Riverscape Patterns of Fish and Habitat in the Chehalis River, Washington

Riverscape surveys can be used to identify fish distributions, understand fish-habitat interactions, and provide information for conservation and restoration. The Chehalis River is a 5,500 km² coastal basin in western Washington. Rain dominated hydrology of the basin results in low flows and warm temperatures during summer, potentially limiting salmonid rearing habitat. To describe summer fish distributions and associated habitat, we conducted a continuous and concurrent snorkel and habitat survey of the mainstem Chehalis River (77 km). These results were compared to a similar survey conducted in the Newaukum River (27 km), a tributary to the Chehalis River. Both surveys began and ended at similar elevations. In both survey areas, we identified three fish species assemblages – juvenile salmonids, native cyprinids, and exotic species – organized in an upstream to downstream pattern. These assemblages were associated with a habitat gradient characterized by narrow channel, coarse substrate, and cool temperatures at the upstream end and wide channels, fine substrate, and warmer temperatures at the downstream end. Juvenile salmonids were generally confined to upper portions of both surveyed reaches, highlighting the importance of these areas for summer rearing. Our results provide important context when considering restoration for areas of the Chehalis River watershed.