A Characterization of Hyporheic Temperatures with Applications for Salmon Habitat Restoration in a Thermally Impaired River

Elevated stream temperatures represent an important stressor affecting Pacific salmon (Oncorhynchus spp.). In thermally impaired streams, upwellings of shallow subsurface (i.e., hyporheic) water have the potential to create patches of cool-water refuge that allow salmon to persist in otherwise unsuitable water temperatures. Since patterns of hyporheic upwelling are influenced by variations in streambed topography, habitat restoration actions such as engineered log jam construction may be used to preserve or promote upwellings. This strategy requires that hyporheic flows remain cooler in summer, relative to the overlying surface stream, but this might not always be the case. Here we characterize the relationship between hyporheic and overlying surface temperatures during a summer low-flow season in a restored reach of the South Fork Nooksack River. Among six sampling sites, we found that one had hyporheic temperatures that were consistently colder than the overlying surface stream, two had hyporheic temperatures that were variable but more moderate than those of the overlying surface stream, and three had hyporheic temperatures that were not cooler or more stable than those of the overlying surface stream. Habitat mapping suggests that thermally stable hyporheic flow paths may be associated with specific combinations of channel geomorphic units, which influence flow path length, depth, and discharge. These findings may be used to identify potential areas of cool-water refuge and guide the design and placement of habitat restoration actions to promote climate adaptation for salmon populations in thermally impaired streams. Key Points Upwellings of cool, subsurface water have the potential to provide summertime refuge for salmon in rivers that are otherwise too warm. Not all upwellings are cool enough to benefit salmon. Habitat mapping can be used to identify locations with cool upwellings and guide salmon habitat restoration efforts.