After passing through Redmond and parts of unincorporated King County, the Sammamish enters Woodinville where it turns northwest and then west.

[2] The total basin drainage area covers approximately 626 km2 (242 mi2), including the surface of Lake Sammamish.

There is also a significant amount of more diffuse drainage, primarily originating from the hills and valley to the west of the river.

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 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.

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 source.

[4] Several species of birds and mammals are plentiful along the river, including waterfowl such as Canada geese, ducks, and the great blue heron.