Climatic effects of the Williston Reservoir on Tsay Keh Dene Nation Territory of northern British Columbia, Canada

Climate change is a major concern for freshwater allocation and management, but there is a lack of attention regarding how water impoundment impacts boreal climate at the intersection of mountain topography. We explore how the local climate was affected by the construction of one of North America’s largest reservoirs, the Williston Hydropower Reservoir, in British Columbia’s Rocky Mountain Trench. High-resolution simulations of the Weather Research and Forecasting model over a 10-year period were used to analyze differences in mean meteorological states with and without the reservoir represented in the landscape. Relative to terrain without the Williston Reservoir, autumn precipitation increased by up to 30 mm (11.5%); however, a substantial precipitation decrease in summer results in a 15% reduction of mean annual precipitation over ridges east of the reservoir. The presence of the reservoir with high thermal inertia in place of vegetated soil reduced convection in the warm season so that heavy precipitation events (> 20 mm) become fewer. An increase in mean annual air temperature within 10 km of the reservoir by 0.2–0.5 °C due mainly to significant warming in autumn was also detected. Overland wind increased by 0.5 m s −1 up to 13 km from the reservoir. We highlight the social importance of these results on the perception of environmental change.