Responders pulled people from their submerged homes, situated them in rafts or choppers and whisked them to safe and dry ground.
The storm's US death toll climbed to 21 on Monday, with North Carolina reporting 11 deaths
, Florida reporting four, and three each in South Carolina and Georgia.
on Monday declared a major disaster in North Carolina and ordered federal aid to supplement state, tribal and local recovery efforts.
Federal funding is available to people in Beaufort, Bladen, Columbus, Cumberland, Edgecombe, Hoke, Lenoir, Nash, Pitt, and Robeson counties.
Flooding in Lumberton
In the eastern North Carolina city of Lumberton, in one of the counties included in the President's disaster declaration, rescuers worked to save 1,500 people stranded by flooding.
"We have people on roofs (there) as we speak," North Carolina Gov. Pat McCrory said earlier Monday.
The Federal Aviation Administration has ordered flight restrictions over the area so helicopters can rescue them.
Kellie Blue, Robeson County, North Carolina, spokeswoman, said around 3,000 people were sent to shelters after the flooding but the county has no precise figure on the total number residents forced from their homes.
She said ice and water were being shipped in because about 26,000 utility customers in the county lacked water.
Utility crews were working to repair water lines but once water mains were fixed, another road was washed out, and lines burst again.
At one point, she said, people were evacuated to a shelter that had to be evacuated because of flooding.
There are 800 residents living in an evacuation shelter at a high school. Four people remain missing in the county, she said.
Homes, restaurants and business have been lost, she said.
"Our board of education is destroyed," Blue said. "It's completely underwater."
The state reported the evacuation of a prison threatened by flooding. Authorities said 797 inmates from the Neuse Correctional Institution in Goldsboro were taken to other prisons in the state system.
The minimum-security facility for male inmates is near downtown Goldsboro and near the confluence of the Little River and the Neuse River.
Taking shelter at a town hall
Authorities in Marion County, South Carolina, used boats and helicopters to rescue the last members of a group of 150 people who sought shelter against the Lumber River in the Nichols, South Carolina, town hall.
People had been in the town hall since 9 p.m. Sunday, according to Kent Williams, who is the Marion County deputy administrator and a state legislator.
"With the Lumber River and all the rain that Matthew dumped on us, it was more than they could handle. That was pretty much the safest place for them to be," Williams said. "The town hall was the high ground."
Rescue teams took them to a middle school in Mullins.
"The water is rising so fast it's scary. The river is not scheduled to crest until ten in the morning. It's the worst we've seen it," Williams said.
Massive power outages
About 500,000 structures in North Carolina didn't have power Monday.
"That's probably well over 1 million people without power," McCrory said. "That's 10% of our population in North Carolina -- all in a very small area. At least 10%."
Matthew's damage exceeded expectations, Duke Energy said. The power company said it now has to rebuild parts of the electrical system
. It said some of the hardest-hit areas "might be without power all week
More than 469,000 South Carolina customers didn't have power Monday, Gov. Nikki Haley said. She said 8,000 linemen were working to get electricity restored.
And Florida, the first state thrashed by Matthew, still had 169,000 customers without power Monday, Gov. Rick Scott's office said.
Matthew also played havoc with the state's voter registration process. On Monday, a court extended
the voter registration deadline because evacuations.
Meanwhile, the state is working to get back to normal. The Kennedy Space Center will reopen on Tuesday at 6 a.m. Employees are being asked to report to work at their regular times. At the same time, recovery operations are continuing and people are asked to be careful on their return.
Dead and missing
The 11 deaths in North Carolina included some victims who drowned after driving onto flooded roads.
"I cannot stress more: If you see a road that's flooded, do not take your car through that road," McCrory said. "If you see a barrier, do not go around that barrier."
Another five people remain missing in North Carolina, McCrory said.
In South Carolina, 66-year-old David L. Outlaw drowned when his wheelchair got pinned down in standing water at a nursing facility's courtyard, Richland County Coroner Gary Watts said.
Along with the 21 official storm-related deaths in the United States, a member of a tree-removal crew in Florida clearing debris in left behind by Hurricane Matthew lost his life on Monday when a large log rolled on top of him. The incident occurred shortly before 6 p.m. in the Halifax Plantation community near Ormond Beach in Volusia County's extreme northeast corner, not far from the border with Flagler County.
The US deaths came after Matthew devastated parts of the Caribbean, killing more than 300 people in Haiti, said Paul Altidor, Haitian ambassador to the United States. Others report much higher death tolls.
orecasters warned of more "catastrophic flooding" as North Carolina residents braced for swollen rivers to top their banks in the next few days.
"River levels will rise above major flood stage this morning and remain elevated well into next week," the National Weather Service said.
n addition to avoiding all flooded roads, keep an eye out for downed power lines and weakened bridges and roads that look like they might collapse, the National Weather Service said.
Once back home, "walk carefully around the outside of your home to check for loose power lines, gas leaks and structural damage," the NWS said. "Stay out of any building if you smell gas."
The agency said carbon monoxide poisoning is one of the leading causes of death after storms involving power outages.
"Never use a portable generator inside your home or garage," the NWS said. "Use battery-powered flashlights. Do not use candles. Turn on your flashlight before entering a vacated building. The battery could produce a spark that could ignite leaking gas, if present."
North Carolina's Department of Public Safety offers additional tips on how to safely remove trees and limbs
Beach house turns into a beach
Grant Lynch garnered national attention last week when he posted video of Matthew's mammoth waves clobbering his Florida beach house
He returned to his Palm Coast home over the weekend to find the house filled with beach sand.
He tried to walk inside the house, whose floors were covered by what looked like more than a foot of sand.