Effectiveness of Instream Wood Treatments to Restore Stream Complexity and Winter Rearing Habitat for Juvenile Coho Salmon

Abstract

AbstractLarge wood and boulder placement projects have become common in the Pacific Northwest to restore complex stream habitat for juvenile Coho Salmon Oncorhynchus kisutch and other salmonids. We evaluated habitat responses at 91 large wood restoration projects in western Oregon from pretreatment to 1 year and 6 years after treatment. Large logs were arranged in jams without cabling or burial in the bottom or banks of the stream. Projects commonly treated 0.5–1 km of stream, but some extended up to 2.3 km. Significant changes in the amounts of large wood, complex pools, and predicted Coho Salmon rearing capacity were observed within 1 year of treatment. Six years after treatment, the amount of large wood, complex pools, and Coho Salmon rearing capacity remained significantly higher than pretreatment levels by 100, 800, and 32%, respectively, and the surface area of pools and gravel increased significantly over pretreatment levels by 15% and 8%, respectively. However, the amount of large wood decreased in a majority of projects during the 6 years after treatment reflecting net export out of the sites and a lack of recruitment from upstream or local sources. Site‐specific responses in stream habitat were positive overall, but variability among sites suggested independent behavior. Responses of the restoration projects were weakly related to channel size, local lithology, or landscape cover, although retention of large wood was associated with lower gradient and coniferous forest cover. Despite the variability in project behavior, the findings indicate that large wood projects may play a role in maintaining and improving stream complexity and Coho Salmon rearing capacity in coast basins of Oregon, potentially compensating for the lack of natural recruitment of wood to the streams. Attention to location within the stream network and treatment details may improve performance of the restoration actions.