Canada was still trying to reach 18,000 people stranded on Thursday after floods and mudslides destroyed roads, houses and bridges in what could be the costliest natural disaster in the country's history.

Receding flood waters were helping rescue efforts, but the downpour blocked off entire towns in the province of British Columbia and cut access to the country's largest port in Vancouver, disrupting already strained global supply chains.

Premier John Horgan declared a state of emergency and said the death toll would rise from the one confirmed fatality.

Many towns are in mountainous areas to the east and northeast of Vancouver with limited access.

Shoppers emptied grocery shelves, although the shortages were as much down to panic buying as disrupted supply chains.

In Ottawa, Minister for Emergencies Preparedness Bill Blair said all river flows in the province were beginning to drop as the rain lightened.

"The situation remains critical, however, but there is in fact an improvement," he told a briefing.

Ottawa has promised to send hundreds of air force personnel to British Columbia, the first of whom have already arrived. Thousands more are on standby.

Defence Minister Anita Anand said the military would be there for at least 30 days.

The flooding also hit the U.S. state of Washington, as President Joe Biden noted before a meeting with Prime Minister Justin Trudeau.

"We've been good friends for a while... we're both keeping our minds close to the families affected by the storms, flooding in the British Columbia area and the Pacific Northwest," he said in the Oval Office.

Residents in Merritt, which has been cut off for almost four days, told CTV on Thursday that waters were starting to drop.

Late on Wednesday, emergency workers temporarily opened a narrow road to Hope, which had also been cut off since Sunday. Once people had left, the road would be closed again, the provincial government said.

At one point the city of Abbotsford, to the east of Vancouver, feared the waters would overwhelm their pumping station and force the evacuation of all 160,000 residents.

Mayor Henry Braun said on Thursday there had been no change in the status of the pumping station and water was receding "at a pretty good clip (rate)" in some parts.

"We continue to move toward the recovery phase of this emergency," he told a briefing, while noting that more heavy rain was forecast for next week.

"We are not out of this by a long shot yet," he said, adding he had been promised help by Trudeau and provincial ministers.

"I take them all at their word. But I've also prepared them for one big bill at the end of this," he said, estimating it would cost up to C$1 billion ($792 million) to repair local damage.

This strongly suggests the final amount will far exceed the C$3.6 billion in insured losses from wildfires that hit Alberta's oil-producing region of Fort McMurray in May 2016.

"Easily the costliest natural disaster in Canadian history. Won't even be close," tweeted University of Calgary economics professor Blake Shaffer, a specialist in climate policy.

The disruption to Vancouver's operations is set to exacerbate existing supply chain issues and could even make Christmas trees harder to find, farmers said.

A massive wildfire in the same region during a heat wave this summer may have left hills devoid of vegetation that contributed to the flooding and mudslides.