Characterizing spatiotemporal variability in water temperature and fluvial habitat to inform river restoration for threatened cold-water fishes

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.

Impact/Purpose

Recreationally and culturally important native trout species have recently declined in the Lower Priest River within the Kalispel Tribal lands of northern Idaho. Potential causes of the declines are changes in flow, temperature, and channel morphology relative to past conditions in the river. The researchers floated the lower 70 km of the river, using temperature probes and acoustic doppler current profiling (ADCP) to produce longitudinal profiles of thermal patterns, water velocity, and depth. They 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 using EPA National Aquatic Resource Survey methods. The results showed that cold groundwater inputs occurred along the river course, but solar radiation warmed it in wide, slow-moving stretches. Good conditions for the targeted fish were found where cold-water refuges coincided with adequate depth and cover. Large deep pools and cover for fish (e.g. overhanging vegetation, brush, large woody debris, and undercut banks) were limited in the river, and cold-water areas associated with tributary junctions were localized. These findings can be used to design restoration efforts focused on increasing instream cover, habitat complexity, and riparian shading can be focused on restoring and enhancing these riverine features at specific locations where cold water enters and is entrained in the river channel. The study demonstrates the importance of high-resolution, spatially extensive assessments of the ecohydraulic landscape to help prioritize the selection of sites for restoration. The results will be directly applied by the Kalispell Tribe, Idaho Fish & Game, and USGS cooperators in planning fish habitat restoration on the Priest River. The river sampling design, methods, and general results serve as a template for cost-effective river fish habitat restoration in other rivers within the Pacific Northwest and some other regions. The Clean Water Act expresses the public interest in restoring the fishability of waters in the U.S. This research identifies factors limiting fishability in a cold-water river fishery that has had recreational importance and cultural importance to the Kalispell Tribe. Furthermore, it provides a template for restoring fish habitat supporting that fishery. The field and analytical methods used to evaluate channel morphology, fish cover, and habitat in this project were adapted from those of EPA’s National Aquatic Resource Surveys, demonstrating the relevance of those methods to the agency goal of assessing and restoring the fishability and biointegrity of waters in the U.S.

Citation

Torgersen, C., F. Mejia, E. Berntsen, T. Andersen, J. Connor, Phil Kaufmann, AND M. Lorang. Characterizing spatiotemporal variability in water temperature and fluvial habitat to inform river restoration for threatened cold-water fishes. 13th International Symposium on Ecohydraulics, Lyon, FRANCE, May 23 - 29, 2020.