Side Channel Restoration in the Stillaguamish River, Snohomish County, and Use by Juvenile Salmon

A Snohomish County salmon recovery project in the Stillaguamish River, Washington, reconnected the historic “North Meander” in 2006 in order to benefit ESA-listed salmonids. This channel restoration project was implemented in a highly converted floodplain where 69% of historical side channel habitat had been lost. The location, previously isolated by a levee (still present), necessitated complex feasibility, design and construction due to mainstem channel incision, diverse adjacent land uses, and project goals for site longevity that required a regulated inlet to limit sediment accumulation, with periodic excavation. The reconnected North Meander (NM),1.3-2.2 hectares depending on flow, increases existing river side-channel salmonid habitat by approximately 18.5% in the river floodplain, or is equivalent to 8% of the estimated historic amount. In the channel, constructed woody debris exceeds 1200 pieces/km and contains wood jam accumulations (10 jams/km), both high for any sampled location in the Stillaguamish watershed. On the streambanks and across the site, 11 acres of new vegetation has been planted and maintained. Post-construction monitoring of fish use has taken place at NM and one comparison site (2008-2010) during the spring juvenile salmon smolt migration period. We used baited minnow traps and beach seines and captured 15 fish species among all dates and years. In particular, migration timing and use of NM by juvenile hatchery Chinook salmon among years was brief relative to the protracted, March-June, juvenile wild Chinook out-migration that began with early fry-migrants (<50mm length). NM total juvenile Chinook salmon density averaged 0.05 fish/m² and peaked at 0.109 fish/m², approximately double the comparison site density. The estimated maximum density of 1,090 fish/hectare suggests Chinook use was within a range comparable to nearby Skagit River off-channel and river edge habitats, and other Stillaguamish River estuary locations. Although we did not measure growth or determine residence time, these results imply relatively high use by Chinook salmon, and a disproportionate benefit to wild fish, consistent with the intent of the project. This project recovers salmonid rearing habitat capacity and provides many habitat features important for juvenile salmon survival during the smolt out-migration period. Sediment, flow, wood, and vegetation will be actively and adaptively managed over time.