Characterizing spatiotemporal variability in water temperature and fluvial habitat to inform river restoration for threatened cold-water fishes
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Aquatic organisms are exposed to multiple stressors due to habitat modification and climate change. Restoring degraded lotic systems requires initial assessment of the ecohydraulic landscape to identify stressors and determine riverine features that can be enhanced. The Lower Priest River in northern Idaho (USA) has had recent declines in bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi), potentially due to changes in flow, temperature, and channel morphology. We identified cold-water refuges and characterized channel morphology by floating the lower 70 km of the river in a Lagrangian fashion using temperature probes and acoustic doppler current profiling (ADCP) to produce longitudinal profiles of thermal patterns, water velocity, and depth. We complemented these spatially continuous data with (1) in-situ thermographs placed every 2 km and at selected pools and tributary junctions, and (2) physical measurements of channel morphology, and instream and riparian cover. Preliminary results revealed that water temperature decreased in a downstream direction due to groundwater and surface water exchange and tributary inflows. Downstream warming was observed in low velocity reaches where travel time was greater and net solar radiation was relatively high, whereas high velocity reaches showed less warming in a downstream direction. Cover for fish (e.g. overhanging vegetation, brush, large woody debris, and undercut banks), large woody debris, and large, deep pools were limited, and cold-water areas associated with tributary junctions were localized. Restoration efforts can be focused on restoring and enhancing these riverine features at specific locations. These findings highlight the importance of high-resolution, spatially extensive assessments of the ecohydraulic landscape to help prioritize the selection of sites for restoration.