Snoqualmie/South Fork Skykomish River Basin

The Lower Snoqualmie River basin begins at Snoqualmie Falls and generally drains north toward Snohomish County. The river meanders in wide loops through a largely agricultural valley floodplain, passing through the unincorporated community of Fall City and the cities of Carnation and Duvall. Aside from these three residential and commercial centers, most of the lower Snoqualmie valley supports rural residential, agricultural, and recreational land uses.

The Snoqualmie River is prone to flooding and typically has annual multiple flood events that inundate local farmland and close low-lying roads. The City of Snoqualmie is one of the most flood prone cities in the United States. Flooding causes significant property, economic, and social losses to residents, businesses, and farms. Impacts include injuries to citizens, health hazards, economic and property damages, lost revenue, and increased demand on public safety and infrastructure-related services.

The Raging River flows into the Snoqualmie River from the southwest at Fall City, downstream of Snoqualmie Falls. The entire basin is located in unincorporated King County; the communities of Preston, near River Mile 5 at Interstate 90, and Fall City, at the mouth, are centers of residential and commercial land use. Timber harvesting has been the main land use in the upper two-thirds of the Raging River basin since the early 1900s. Residential development exists throughout the Raging River valley bottom.

In November 1990, record flows (6,220 cubic feet per second) overtopped the right bank along Preston-Fall City Road and nearly filled the entire opening under the Preston-Fall City Bridge in Fall City.

The S.F. Skykomish River basin includes Beckler River and Miller River which join the Skykomish River. Formed at the confluence of the Tye and Foss Rivers about 13 river miles upstream of the King and Snohomish county line, the S.F. Skykomish River is a relatively unpolluted and free-flowing river, with no significant dams in the watershed. It includes several waterfalls and feeds the Snohomish River which empties into Puget Sound at Port Gardner in Everett. The State of Washington has designated many portions of the South Fork Skykomish as scenic.

Homes and other structures at many locations along the South Fork Skykomish River have suffered damage from deep and fast moving water resulting from floods. The largest flood on record in Gold Bar (Skykomish County) occurred in November 1990, when South Fork Skykomish River flows reached 102,000 cubic feet per second. This flood also inundated the Town of Skykomish in King County. During significant flood events, homes in the Town of Skykomish have been struck by flood-borne debris moving at high speeds.

The Tolt River is a major tributary that enters the Snoqualmie River from the east, near the City of Carnation. Its headwaters are at the crest of the Cascades. Land use in the Tolt River valley is primarily residential with lower density development in the upstream valley and higher density development downstream. The upper reaches of the Tolt River basin are mostly within the Forest Production District, where timber harvesting has occurred on an ongoing basis since the early 1900s. The City of Seattle operates a water supply and hydroelectric power dam on the South Fork Tolt River, which was completed in 1963.

Most of the Tolt River basin is in unincorporated King County. The City of Carnation is located along the north bank of the river. Flood and erosion hazards affect unincorporated areas and incorporated areas. Levees line both banks from about River Mile 2 to the mouth. The State Route 203 Bridge crosses the Tolt River at River Mile 0.55 and the Snoqualmie Valley Trail Bridge, formerly a railroad bridge, crosses the Tolt River at River Mile 1.1.

Major floods on the Tolt River have occurred in 1990, 1995 and 1996, in some cases damaging levees and necessitating repairs.

The Snoqualmie River’s three forks (North Fork, Middle Fork and South Fork) begin in the high peaks of the Cascades, follow steep watercourses through the mountains to the confluence near the foot of Mount Si, and combine to form the mainstem Snoqualmie River. The river flows through the City of Snoqualmie and over Snoqualmie Falls.

Land uses along the Snoqualmie River in North Bend and Snoqualmie primarily consist of residential and commercial uses. Rural residential and forestry dominate the upper basin. Flows along the forks are unregulated, with no major reservoirs in the system. Several hydroelectric facilities divert flows, including a dam operated by Puget Sound Energy immediately above Snoqualmie Falls. All of the hydroelectric facilities in this sub-basin lack sufficient storage volumes to control downstream flooding.

The highest flows recorded at the Snoqualmie River were 78,800 cubic feet per second in November 1990. Floods in 1995 and 1996 caused damage to King County levees and revetments in this area. Most of this damage has been repaired.