(CNN) -- Florence's catastrophic encore could bring some of its worst destruction yet on Sunday.
"We're going to get hammered," said Kevin Arata, spokesman for the city of Fayetteville, North Carolina. "The worst is still yet to come."
Never mind that Florence weakened to a tropical depression Sunday morning. That just means the wind speeds calmed down slightly. The real dangers are Florence's relentless rainfall and historic flooding, which keep getting worse.
Florence has already killed 15 people in two states. Volunteer rescue groups such as the Cajun Navy and emergency workers are scrambling to save hundreds of people still trapped by rising floodwater.
"We actually just rescued over 200 people, and we've got about another 300 to 500" left to rescue, United Cajun Navy President Todd Terrell said Sunday. "The water is coming up really fast."
And in Lumberton, North Carolina -- a city submerged by Hurricane Matthew in 2016 -- residents are bracing for potential disaster as the Lumber River threatens to burst through a recently patched-up gap in the levee system.
• Numerous road closures: Large sections of I-95 and I-40 in North Carolina are closed because of flooding, including a roughly 55-mile stretch of I-95 from Westry to Four Oaks. Road closures are mounting, and it's so bad that the state transportation department gave this notice on its website: "Travel not advised in or through North Carolina."
• Widespread power outages: About 703,000 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.
• Florence is lingering in the Southeast: As of late Sunday morning, 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.
• Much more flooding to come: By storm's end, Florence will have left up to 40 inches of rain in southeastern North Carolina and the northeastern tip of South Carolina, the hurricane center said. Some other parts of North Carolina South Carolina will be left with up to 20 inches of rain. That rain will cause significant river flooding, with some rivers not cresting until later this week.
• Watch out for landslides, too: "In addition to the flash flood and flooding threat, landslides are possible in the higher terrain of the southern and central Appalachians across western North Carolina into southwest Virginia," the hurricane center said.
• Storm victims report price gouging: 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.
'Let's get in the truck and get out of here'
Residents in Lumberton watched nervously as the Lumber River swelled about 11 feet higher than flood stage. The city was submerged for days after 2016's Hurricane Matthew.
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.
"Let's get in the truck and get out of here," Hunt said as his family quickly left their boarded-up home.
Hunt said Matthew caught them by surprise with flooding in the middle of the night. He's not waiting for that to happen again.
By Sunday afternoon, the Lumber 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.
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.
"Any direction you try coming into the city, from 20 to 40 miles out, roads are impassable," Mayor Bill Saffo said. "Anyone trying to get in here -- don't try, you will be turned away. Highway Patrol won't let you."
That means the city is having trouble getting fuel and other critical supplies, too. 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.
Causes of death include electrocution and fallen trees
The death toll from Florence rose Sunday, with authorities saying 15 deaths have been linked to the storm:
-- 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, North Carolina. When emergency responders tried to reach her, their path was blocked by fallen trees.
-- 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.
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."
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