What is an atmospheric river? A look at the force behind the flooding in B.C.

Section of the Coquihalla at Juliet closed between Hope BC and Merrit due to mudslide Courtesy of BC Transportation/Twitter

Parts of British Columbia have been hit with intense flooding that has washed out highways, destroyed infrastructure, and brought evacuation orders to areas like the Fraser Valley. What some officials are calling “the worst weather storm in a century” is the result of an atmospheric river—a natural occurrence that can have devastating consequences.

What is an atmospheric river?

We usually think of rivers on the ground, but they can also take shape in the sky, forming a long band of concentrated moisture that turns into heavy rain or snow once it makes landfall. Dr. Brent Ward, a professor in the earth sciences department of Simon Fraser University, says atmospheric rivers used to be called “pineapple expresses” since they often originated near Hawaii, but we now know they can form in more places. 

Though it’s common for atmospheric rivers to make landfall in B.C.—there have already been five this year, Ward says—they usually don’t cause this level of destruction. “They’ll hit Vancouver Island and there’ll be landslides and high rivers, but often it’s outside of Tofino, so people don’t notice it as much,” Ward says.

Why do atmospheric rivers happen?

Atmospheric rivers are a key part of the Earth’s water cycles as they transport moisture from the tropics to the Northern and Southern hemispheres. California, for example, relies heavily on atmospheric rivers as a source of rainfall. As with a tornado or hurricane, an atmospheric river doesn’t always make landfall, but experts have found that climate change increases the chances it will, along with the intensity of its impact. 

One 2018 study suggested atmospheric rivers that drift over the Northern hemisphere could increase in size by up to 50 per cent. This is largely due to the warming atmosphere, and as Ward explains, when ocean temperatures increase, it’s easier for water to evaporate up into these sky-bound rivers.

What’s different about the one that hit British Columbia?

The province broke precipitation records over the past few days, but Ward says the pattern of this atmospheric river was also unusual. “It came in a little more East-West,” he says. Once it tracked into the mountainous valleys surrounding Vancouver, “it kept piling up against the mountains, and then dumped all this precipitation at once.”

The intensity of wildfires this summer also played a role. As Ward explains, there’s a strong link between intense forest fires and the chance for debris flows (the technical term for mud or landslides). 

“When we get fires, they’re bigger and hotter, so they’re burning more of the trees and a lot of the organics in the soil,” Ward explains. If forests burn to that extent, the crucial layer which helps retain moisture is stripped away. As a result, a waxy substance is formed over top of the soil, known as a hydrophobic layer. When water hits this, it slides along and doesn’t soak in; Ward likens it to watering a very parched garden. 

“Once the water flows off a slope and hits a steeper area, it’s going fast enough to erode that hydrophobic layer, and sometimes that’s enough to trigger a debris flow,” he says. Ward points out that areas with some of the most destructive debris flows, such as in the town of Merritt, were also where the worst fires happened last summer. He and other environmental experts call this the idea of “cascading hazards.”

How long will this atmospheric river last?

Much of the intense torrential rain has tapered off in B.C., meaning, for now, the worst of this atmospheric river’s landfall in the province has passed. However, its effects can extend to the rest of Canada and beyond, bringing intense rain, snow, wind, and other storm conditions.

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