Defining Cold Water Refugia at the Confluences of the South Fork of the Eel River

Global warming will raise temperatures in terrestrial landscapes and rivers worldwide, which may pose a threat to the survival of many species that depend on cool temperatures. Although temperature increases are likely to be ubiquitous at large scales, fine scale heterogeneity in temperature rise creates micro-sites in which cooler temperatures persist - thermal microrefugia. Microrefugia already account for species persistence during extreme heat events- for example, brown trout use cold water pools to survive summer heat waves. Identifying, characterizing, and protecting thermal microrefugia are high priority conservation targets for species protection in the face of anthropogenic climate change. Here, we explore the challenge of defining the existence of thermal microrefugia in the context of Steelhead Trout (Oncorhynchus mykiss) in the South Fork Eel River in northern California. Cold-water microrefugia for O.mykiss can form when rivers are protected from warming by: riparian shading, the thermal inertia of deep pools, and cold water inflows into warmer areas, e.g. from cooler tributary streams, groundwater springs or seeps, or hyporheic exchange, from localized cold zones. The variation in temperatures at which salmonids become reliant on cold water refugia, has led to a number of different physical criteria to differentiate thermal micro-refugia from the main channel environment. Sometimes areas cooler than the main channel waters by 2 degrees Cecilius are considered refugia, while other times a physiological thermal threshold, such as 21 degrees Cecilius for O.mykiss, defines the refuge. Here, we illustrate the implications of multiple definitions for the characterization of the volume and temporal variability of inferred thermal microrefugia at two contrasting and intensively monitored confluences in the South Fork Eel River. We compute the water volume at the confluences where (i) water is cooler than 2 degrees Cecilius of ambient conditions, and (ii) where water is cooler than 21 degrees Cecilius. We then explore how the refugia characterized at these two confluences vary from each other on hourly, daily, and seasonal timescales. Finally, we consider the implications of the contrasting refugia definitions for conservation and for management decision-making.