Atmospheric Rivers Increase Future Flood Risk in Western Canada's Largest Pacific River

Snow-dominated watersheds are bellwethers of climate change. Hydroclimate projections in such basins often find reductions in annual peak runoff due to decreased snowpack under global warming. British Columbia's Fraser River Basin (FRB) is a large, nival basin with exposure to moisture-laden atmospheric rivers originating in the Pacific Ocean. Landfalling atmospheric rivers over the region in winter are projected to increase in both strength and frequency in Coupled Model Intercomparison Project Phase 5 climate models. We investigate future changes in hydrology and annual peak daily streamflow in the FRB using a hydrologic model driven by a bias-corrected Coupled Model Intercomparison Project Phase 5 ensemble. Under Representative Concentration Pathway (8.5), the FRB evolves toward a nival-pluvial regime featuring an increasing association of extreme rainfall with annual peak daily flow, a doubling in cold season peak discharge, and a decrease in the return period of the largest historical flow, from a 1-in-200-year to 1-in-50-year event by the late 21st century. Plain Language Summary Snow-covered areas of the globe are particularly sensitive to global warming. Future projections using global climate models generally show that as the ratio of snow to rain declines, river flows peak earlier in the year with reduced volume. These models also capture the phenomenon of "atmospheric rivers": long, meandering plumes of water vapor often originating over the tropical oceans that bring sustained, heavy precipitation to the west coasts of North America and northern Europe. The present-day frequency of landfalling atmospheric rivers on the Canadian west coast is projected to increase nearly fourfold by the late 21st century, with a proportionate increase in extreme rainfall events. Our work is the first to directly investigate the impact of these "rivers in the sky" on "rivers on the land" using climate model projections. Focusing on the Fraser River Basin, Canada's largest Pacific watershed, and using a business-as-usual industrial emissions scenario, we show that the basin transitions from one where peak flow results from spring snowmelt to one where peak flow is often caused by extreme rainfall. Our modeling suggests that extreme rainfall events resulting from atmospheric rivers may lead to peak annual floods of historic proportions, and of unprecedented frequency, by the late 21st century in the Fraser River Basin.