SIDE CHANNEL USE BY SALMONIDS IN THE SAUK RIVER, WASHINGTON

Habitat use by salmonids is a critical concern for river restoration efforts in the Pacific Northwest. This analysis examines the distribution of salmonids within side channels of the Sauk River, Washington, during seasonal low-flow conditions. The Sauk is an unconstrained low-gradient river, characterized by numerous meanders and side channels, with an unconsolidated alluvial substrate. Nighttime snorkel surveys were conducted on two types of side channels: surface-fed channels with a direct connection to the Sauk River, and groundwater-fed channels. The surface-fed channels had greater discharges and higher mean velocities than the groundwater-fed channels. Juvenile Oncorhynchus kisutch were found to be the dominant salmonid in groundwater-fed channels (p < 0.05), while juvenile O. mykiss and O. clarki were dominant in surface-fed channels (p < 0.05). All three species were found in significantly greater numbers in pool habitats (p < 0.05), as compared to either glide or riffle habitats. A possible reason for the observed pattern of side channel use may relate to body form or differential channel hydraulics. These observations are particularly relevant because they show that individual side channel characteristics, such as water source and associated flow regime, may alter the distribution of juvenile salmonids. Therefore, channel restoration efforts that focus on the use of side channels as a primary objective need to consider the preferences of salmonids in terms of differential habitat utilization. This issue relates directly to the necessity of maintaining high levels of habitat diversity within the context of dynamic fluvial systems.