Atmospheric Rivers & Flooding
Atmospheric rivers are long, narrow flows of moisture-laden air that are often thought of as “rivers in the sky.” The atmospheric rivers that affect British Columbia and the west coast of the United States originate in the tropical and sub-tropical Pacific Ocean, sometimes carrying water vapour from as far away as Hawaii. Atmospheric rivers frequently occur during the fall and winter in British Columbia. Annually, they contribute to 20%-25% of the rainfall on BC’s coast.
When atmospheric rivers move over land and encounter mountain ranges, the water vapour rises and cools in the atmosphere, transforming into intense rain or snow, depending on the location and time of year. On BC’s south coast, with its steep mountain ranges, atmospheric rivers frequently discharge precipitation on the western, coastal side of the slopes. Since atmospheric rivers are narrow, one watershed may face considerable impact from an atmospheric river while an adjacent one is hardly affected.
Although atmospheric rivers are predominantly beneficial, they can also be hazardous. Atmospheric rivers that carry significant water vapour, bring heavy winds, and stall over vulnerable watersheds can be dangerous. Resulting precipitation can overwhelm urban stormwater infrastructure systems, streams and rivers, leading to localized and widespread flooding. Warmer air brought by atmospheric rivers can also raise freezing temperatures to higher elevations, further contributing to flooding through rain-induced snowmelt
2021 Flooding in BC
In November 2021, a series of atmospheric rivers caused considerable damage in BC’s Lower Mainland and Interior regions and neighbouring Washington State. These rainfall events caused landslides and flooding of streams and rivers, with consequences that included five human fatalities, damage to property and infrastructure, and the disruption of transportation routes. The overflow of the Nooksack River in Washington State led to a dike failure and considerable flooding of Sumas Prairie in the Fraser Valley where at least 640,000 animals kept as livestock on farms died.
Although prediction models can alert members of the public when an atmospheric river is several days away, its intensity and exact path are difficult to determine until it is close to impact. Factors before and after the atmospheric river can also influence its impact. In the weeks before the mid-November 2021 atmospheric river, the Lower Mainland experienced several intense storms that saturated the soil. Subsequent atmospheric rivers complicated emergency response efforts and continued to threaten critical infrastructure since watercourses and the surrounding environment did not have enough time or capacity to absorb the excess water.
Flooded barns, Abbotsford, November 2021. Photo: Province of British Columbia
Climate Change and Atmospheric Rivers
Although the extent to which climate change is impacting atmospheric rivers is uncertain, a warmer atmosphere has greater potential to carry more water vapour. As a result, extreme precipitation may occur more frequently, contributing to flooding in the area unless infrastructure, land use and flood management practices adapt accordingly to reduce the associated risks.
Currently, the Fraser River is a freshet-dominated river, meaning that peak water flows – and resulting floods – tend to occur in the spring and early summer due to snowmelt from the mountains. Researchers have recently found that some peak flows may start to shift to the fall. The probability of extreme rainfall events may increase as more precipitation falls as rain rather than snow.