An “atmospheric river” caused major flooding and mud and rock slides in southern British Columbia on Sunday and Monday, trapping families in cars without food or medications, shutting down highways, downing power lines and placing thousands of people under a sweeping evacuation order.

In one video posted to Twitter, a house is shown floating down a brown, overflowing river, before breaking apart on a bridge. Other footage shows inundated highways and bridges, and the blinking lights of vehicles partially submerged near Vancouver.

Landslides trapped as many as 100 vehicles on Highway 7 overnight, a scenic route running east of Vancouver, Mike Farnworth, the province’s public safety minister, told reporters on Monday. People stuck inside cars pleaded for officials to deliver food, insulin and diapers as they waited to be rescued, local media reported.

“This is a significant storm,” Farnworth said, adding that gusty winds could hinder rescue efforts. “It ranks up there. Is it the largest? We don’t know yet.”

Thousands of people are under evacuation order in southern British Columbia on Nov. 15 due to major flooding caused by an “atmospheric river.” (Justin Scuiletti/The Washington Post)

The 7,000 residents of Merritt, a city some 170 miles northeast of Vancouver, were evacuated Monday, after floodwaters inundated two bridges across the Coldwater River and blocked access to a third. Flooding also shut its wastewater treatment plant, and officials warned of a “risk of mass sewage backup and personal health risk.”

BC Emergency Health Services said on Twitter that it had transported nine people with minor injuries to the hospital Sunday night after a landslide near the city of Agassiz, roughly 77 miles east of Vancouver. It was not clear whether there were more injuries or any deaths related to the flooding.

“To the people of Merritt, and to all British Columbians affected by the flooding: please stay safe,” Prime Minister Justin Trudeau said on Twitter. “We’re ready to provide whatever assistance is needed as you deal with and recover from the flooding and this extreme weather.”

Lt. Cmdr. Tony Wright, a spokesman for the Joint Rescue Coordination Center in Victoria, British Columbia, said that the Canadian Forces center, which leads and directs search-and-rescue efforts in the region with the Canadian Coast Guard, has three CH-149 Cormorant helicopters helping to rescue stranded motorists on Highway 7.

“The helicopters are ideally suited for hoisting people out of difficult areas,” Wright said. “They’re prioritizing their work on places where they might need to hoist people out instead of landing, loading people and getting them out. … It’s not to say that they couldn’t do all that, but that’s what they’re prioritizing right now.”

The flooding in British Columbia is the latest extreme weather event to hit the region, which experienced deadly heat waves and devastating wildfires this summer, including a blaze that erased the entire town of Lytton off the map. It was poised to raise more questions about whether the province could have been more prepared.

“Landslides … are unpredictable and they happen,” Farnworth said, defending the government’s response. “What’s important is that once they do and we realize … Search and Rescue is called out right away.”

One woman from Abbotsford told the Vancouver Sun that she was forced to sleep in her car on Highway 7 with her two young children Sunday night, conserving gas by turning on the engine for just a few minutes at a time to keep the kids warm. She and others had resorted to collecting rainwater to drink, the newspaper reported.

In a video released Sunday by the British Columbia transportation ministry, muddy water streams down part of the province’s Highway 5, taking up more than a full lane in some parts. Another image shared by the ministry shows mud and debris blanketing the province’s Highway 1.

Highway 5 and other highways in the region remained fully or partially closed on Monday, and flood watches were still in effect for the Fraser Valley and other areas. A flood warning was in place for the Coquihalla River. Public weather alerts also warned of heavy snowfall along the Coquihalla Highway on Monday night.

Abbotsford, a city just above the U.S. border, had also asked residents of certain areas to evacuate due to mudslides and flooding. The British Columbia government urged evacuees to register with emergency support services.

The deluge of water came from an atmospheric river — a winding stream of intense water vapor likened to a river in the sky — which dumped precipitation on the province’s southern coast on Sunday and Monday, and drenched the Pacific Northwest in the United States. The unusual rainfall caused rivers in the region to rise.

Armel Castellan, a meteorologist with Environment Canada, told reporters on Monday that the current flooding followed two months of unusually heavy rain in the area. He said that more than six inches of rain has fallen in some of the affected areas and that more power outages were likely.

“We’re seeing what has fallen in the last 24 to 36 hours as being equivalent to and surpassing some monthly November statistics for some locations,” Castellan said, “which is saying a lot considering November is our wettest month.”

Atmospheric rivers typically pose flood and storm risks and have so far remained relatively stable in the face of climate change, according to a Yale University study this year. But the study predicted that rising greenhouse gas emissions could cause precipitation from atmospheric rivers to rise 20 millimeters, or 0.8 inches, per month.

A NASA-led study in 2018 projected that while climate change could slightly reduce the number of atmospheric rivers in the future, the warming of the planet will probably cause them to be longer, wider and more intense.

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