Surface Water Management Division

Projects

Twin Rivers Slough Restoration Project

Just southwest of the city of Monroe, the Skykomish and Snoqualmie rivers converge to form the Snohomish River. The area where these river waters mix and converge, called a confluence, is visible from Highway 522 where it crosses the Snohomish River.

Located in an active flood plain, this location experiences large-scale flood events. In 2003, SWM completed an engineering, ecological, and geomorphic analysis of the Snohomish River confluence to better understand how this stretch of the river and the landscape around it is impacted during high water and floods, and how to protect and enhance its water quality and aquatic habitat.

A ¾-mile piece of land along the north bank of the confluence had originally contained a natural, narrow side channel of the river. SWM’s analysis identified this property, now known as Twin Rivers Slough, as an opportunity to reopen the side channel, creating a place where the river could flow during high water and floods and where adult and juvenile salmon could seek refuge.

Twin Rivers Slough is a vital last way-point for juvenile salmon to adjust to the river’s naturally changing temperature, salinity, and food sources as they begin their transition into the brackish waters of the Snohomish River Estuary. The estuary, where the river meets the tidal influence of salt water, is where salmon complete their acclimatization to salt water before heading out to sea.

To reopen and restore Twin Rivers Slough, SWM had to stabilize the river bank and protect newly planted vegetation during large-scale floods.

To reduce the erosive speed of flood waters, SWM used an innovative technique known as “flood fencing”. Acting like a stand of trees, large logs are planted vertically along the riverbank. Native cottonwood logs were salvaged from local construction sites for this use. The cottonwood logs can root and grow in place to withstand strong currents, catch driftwood and sediment, and slow the water down.

SWM used large-scale trees in the restoration plantings as another cost-saving measure. Dotted throughout the area trees, up to thirty feet tall, were planted to form islands. In certain circumstances, this method has proven more cost effective than using smaller seedlings. During high water and floods these islands provide pockets of protection for fish and wildlife to naturally congregate.

The large-scale plants and flood-fencing withstood the record floods of November 2006 and February 2007 with less than typical damage for newly implemented restoration projects.

As flood waters receded, juvenile salmon were spotted in the newly restored Twin Rivers Slough.

Benefits

• Minimizes damage from flooding and erosion
• Provides refuge for adult and juvenile salmon during high river flows
• Improves water quality and aquatic habitat along ¾-mile of the Snohomish River

Project partners

• Salmon Recovery Funding Board

Contact

Bob Aldrich, Project Specialist, 425-388-6424