Lower Miller River Floodplain Restoration Project

Project overview

This project will plan and implement river restoration actions in the lower mile of the Miller River near Skykomish, WA to improve habitat for threatened salmon species. Project actions being considered include removal of old levees and revetments to restore natural river processes, construction of modern flood protection facilities to protect adjacent property and infrastructure, excavation to better connect the river channel with floodplain habitats, construction of log jams to increase the complexity of habitat conditions, and replanting of riparian areas. Several project alternatives are being considered and an alternative will be selected in the spring of 2024. Construction is anticipated in 2027.

The lower mile of the Miller River is a highly dynamic and ecologically rich setting, but it has been significantly harmed by past human actions such as logging and flood control. This project presents a rare opportunity to restore up to 120 acres of headwater habitat, which is important to maintain summer low flow further downstream, reduce water temperature, trap large wood and sediment, and increase the amount of habitat for juvenile salmon. These functional headwater habitats will become increasingly important to restore and protect given the expected impacts of climate change in the Snohomish Basin.

Over the past 100 years, numerous public investments have been made in the construction and frequent repair of bridge, roadway, and flood protection infrastructure at this site. Yet the efforts to control the river were ultimately unsuccessful and Old Cascade Highway washed out for the final time in 2011. Due to the prohibitive cost of a replacement bridge across this dynamic and hazardous alluvial fan setting, King County permanently closed this portion of Old Cascade Highway. Removal of remnant infrastructure would reduce flood risks and long-term maintenance costs and would immediately connect critical off-channel habitat for fish. Modern flood protection infrastructure will be constructed as needed to avoid any significant increase in flood-related risks to private property or infrastructure, such as to the BNSF railroad line.

King County is currently conducting an analysis of potential restoration alternatives at the site. A preferred project alternative will be selected in spring 2024 following consultation with the nearby community, tribal governments, project sponsors, and other interested parties.

Project documents

• History of Old Cascade Highway at Miller River

Project funding

Design work has been funded by the following sources:

• Washington State Recreation and Conservation Office (Puget Sound Acquisition and Restoration Fund)
• King County Flood Control District (King County Cooperative Watershed Management Grant)
• King County (Parks Open Space River Corridor Grant and Surface Water Management Fee)