In response to projected global warming, nearly half of the Fraser River basin, Canada’s largest Pacific watershed, is expected to transition from one that is snow-dominated to a primarily rain dominated regime by the end of this century, according to a new hydrology study based at the University of Northern British Columbia. 

The research reveals that the Fraser River basin transitions from one where peak annual streamflow results from spring snowmelt to one where the peak stream flow is often caused by extreme rainfall. 

Dr. Stephen Déry, a UNBC Environmental Science Professor and Dr. Siraj Ul Islam, a UNBC Environmental Science Research Associate, ran a hydrological model for 150 years, from 1950 to 2099 using future climate projections from 21 global climate models.

The projections show in a future, warmer climate that as the ratio of snow to rain declines, river flows peak earlier in the year with reduced volume. Runoff in the cold season (fall and winter) at the outlets of the Fraser River and its major tributaries increase substantially and its year-to-year variability more than doubles compared to the historical time period.

The research is published in two scientific journals: Geophysical Research Letters (Atmospheric rivers increase future flood risk in western Canada's largest Pacific River, https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL080720); and the Hydrology and Earth System Sciences (Quantifying projected changes in runoff variability and flow regimes of the Fraser River Basin, British Columbia, Hydrology and Earth System Sciences, 23, in press. https://www.hydrol-earth-syst-sci.net/23/811/2019/

Dr. Déry and Dr. Islam are the co-authors of the papers.

The marked increase in cold season runoff is most likely linked to heavy rainfall from more frequent landfalling “atmospheric rivers” – long, meandering plumes of water vapour often originating in the tropical oceans that bring sustained, heavy precipitation to the west coasts of North America and northern Europe, often what are known as Pineapple Express storms.

“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,” said Dr. Islam. “The results presented in both papers could provide useful guidance for future water management and decision-making in the Fraser River Basin.”

Despite the steady decline in the snowpacks in the Fraser River Basin in the coming decades, modelling suggests the projected increase in cold season rainfall may lead to peak annual floods that rival or exceed historic record levels by the end of this century.