Edgecomb Creek Stream Restoration Design

SWCA designed stream restoration options for Edgecomb Creek in Snohomish County as part of the on-site mitigation for converting the once agricultural land to a multi-unit commercial development. The primary goals of the project were to restore channel stability and create a self-maintaining, quasi-stable geomorphic state; restore fish passage, floodplain connection, a riparian vegetation buffer, and aquatic and riparian habitat diversity; and reduce anthropogenic impacts to Edgecomb Creek.

The design was coordinated and reviewed by the Washington Department of Fish and Wildlife and the sovereign Tulalip Tribes. The project will be monitored for 10 years as part of the CWA Section 404 permit requirements to ensure that the stream corridor remains stable and continues to provide adequate ecological function both for the target salmonid species as well as other native aquatic fauna.

The restoration of Edgecomb Creek will provide several improvements in stream ecological functions over the existing degraded stream channel. The majority of the existing stream channel consists of excavated linear ditches in agricultural fields that lack substantial native riparian trees and shrubs, meanders, cobbles or sorting, riffle or pool structures, large woody debris, or floodplain connectivity. The restored stream channel will consist of a meandering channel connected to side channels and wetland habitats within a riparian corridor containing native forest, shrub, and emergent plant communities. Side channels will have different degrees of hydrological connectivity to Edgecomb Creek. Flowthrough side channels will provide off-channel habitat throughout much of the year, while dendrite side channels will provide seasonal off-channel habitat.

In addition, the mainstem and side channels will be enhanced with large woody debris, small woody debris, streambed gravels, and pool and riffle creations. Fish accessibility to the site and upstream reaches of Edgecomb Creek will be improved by the replacement or removal of culverts that currently act as partial fish passage barriers. Once established, riparian habitat corridor will provide immediate and long-term benefits for salmonids and other fish through native plantings that will provide streambank stability, stream shading, stormwater filtration, and wood recruitment; a complex channel system with natural channel sinuosity, pool and riffle structures, and side channels that will provide spawning, rearing, and foraging opportunities; and connectivity to wetland and floodplain habitats that will provide additional water quality improvements, hydrologic regulation, and flood refugia benefits.