By Holly Yan, Cassie Spodak and Jason Hanna, CNN

(CNN) -- Florence's merciless deluge has already killed 18, trapped hundreds and made parts of North and South Carolina impassable -- and authorities say the worst flooding is yet to come.

The tropical depression will keep dumping rain over parts of North Carolina for the next few days, with numerous rivers expected to crest at major flood stage.

Flooding already is so bad in North Carolina that the state transportation department is telling people not to travel in the state. Numerous highways, including sections of I-95 and I-40, are closed, and road flooding has virtually cut off the coastal city of Wilmington.

More than 900 water rescues have been reported in North Carolina alone, the governor's office said -- but many more people need help. The volunteer United Cajun Navy rescue group says it was helping in Leland, where about 200 people have made calls for help, after it made numerous rescues in Wilmington.

"We're just chasing the water," United Cajun Navy President Todd Terrell said Sunday.

And in Lumberton, North Carolina -- a city submerged by Hurricane Matthew in 2016 -- residents are bracing for potential disaster as the Lumber River seeps through a patched-up gap in the levee system.

What to expect

As of Sunday evening, Florence was centered about 25 miles south-southeast of Greenville, South Carolina, the National Hurricane Center said. It was moving west at 14 mph, whipping 35 mph winds.

By the storm's end, up to 40 inches will fall in southeastern North Carolina and the northeastern tip of South Carolina, the National Hurricane Center said. Other parts of the Carolinas will be left with up to 20 inches of rain, causing significant river flooding, with some rivers not cresting until later this week.

Up to 6 more inches of rain could fall in parts of North Carolina and Virginia from Sunday evening to Tuesday evening, forecasters said. The storm should move up into West Virginia, Pennsylvania, New York and parts of New England by Tuesday, dropping 2 to 4 inches of rain there.

About 532,338 customers in North Carolina and 61,000 in South Carolina don't have power. But the number of actual people without power is far greater, since a single customer can represent an entire family.

'Let's get in the truck and get out of here'

Lumberton was still recovering from Hurricane Matthew when Florence swept through. Now, workers are racing against rising flood waters to shore up the city's main levee system before the river crests.

By Sunday afternoon, the river had reached nearly 24 feet. It is expected to crest late Sunday or early Monday around 25.7 feet. If the river gets higher than 26 feet, "all bets are off," city public works deputy director Corey Walters said.

At one point Sunday afternoon, part of a makeshift barrier meant to plug a low point in the city's main levee system gave way and river water leaked through -- prompting workers to try to shore it back up with construction equipment.

Bobby Hunt's house is still damaged from Matthew. As the river kept rising Sunday, he knew it was time to flee. Hunt said Matthew caught them by surprise with flooding in the middle of the night. He's not waiting for that to happen again.

One family riding out the storm in a cluster of homes on the outskirts of Lumberton was content to leave their fate in God's hands. "I just feel comfortable staying. We have our dogs and our property, so we stay," 57-year-old Kenan Chance, said.

Their homes are still damaged from Matthew, but they survived, members of the family said. Whether they stay or go, they'll have damage to contend with.

Her father, 84-year-old Rembert Walters, says that if his relatives dispersed to different havens, he'd spend all his time worrying about them.

"We're not going to move until this thing's over. It's going to wipe us out or kill us. Or bury us, or something," he said as floodwater crept upon his porch. "'Cause we don't run every time this thing happened."

Betty Walters says she has health issues that are hard to manage at a shelter. "It's just better for us to stay at home," she said. "I trust in the Lord. He calmed the waters and he's going to look after us."

The fear of sudden, massive river flooding isn't limited to Lumberton. Forecasters say many rivers across the state still haven't crested -- some won't crest until late Sunday or Monday.

North Carolina Gov. Roy Cooper said it's not too late to go to an emergency shelter. More than 15,000 people are staying at 150 emergency shelters. And if those shelters fill up, he said, the state will open up more.

Wilmington is basically cut off

The coastal North Carolina city of Wilmington, population 117,000, is so deeply submerged that no one can get in -- not even aid workers carrying fuel and critical supplies.

FEMA crews and power company trucks were turned away Saturday night because of the flooding, Saffo said.

The Wilmington-based Cape Fear Public Utility Authority urged residents to fill bathtubs and containers with water in case the utility doesn't have enough fuel to keep its water treatment plants running.

Nearby Pender County, north of Wilmington, also is running out of fuel, Commissioner Jackie Newton said. Near the community of Wards Corner in that county, US 421 was a virtual lake, with waters lapping up to homes on either side, video from a CNN crew showed.

The North Carolina attorney general's office has received more than 500 complaints of price gouging -- including for hotel rooms, gas and water. Authorities have launched investigations.

Causes of death include electrocution and fallen trees

The death toll from Florence rose Sunday, with authorities saying 18 deaths have been linked to the storm:

-- A 3-month-old baby who died after a tree fell on a mobile home in Dallas, North Carolina.

-- A man who died when a his truck hit an overpass support beam on Interstate 20 in Kershaw County, South Carolina.

-- A man who drowned in an overturned vehicle on a flooded road in Georgetown County, South Carolina.

-- Three people who died in flash flooding or swift water on roads in Duplin County, North Carolina

-- Two people who died in a storm-related fire in Cumberland County, North Carolina

-- A mother and a child who were killed when a tree fell on their house in Wilmington, North Carolina

-- Two people who died from carbon monoxide poisoning in Horry County, South Carolina.

-- A woman in South Carolina who struck a downed tree while she was driving.

-- A woman who suffered cardiac arrest in Hampstead, in North Carolina's Pender County. When emergency responders tried to reach her, their path was blocked by fallen trees.

-- Another person in Pender County, according to the county's emergency management director. No details were immediately available.

-- A man who was killed while checking on his dogs in Lenoir County, North Carolina.

-- Another man in Lenoir County who was electrocuted while trying to connect two extension cords.

-- An 81-year-old man who fell and struck his head while packing to evacuate in Wayne County, North Carolina.

As much of North Carolina faces flooding for days, Gov. Cooper said the risk of more deaths is quite real.

"Remember: Most storm deaths occur from drowning in fresh water, often in cars," he said. "Don't drive across standing or moving water."