How a U.S. river swelled catastrophic floods in B.C. — and why little can be done to stop it happening again

Two years ago, what would soon become a historic atmospheric river made landfall in B.C. as nearly a month's worth of rain pounded down on the province in less than 48 hours.

Over a terrifying few days, mass evacuations were ordered, thousands of animals were lost, and homes were flooded to their upper levels. Every highway connecting the Lower Mainland to the rest of the province was badly damaged or destroyed.

Over 1,000 travellers became trapped in Hope for days, while hundreds others spent frigid nights on the highway, trapped between mudslides. Across the province, five people died. The floods were the most costly weather event in B.C. history, resulting in over $675 million in damage.

B.C.'s Fraser Valley, a relatively flat area named for the temperamental Fraser River and surrounded by steep mountain ranges, bore much of the brunt of the disaster. What ultimately put the prairie underwater wasn't just the rain falling from the sky, but floodwaters from the Nooksack River in Washington state.

"They have to do something with the American river," said Ben Timms, a Fraser Valley resident of over 40 years, who fled the area as he watched floodwaters rise two years ago.

"It breached all its dikes, and that's where it hit the fan."

When the Nooksack River in Washington state overflows, floodwaters pour into the Sumas River, then cross the border into Abbotsford. Most of the time, the Barrowtown Pump Station keeps that water at bay.

But as floodwaters surged in 2021, the Sumas River breached a dike, causing disaster. Damages were significantly less profound on the American side of the border.

Last month, B.C. and Washington state signed an agreement to address flooding stemming from the Nooksack River. 

The generality of the agreement, which came with no timeline or monetary guarantees, highlights the complexity of managing waters that flow across international borders — and the limited jurisdiction of Canadian officials hoping to mitigate another disaster at home.

Abbotsford Mayor Ross Siemens said nearly a year after the floods the municipality has made "really good progress" on fixing what it can within city limits. 

"The major breaches have been fixed, 17 kilometres of dike have been raised by half a metre ... We're in a better spot than we were in 2021," he said. 

Siemens was one of the local B.C. politicians who signed the trans-boundary agreement, along with B.C. Premier David Eby, Washington Governor Jay Inslee, local leaders from American counties, and First Nations communities on both sides of the borders.

He acknowledged the difficulties of mitigating future disasters when the potential source of a future flood lies not only out of his own jurisdiction, but out of his country's.

"Understanding the complexity and the challenges on the north side of the border are paramount. I think they got the message loud and clear," he said. 

South of the border, in the city of Sumas in Whatcom County, Mayor Bruce Bosch said major efforts to prevent another disastrous flood simply aren't moving fast enough.

"If it were up to me, I'd be out there with a shovel right now," he joked. 

Sumas, with a population of just over 1,600 was the most devastated city on the American side, with 85 per cent of homes and businesses damaged, some beyond repair. But Bosch said he and other small-town mayors weren't aware of talks about the non-binding trans-boundary agreement, and only learned of it afterwards. 

"We haven't been invited to that yet. It was a little embarrassing actually. That the leadership doesn't know what's going on," he said. 

"We're in preparation for the next one, waiting for the powers-that-be to make the changes to protect Sumas," he said. 

In the 1990s, a cross-border task force was formed to research solutions after the Nooksack River overflowed its banks, flooding the Sumas Prairie and Highway 1. In the end, no actions were taken.