CREATING GROUNDWATER-SOURCED THERMAL REFUGES IN RIVERS TO ADAPT TO A WARMING WORLD

Climate change is causing widespread river warming and a loss or fragmentation of cold-water habitat for aquatic species such as brook trout and Atlantic salmon. In the context of high summer water temperatures, these aquatic species seek out cold-water plumes known as ‘thermal refuges’ during periods of thermal stress. Thermal refuges are often found in rivers at the mouths of groundwater springs or groundwater-dominated tributaries as groundwater is more thermally constant than air or surface water. These refuges are known to promote resilient and diverse aquatic ecosystems that can withstand both short-term heat waves and potentially long-term climate warming, but thermal refuges are not distributed uniformly in space. In the Canadian Maritimes, river warming and loss of cold-water habitat has contributed to the decline in Atlantic salmon, triggering their distinction as ‘endangered’ in the Species at Risk Act within several watersheds. Thus, proactive human alterations, such as engineered thermal habitat creation or restoration, is an emerging research topic in this region and in other locations at the latitudinal or altitudinal limit of cold-water fish distribution.

The objective of this study was to implement engineered thermal refuge designs in warm rivers in Nova Scotia, Canada. In summer 2023, we created a thermal refuge using a municipal well and a pump to deliver groundwater to a discrete point in a river at a flowrate of 8.89 liters per second and temperature of 9ºC, when the river was up to 30ºC. The spatial extent and thermal anomalies were monitored via the use of a drone equipped with a thermal infrared camera and water temperature loggers. The thermal plume continued downstream for at least 60 meters. Time-lapse cameras were installed to monitor fish aggregation to measure the success of the active thermal refuge system. The findings from this study will benefit future projects aiming to proactively maintain thermal diversity in warming rivers.