The Thermal Sensitivity and Importance of Groundwater-Sourced, Cold-Water Thermal Refugia in a Warming Climate: Insight from the Miramichi River, New Brunswick, Canada

Salmonid reliance on cold-water thermal refugia is expected to increase in future decades as rivers and streams warm. However, the potential for groundwater-sourced thermal refugia to also experience future warming is generally overlooked. Results from several recent groundwater temperature modeling studies are presented to illustrate the thermal sensitivity of shallow groundwater to atmospheric climate change. The timing and magnitude of groundwater warming is shown to depend on the climate scenario, soil thermal properties, groundwater flow, and aquifer configuration. Groundwater-sourced thermal refugia are also threatened by deforestation, excessive groundwater pumping, and aggregate extraction (i.e. sand and gravel pits).

These imminent threats to thermal refugia integrity, and hence cold-water fish populations, may be mitigated by preserving existing thermal refugia, augmenting thermal anomalies that are not utilized as refugia, and creating new thermal refugia in uniformly warm river reaches. Existing thermal anomalies may be enhanced by controlling advective thermal mixing between cold-water tributaries and the river mainstem flow, installing riparian shading, and adding temporary structures for protection from avian predators. New refugia may be created by temporarily pumping groundwater to discrete points within the river during periods of thermal stress. These activities should be informed by local ecological, hydrological, and hydrogeological knowledge.