to Bothell, where it connects to the Burke-Gilman Trail to Seattle.
The Sammamish River basin includes Lake Sammamish – the main source of the river – and four major tributaries and smaller drainages that enter along the length of the river before it discharges to Lake Washington. The total basin drainage area covers approximately 626 km² (242 mi²), including the surface of Lake Sammamish. Lake Sammamish is the largest single tributary basin at 251 km² (97 mi²); again including the lake surface. Big Bear Creek is the largest of the four major tributary basins that drain to the river at 130 km² (50 mi²). The remaining direct drainages to the river include Little Bear, North, and Swamp Creeks with drainage areas of 39, 74, and 64 km², respectively. There is also a significant amount of more diffuse drainage, primarily originating from the hills and valley to the west of the river (68 km²). Although all of the drainages to the Sammamish River have urbanized levels of development (primarily low to medium density development), the Lake Sammamish basin remains almost 60 percent forested (based on 1995 land cover data – excluding the lake surface), albeit primarily second and third growth timber. Among the major stream inflows, Big Bear Creek remains the least developed with 40 percent forest cover based on 1995 data. Little Bear has approximately 34 percent forest cover and North and Swamp Creeks have approximately 20 percent forest cover.
The inflow from Lake Sammamish at the upstream end of the river is controlled by a broad-crested weir. From the lake outlet to Bothell, the river travels over a broad, low-gradient glacially formed valley generally underlain by pre-Vashon (>15,000 years before present) till, Vashon recessional lacustrine deposits, Vashon recessional sand and gravel, recent flood plain deposits and more recent fill (Liesch et al. 1963). As is typical of this geologic setting, the river historically had a single-channel meandering pattern , oxbow lakes, infrequent meander cut-off avulsions and extensive floodplain wetlands (Collins et al 2003). Over time, these typical Pleistocene valley rivers built their channel (aggraded) through fluvial deposition during flooding events (Collins et al. 2003). Flooding events in these valleys historically spilled water into valley wetlands which remained saturated for long periods of time due to the low valley gradient and low elevation relative to the river banks (Collins et al. 2003). Historically, flooding in the Sammamish valley was further exacerbated by the backwater effect of seasonal high water levels in Lake Washington, which may have inundated much of the valley during the wet season (Stickney and McDonald 1977).
The Sammamish River of the early 1800s has been extensively modified as a result of various human efforts to use the channel for navigation, to utilize the floodplain for agriculture, and to reduce flooding in the Sammamish River valley and Lake Sammamish. Prior to major hydrologic modifications that began in the early 1900s, the river was wider and deeper and generally flowed more slowly due to a relatively small difference in elevation between Lake Sammamish and Lake Washington (Chrzastowski 1983). Ajwani (1956) described the historic river floodplain as a “willow-in[f]ested marsh” (p. 58). During this time, the river was also known as Squak Slough and was navigable over its entire length by shallow-draft steamers and used to float logs and coal barges from Lake Sammamish to Lake Washington (Chrzastowski 1983, Stickney and McDonald 1977). The river was also longer and more meandering prior to major channel modifications during the 20th century. The U.S. Army Corps of Engineers reported that the river was approximately 27 km (17 mi) long as part of a study of the feasibility of constructing a canal and lock system between Lake Sammamish and Lake Washington (U.S. Engineer Office 1891) . This is consistent with the Sammamish River channel defined in a 1893-1895 USGS topographic map. The length of the river currently is about 22 km (13.5 mi) measured between the Lake Sammamish weir and the 68th Ave NE (Juanita) Bridge at the confluence with Lake Washington in Kenmore, WA.
Although early drainage and navigation improvement efforts likely affected the form and function of the river, the most significant modifications occurred as a result of two largely federal-funded navigation and flood control projects. The first major change occurred as a result of the reduction in the mean level and seasonal elevation range of Lake Washington in 1916 as part of the development of the Lake Washington Ship Canal and Lock system, which officially opened June 16, 1917. This effectively increased the elevation difference between Lake Sammamish and Lake Washington and increased the flow rate of the river and may have reduced flooding in Lake Sammamish to some extent, although no reliable references to effects of the 1916 project on Lake Sammamish water surface levels has been identified at this time. Ajwani (1956) states that the level of Lake Sammamish was only 1.22 m (4 ft) above the level of Lake Washington prior to the lowering of Lake Washington in 1916 by 2.68 m (8.8 ft). However, the U.S. Engineer Office (1891) states that the elevation difference between the two lakes was 2.93 m (9.6 ft). The current normal pool elevation difference between the two lakes is currently about 4.0 m. Based on Ajwani (1956) the level of Lake Sammamish probably changed little in 1916 (1.22 m historic difference + 2.68 m drop in Lake Washington = 3.9 m, which is very close to the current difference). However, using the U.S. Engineer Office (1891) elevation difference suggests a drop of about 1.61 m (5.3 ft) in the level of Lake Sammamish in 1916.
The second major change resulted from a King County/ACOE channel dredging, straightening project completed in November 1964 that included construction of a weir at the Lake Sammamish outlet (USACOE 1965, USACOE 1964, USACOE 1962). This project practically eliminated flooding in the Sammamish River valley and reduced maximum flood elevations and seasonal water surface elevations in Lake Sammamish. The weir was modified in 1998 to improve passage for anadromous salmon during low flow.
