Fish, as ectotherms, rely on the temperature of the water they are in to regulate physiological processes such as development and metabolism. As global temperatures rise under climate change, water temperature is also expected to rise and pose a threat to fish populations. Water that is too warm increases energy use, speeds up development, and results in smaller size at maturity. The solubility of oxygen in water also decreases at warmer temperatures, which results in less oxygen available to fish.

While average temperatures may rise, there is significant inconsistency in water temperature on a smaller scale that is difficult to detect using models. Such inconsistency is important for fish during warm seasons, as they maintain lower body temperatures by moving between cold patches of the river. This study uses spatially continuous remote sensing data to map these cold patches at a high resolution, and contributes a powerful new framework to ecological climate adaptation planning.

Salmonids such as Pacific salmon and steelhead trout are important cultural, ecological and economic resources in the Pacific Northwest and Alaska. In order to mitigate the effects of climate change on salmonids, it will be necessary to have a more nuanced understanding of the role of microclimates on their ability to acclimate, in addition to considering the effects of general climate trends on their ecology.

The first objective of this project was to characterize thermal heterogeneity in rivers. We used spatially continuous thermal infrared data to describe maximum surface temperatures for 6,106 km of rivers, focusing on the Snoqualmie River in Washington and the Siletz River in Oregon. We then compared these findings with temperatures predicted by the NorWeST spatial stream network model to determine the importance of data resolution in detecting cold patches. Finally, we presented a conceptual model and suggested response variables for how potential drivers of cold patches may be identified in future research.

The second objective was to assess potential future thermal heterogeneity. We did this with three different methods: a simple increase of 2˚C over observed thermal infrared data; a random forest regression model using three climate covariates for a 2080s scenario; and a process-based model, which coupled the Distributed Soil Vegetation Hydrology Model with the water temperature River Basin Model.

The final objective was to illustrate salmon vulnerability in the two case study watersheds. We estimated current and potential future sensitivity and exposure of adult and juvenile salmonids to warm water using a homogenous 2˚C increase in water temperature. Vulnerability ratings of each life stage for five species ranged from zero to six in the Snoqualmie watershed, and from zero to four in the Siletz watershed.

Envisioning, Quantifying, and Managing Thermal Regimes on River Networks. Water temperatures fluctuate in time and space, creating diverse thermal regimes on river networks. New data and tools can identify particular facets of the thermal landscape that describe ecological and management concerns and that are linked to human actions.

Rethinking the Longitudinal Stream Temperature Paradigm: Region-wide Comparison of Thermal Infrared Imagery Reveals Unexpected Complexity of River Temperatures. Our objectives in this article are (a) to illustrate how advances in data collection and quantitative modeling are refining our vision of the thermal landscape of rivers, including an introduction to the idea of a thermal facet or nuanced aspect of the thermal landscape that is ecologically important; (b) to highlight foundational and new science on the biological importance of thermal variability and on how humans are altering the spatiotemporal complexity of thermal landscapes; and (c) to synthesize needs and opportunities in riverine management and science resulting from this emerging understanding of variation in thermal landscapes over time and space.

Projecting Spatiotemporally Explicit Effects of Climate Change on Stream Temperature: A Model Comparison and Implications for Coldwater Fishes. We used a physical process-based model to demonstrate the implications of climate change for streamflow and water temperature in two watersheds with distinctive flow regimes: the Snoqualmie watershed (WA) and Siletz watershed (OR), USA. Our model incorporated a downscaled ensemble of global climate model outputs and was calibrated with in situ and remotely sensed water temperatures. We compared predictions from our processed-based model to those from a publicly available and widely used statistical model.