ABSTRACT

Restoration efforts to improve stream habitat for salmonids often include the addition of large wood (LW), yet the success of these efforts in streams of different size and geomorphic settings remains poorly understood. In addition, success is rarely directly measured in biological terms due to the difficulty of collecting fish data at the appropriate scales. We present the results of a large restoration effort conducted in 2016 in a coastal watershed in Oregon, USA where 680 LW pieces were added to the stream channel to improve fish habitat. We combined hydraulic modelling in three sites (5-12 km2), annual topographic surveys up to 4 years after LW placement, detailed topographic surveys before and after the restoration in 7 additional sites (3-22 km2), and Coho Salmon smolt production and adult returns since 1997. Surveys and hydraulic modeling indicated a 30% decrease in velocity after the LW additions, as well as improved flood plain connectivity, increases in channel width and the creation of secondary channels. However, these changes are channel size dependent; larger sites responded more quickly to LW addition, but changes in smaller sites may be more enduring due to lower wood mobility. Results from the first 2 years of fish monitoring following the LW addition showed a 29% increase in Coho Salmon smolt abundance compared to the 10-year pre-treatment average. Smolt abundance at control sites showed a smaller increase. Overall, the fish response has been commensurate with observed changes in channel form and results from hydraulic modeling.