CREATING GROUNDWATER-SOURCED, COLD-WATER HABITAT IN WARMING RIVERS

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. During periods of thermal stress when water temperatures are high, cold-water fishes seek out thermal refuges to alleviate thermal stress. These refuges are known to promote resilient and diverse aquatic ecosystems that can withstand short-term heat waves and potentially long-term climate warming, but they 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 designation as ‘endangered’ in the Species at Risk Act within several watersheds. As a result, proactive human alterations of rivers, such as thermal habitat creation or restoration, is an emerging research topic in this region.

The objective of this study was to investigate two proof-of-concept engineered cold-water refuge designs. In summer 2022 and 2023, we created two thermal refuges by redirecting a portion of the warm river water through an underground trench system filled with gravel to provide the subsurface residence time to cool the water. In summer 2023, we created a thermal refuge using a municipal well and a pump to discharge groundwater to the river at a flowrate of 9 L/s and temperature of 9ºC, when the river was up to 30ºC. The spatial extent and thermal anomalies were monitored via water temperature loggers and a drone equipped with a thermal infrared camera. Time-lapse cameras were installed to monitor fish presence and movement and assess the success of the ‘pumped’ thermal refuge system. The findings from this study will benefit future projects aiming to proactively maintain or enhance thermal diversity in warming rivers.