Adult Coho Salmon use of restored side channel habitat

We examined the use of restored side channel habitat on the Salmon River, Oregon, by adult Coho Salmon Oncorhynchus kisutch. Our objectives were to determine whether redd densities of Coho Salmon in the restored side channels reached a level comparable to that of natural side channel habitat and to estimate the rate at which the use of the newly available side channel habitat increased after the restoration effort.

We restored year-round flows to side channels by excavating bermed or depositional substrate material at side channel entrances and constructing bar-apex wood jams at the entrances to maintain channel scour. Additionally, we added large wood to the side channels to increase the complexity of the targeted habitat. We compared yearly redd densities in the restored side channels with those of a nearby natural side channel during 2009–2019. To estimate the rate at which spawning was reestablished in the restored side channels, we fit a linear model to the relationship between year and the log ratio between spawners in the restored side channel and spawners in the Sandy River basin.

Redd densities of Coho Salmon were initially low, averaging 7.7 redds/km in the first 7 years, but by the last 4 years of the study period, the average density of 21.3 redds/km was comparable to that of a nearby established natural side channel. As a percentage of the Sandy River basin population, spawners in the restored side channels increased at an estimated rate of 24% per year, from an average of 1.3% in the first 4 years to an average of 10.5% in the last 2 years.

Eleven years of monitoring 2.7 km of restored side channels provided compelling evidence that the implementation of federal and state recovery plan objectives to restore impaired side channel habitat will benefit Coho Salmon and demonstrated the importance of long-term monitoring when estimating the response of spawning Coho Salmon to side channel reconnection.