Torrential rain in parts of British Columbia set off landslides on Sunday and Monday, choking off highways, trapping dozens of vehicles and prompting evacuations and flood warnings.
Residents of Merritt, a Canadian city of more than 7,000 people about 170 miles northeast of Vancouver, were told on Monday to leave their homes immediately after heavy rain caused the Coldwater River to spill its banks.
Flooding then disabled the city’s waste water system, a municipal notice said, warning that anyone who stayed faced a “risk of mass sewage backup” that could threaten their health.
“Merritt residents cannot use any water, including running faucets and flushing toilets,” British Columbia’s minister of public safety, Mike Farnworth, said at a news conference on Monday.
Merritt officials said floodwaters also inundated two bridges that spanned the river, which flows for 59 miles from the Cascade mountain range, and overwhelmed a third bridge, making it impassable.
It was not clear early on Monday whether there were injuries or deaths from any accidents related to the severe weather.
“To the people of Merritt, and to all British Columbians affected by the flooding: please stay safe,” Prime Minister Justin Trudeau said on Twitter on Monday. “We’re ready to provide whatever assistance is needed as you deal with and recover from the flooding and this extreme weather.”
Bill Blair, the minister of emergency preparedness, said the government was “closely monitoring” conditions in southern British Columbia.
In Agassiz, a small community east of Vancouver, Mayor Sylvia Pranger declared a state of emergency after a landslide occurred and a flood watch warning was issued for the District of Kent, which includes Agassiz. The landslide blocked traffic along two roadways, officials said: Rockwell Drive, stretching about 1.5 miles long, and the Lougheed Highway, which runs for about 95 miles and is also known as Highway 7.
Martina Martinkova, who was driving with her daughter on that highway near Agassiz, spent more than half a day in her car, which was one of at least dozens of vehicles brought to a standstill by the muck.
In an interview aired by the Canadian Broadcasting Corporation, Ms. Martinkova, sitting in her dark car with her child peering over her shoulder from the back seat, said people in vehicles around her were sharing food and water throughout the ordeal.
“We were very lucky it didn’t hit us,” she said of the landslide, adding that the group had fruit and Coke. “It is very scary.”
Paul Doel, who became trapped with his family in a pickup on Sunday night because of two mudslides north of Hope, British Columbia, told the CBC that he and other stranded motorists had “built a little bit of a community.”
He said that at the largest of the two slides, “it looks like the side of the mountain has just come apart,” leaving a huge pile of debris along a large stretch of the highway.
On Sunday, before he became stuck, Mr. Doel said that the heavy rain had washed out several sections of the highway and created deep potholes that destroyed the tires of several vehicles.
About 150 people were trapped in Mr. Doel’s group, he said, including health care workers and a highway department crew member. Despite the long hours without word from the authorities, he said no one had panicked. “We’re just hanging out,” he said, adding: “We’ve got internet so that’s saving a lot of people.”
Mr. Farnworth told reporters on Monday that search and rescue crews were trying to determine “what the best way is to reach the people who are stranded in their vehicles.”
He said that about 80 to 100 vehicles became trapped on Highway 7, and that emergency workers were still hampered by “very difficult weather.” Air rescues were possible, he said, but were being complicated by strong winds.
“They will be doing an assessment, and they will determine the best way, the safest way to be able to extricate people who are trapped between those slides,” Mr. Farnworth said of the crews.
The heavy rainfall extended into the Pacific Northwest of the United States, including Washington State. The weather system was caused by an atmospheric river, part of a convergence of storms that was so vast that it swept from California into Washington and southern British Columbia.
On Monday, the National Weather Service said heavy rain in Washington would result in river flooding and a threat of landslides, along with power outages caused by strong winds. Heavy rain would push east in the afternoon, the service said.
The severe weather comes after weeks of wildfires in the region. The authorities have warned that areas where vegetation has been shorn by fires could become susceptible to streams of rushing mud during heavy rains. Merritt had experienced record high temperatures and wildfires during the summer.