Why is B.C. facing the prospect of a flood disaster again?

British Columbia's Lower Mainland is facing flooding, severed highways and evacuations due to the double impact of an atmospheric river and cross-border water flows. Here's an explainer of why the area has been repeatedly hit by similar disasters.

WHAT IS AN ATMOSPHERIC RIVER?

A long, thin, horizontal "corridor" of water vapour typically associated with a low-level jet stream ahead of the cold front of a cyclonic storm outside the tropics. Atmospheric rivers are associated with heavy precipitation in places where they are forced upwards and they transport more water than double the flow of the Amazon River, on average. They transport heat and moisture from the tropics north, giving them the nickname "pineapple express."

Environment Canada says about 30 to 40 atmospheric rivers hit coastal B.C. every year. Normally, that's a good thing, since they replenish snowpacks and water supply. But if they are particularly strong, long-lasting, or occur repeatedly, their impacts can be disastrous, linked to major flooding in 2021, then again in January and October 2024. 

WHAT HAPPENED THIS WEEK?

An atmospheric river brought heavy rainfall to southern B.C., with up to 145 millimetres of rain being recorded in parts of the Fraser Valley from Wednesday to 4 a.m. Thursday. The rain has since tapered off in B.C. But the storm also pummelled Washington state, which has declared a state of emergency, with the Nooksack River breaking its banks on Wednesday.

WHY DOES THE NOOKSACK RIVER MATTER TO B. C.?

When the Nooksack overflows, there's a chance its waters will spread to British Columbia, which happened in 1990 and in 2021, causing extensive destruction. Four years ago, the Nooksack's waters flooded the low-lying Sumas Prairie in Abbotsford, causing billions worth of damage and devastating livestock.

Emergency officials have said that outflows from the Nooksack overnight on Wednesday rivalled those of 2021, although many variables will determine their impact as they flow into B.C.

HOW IS THAT RISK HANDLED?

An extensive dike system and the Barrowtown pump station in Abbotsford protects the Sumas Prairie. The station pumps water out of the low-lying lands into the Fraser River via the Sumas Canal. But it was almost overwhelmed in 2021. The provincial government has since been upgrading the facility, constructing protective walls and improving the machinery, but this work is not yet complete.

Picture Courtesy: THE CANADIAN PRESS/Darryl Dyck