Storm Florence has left dozens of communities devastated as it moves slowly through the Carolinas.

On Sunday evening the death toll stood at 15, including a mother and a baby who were killed when a tree fell on their home in Wilmington, North Carolina.

A pickup truck ran off Interstate 20 in South Carolina and struck an overpass support, killing the driver. Authorities said a man drowned after his pickup truck flipped into a drainage ditch along a flooded South Carolina road and two people died from inhaling carbon monoxide from a generator in their home.

And while Florence has been downgraded to a tropical depression, it still threatens to cause widespread, catastrophic flooding as rivers swell towards record levels, officials said.

“I cannot overstate it,” said North Carolina governor Roy Cooper. “Floodwaters are rising, and if you aren’t watching for them, you are risking your life.”

Thousands of people have been ordered to evacuate for fear the storm could bring the most destructive flooding in the state’s history.

National Guard troops have been deployed to help in the effort while North Carolina’s utility company, Duke Energy, is attempting to restore power to an estimated 900,000 homes and businesses.

More than 450 people had to be rescued from New Bern after homes were flooded, trees were uprooted and boats were swept on to land.

On Saturday Donald Trump declared North Carolina a “major disaster” and the Federal Emergency Management Agency (Fema) announced funding would be made available to help people recover from damage to their homes and businesses.

In the city of Fayetteville, North Carolina, authorities told thousands of residents near the Cape Fear River and Little River to get out of their homes by Sunday afternoon because of the flood risk.

“The worst is yet to come,” said the city’s mayor Mitch Colvin. “If you are refusing to leave during this mandatory evacuation, you need to do things like notify your legal next of kin because the loss of life is very, very possible.”

Eleven deaths in North Carolina have been linked to Florence and three deaths have been reported in South Carolina, including a couple who died of carbon monoxide poisoning after using a generator inside their home during the storm.

Horry County Chief Deputy Coroner Tamara Willard said the bodies of 63-year-old Mark Carter King and 61-year-old Debra Collins Rion were found in Loris on Saturday afternoon.

Florence blew ashore in the Carolinas as a 400-mile wide category one hurricane on Friday morning with 90mph winds.

By Saturday it had been downgraded to a tropical storm and winds had dropped to around 35mph as its centre passed by Columbia in South Carolina on Sunday morning.

Florence was expected to turn north towards Ohio on Sunday before heading northeast back towards the coast as it gradually weakens in strength.

“This is still a catastrophic, life threatening storm,” said Zack Taylor, a meteorologist with the National Weather Center’s Weather Prediction Center.

“It has already dumped 20-to-30 inches of rain on parts of the Carolinas with more to come. And many of the rivers will see prolonged flooding, some not cresting for a few days.”

The US president plans to visit the region this week and he tweeted his “deepest sympathies and warmth” to the families and friends of those who had lost their lives.