Flooding in the Snoqualmie-South Fork Skykomish Watershed

The Snoqualmie watershed is often divided into the upper and lower Snoqualmie. These two sections are split by Snoqualmie Falls, a 262-foot high waterfall.

Above the falls, the South and Middle Fork Snoqualmie River flow on either side of the City of North Bend and adjacent unincorporated communities. After the North, Middle and South Forks join, the Snoqualmie River flows west through the City of Snoqualmie and over Snoqualmie Falls.

At the base of the falls, the watershed is relatively steep, but it soon becomes a broad, flat valley. The lower Snoqualmie River meanders in wide loops flowing north by the community of Fall City and the cities of Carnation and Duvall. There is a robust farming community in the lower Snoqualmie valley.

The Tolt and Raging rivers join the Snoqualmie River below the falls. Both rivers carry sediment to the Snoqualmie valley. Sediment can change how the Snoqualmie River moves across its floodplain in the valley and its flood flows.

The South Fork Skykomish River is located along King County’s northern border. The river flows west through the Town of Skykomish and the community of Baring before entering Snohomish County.

Flood risks in the watershed

• The rivers in the Snoqualmie-Skykomish watershed are the most flood-prone in King County. This is due to historical townsites being located along the rivers and that the flood structures, built largely in the 1960s, do not provide complete levels of flood protection to nearby communities.
• Significant rainfall is the primary cause of major flooding throughout the watershed. Slow-moving floodwater is typical in the lower Snoqualmie valley, but faster flows are possible.
• Slow-moving floodwater can still be dangerous for people, and can cover roads, damage buildings, and cut off utilities.
• The steep channels of the Tolt, Raging, and South Fork Skykomish rivers as well as three forks of the Snoqualmie River above Snoqualmie Falls are subject to erosion. This can cause channel migration that can threaten structures near the active channel.
• The cities of North Bend and Snoqualmie both experience regular flooding. Deep floodwaters can inundate low-lying residential and commercial areas, close roads, and isolate communities.
• Flooding from the South Fork Skykomish can cover roads and isolate communities.

Flood infrastructure in the watershed

There are more than 200 levees and revetments maintained by King County in the Snoqualmie-Skykomish watershed. A levee is a raised structure built along a riverbank to contain, control, or divert the flow of water and protect the land along the river from temporary flooding. A revetment is a structure built along a riverbank to reduce erosion risks.

The segments of the Tolt River near Carnation and the South Fork Snoqualmie River near North Bend have nearly continuous levees and revetments. However, these structures provide inconsistent levels of flood risk reduction and floodwater may overtop levees in certain locations.

The City of Seattle owns and operates the Tolt Dam, located upstream from Carnation. The dam and reservoir provide drinking water and generate power. Any flood risk reduction provided by the dam is incidental.

Snoqualmie Falls has a unique and important influence on flooding in the Snoqualmie watershed. Two large flood risk reduction projects, constructed between 2004 and 2012 by the U.S. Army Corps of Engineers and Puget Sound Energy, altered the opening where the river flows over the falls. The projects resulted in a slightly higher peak 100-year water level downstream of the falls and lowered the peak 100-year water level upstream in the City of Snoqualmie. The peak 100-year water level is the height that water is expected to rise during a flood event that has a 1% chance of happening in any given year.

King County completed a Snoqualmie River Hydraulic Study evaluating these projects in 2016. King County also completed a companion Snoqualmie River Hydrologic Study in 2018 that investigated issues related to river gages, historical trends, basin hydrology, and flood events.

Want to learn more about the future of your watershed?

Visit the flood plan

Flooding in the Snoqualmie-Skykomish Watershed is common and expected to increase due to climate change. The University of Washington Climate Impacts Group estimates that by the 2080s greater rainfall and snowmelt will cause the Snoqualmie River to have 15-25% more water October through March. Similarly, the South Fork Skykomish will have 17-33% more water during the same timeframe. The increased amount of water is likely to cause bigger and more frequent floods.

Typical approaches to reduce flood risks in this watershed include:

• Maintaining and repairing existing levees and revetments.
• Buying flood and erosion prone properties from willing sellers. This permanently removes people and homes from harm’s way.
• Elevate eligible homes and structures to raise the living floor above the base flood elevation. This reduces future flood damage.
• Build levee setback projects to reconnect floodplains allowing more space for flood water to move and be stored naturally.

Capital Investment Strategies have been developed for the Tolt River and South Fork and Middle Fork Snoqualmie Rivers.  These will be part of a reprioritization process in 2023 and 2024 at the direction of the King County Flood Control District. Projects will be reprioritized to ways to achieve multi-benefits using an integrated floodplain management approach. The process will have a focus on equity, social justice, and environmental justice.

The Tolt River Corridor Planning and Capital Investment Strategy

South Fork Snoqualmie River Corridor Planning and Capital Investment Strategy

Middle Fork Snoqualmie River Corridor Planning and Capital Investment Strategy