Identifying thermal refuges for salmon in the Coldwater River

As the climate warms, the interaction between surface water and groundwater provides a critical buffer for salmonids facing thermal stress. In the Coldwater River, summer water temperatures often exceed 25 ºC, well above the survival threshold for the system’s vulnerable Chinook, Coho, and Steelhead populations. Using a thermal infrared drone, we conducted video overflights to map potential thermal refuges across the lower Coldwater River, followed by snorkel and habitat surveys to verify fish abundance and habitat quality. We identified 38 cold-water patches, primarily composed of side channels and alcoves. We also identified three notable 3-4 km gaps between cold-water patches which may act as thermal barriers to both upstream or downstream migration. While cold-water patches provided water temperatures up to 10 ºC cooler than the mainstem, snorkel surveys revealed these sites were predominantly used by juvenile salmonids, as the shallow depths and low velocities were unsuitable for adults. Furthermore, high solar exposure at these sites resulted in warming of up to 5 ºC from upwelling source to downstream end, likely as a function of low riparian vegetation. In conclusion, drone-based thermal video overflights were effective at mapping cold-water patches across a large section of river. Site visits revealed many of these habitats are degraded and offer poor habitat, limited to juvenile fish. More research is necessary to identify the natural river processes that maintain and create thermal refuges and determine practical methods of improving the ecological function of cold-water patches.