Detection of Multi-Scale Salmonid Thermal Refugia from Airborne Thermal Infrared (TIR) Imagery

During summer high temperature events, salmonid species seek out areas of cool river water to alleviate thermal stress. Such thermal refugia are traditionally hard to detect, and their in-river abundance and spatial patterns are largely unknown. Although previous research has examined TIR imaging as a means to sense river temperatures, few have focused on the detection of thermal anomalies typically used by salmonids, with the majority of literature detailing the more general application of thermal imaging to river temperature measurement.  This paper presents results from several thermal image acquisition flights undertaken in Quebec, Canada between 2009 and 2011. To acquire data suitable for resolving riverine thermal anomalies, we developed a custom-designed system capable of simultaneously acquiring high resolution thermal (c. 20cm GSD) and optical (c. 3cm GSD) imagery.  Preliminary findings show that riverine thermal anomalies range from localised ‘coldspots’ of cool water on the scale of 1s to 10s of meters up to larger ‘reach-scale’ thermal anomalies that persist both spatially, over several hundreds of meters, and temporally, over several surveys.  Local-scale thermal anomalies appear to be primarily tributary driven and highly discharge-dependent, with summer low flows resulting in the virtual cessation of several thermal anomaly plumes.  Larger ‘reach-scale’ warming and cooling trends can be linked to local topography, and channel form, whereby steep bounded valleys with deep narrow channels appear to coincide with water cooling while wider, open terrain channels coincide with warming trends.  Variations in relative groundwater and hyporheic inputs associated with these two distinct riverine units are also likely to supplement these warming or cooling trends.  Furthermore, we hypothesise that the presence of emergent boulders at low river discharge may further contribute to reach-scale river warming by acting as heat sinks by temporarily storing and emitting solar radiation.