Sammamish River - River Modifications

River Modifications

The Sammamish River of the early 1900s 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. Prior to these modifications, the river was known as Squak Slough (after a modified pronunciation of the native village at Issaquah) and was navigable over its entire length by shallow-draft steamers and was used to float logs and coal barges from Lake Sammamish to Lake Washington. The mouth of the river was east of its present position.

Although early drainage and navigation improvement efforts in the first couple of decades of the 20th century 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. It also moved the mouth of the river westward.

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. 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.