Quantifying Small-scale Temperature Variability using Distributed Temperature Sensing and Thermal Infrared Imaging to Inform River Restoration

Abstract

Abstract. Watershed-scale stream temperature models are often one-dimensional because they require less data and are more computationally efficient than two- or three-dimensional models. However, one-dimensional models assume completely mixed reaches and ignore small-scale spatial temperature variability, which may create temperature barriers or refugia for cold water aquatic species. Fine spatial- and temporal-resolution stream temperature monitoring provides information to identify river features with temperature ranges that differ from the reach average. We used a distributed temperature sensing system to observe small-scale stream temperature variability, measured as temperature range through space and time, within two 400 meter reaches in summer 2015 in Nevada's East Walker and mainstem Walker Rivers. In addition, thermal infrared aerial imagery collected in summer 2012 quantified the spatial variability of river temperatures throughout the Walker Basin. Both the distributed temperature sensing data and thermal infrared aerial imagery were used to corroborate prior temperature model results. Additionally, these data highlighted that beaver dams and irrigation return flow channels maximize thermal variability and can provide thermal refugia, while groundwater seeps provide small cooler areas and diversion canals often create warm local temperatures downstream. To extend temperature predictions and obtain a better understanding of thermal variability at the watershed-scale, temperatures bounds from observations by river features were added to the longitudinal temperature predictions. These results show that while bulk stream temperatures are often too warm to support trout and other cold-water species, thermal refugia may exist to improve habitat connectivity and passage for migratory species between Walker River and Lake. Overall, river restoration efforts should focus on maintaining and enhancing features that create this thermal variability and habitat connectivity.