Area residents ride down N.C. Highway 53 that is flooded from the Northeast Cape Fear River in Burgaw, N.C., Wednesday. The river had severe flooding due to rains from Hurricane Florence.

WILMINGTON, N.C. — As rivers swollen to record levels started to recede Thursday in North Carolina, officials tried to head off potential environmental disasters and prepared for more record flooding downstream in South Carolina.

Roads were still clogged with people trying to make it back to where the floods had creeped back, leaving silty mud on walls and floors. Crews closed some bridges and reopened others as trillions of gallons of water continued its long, meandering journey to the Atlantic Ocean.

Potential environmental problems remained. Duke Energy issued a high-level emergency alert after floodwaters from the Cape Fear River overtopped an earthen dike and inundated a large lake at a closed power plant near Wilmington, N.C. The utility said it did not think any coal ash was at risk.

State-owned utility Santee Cooper in South Carolina is placing an inflatable dam around a coal ash pond near Conway, saying the extra 2.5 feet should be enough to keep floodwaters out. Officials warned human, hog and other animal waste were mixing in with floodwaters in the Carolinas.

South Carolina Gov. Henry McMaster estimated damage from the flood in his state at $1.2 billion in a letter that says the flooding will be the worst disaster in the state’s modern history. McMaster asked Congressional leaders to hurry federal aid.

North Carolina Gov. Roy Cooper said he knows the damage in his state will add up to billions of dollars, but said with the effects on the storm ongoing, there was no way to make a more accurate estimate.

Florence is blamed for at least 41 deaths in North Carolina, South Carolina and Virginia. Well over half of those killed were in vehicles.

In North Carolina, many places that flooded in Hurricane Matthew in 2016 were again inundated.

The storm continues to severely hamper travel. Parts of the main north-south route on the East coast, Interstate 95, and the main road to Wilmington, Interstate 40, remain flooded and will likely be closed at least until nearly the end of September, North Carolina Department of Transportation Secretary Jim Trogdon said.

More than a thousand other roads from major highways to neighborhood lanes are closed in the Carolinas, officials said. Some of them have been washed out entirely.

In South Carolina, the flood was far from over.

The Waccamaw River has started its slow rise in the city of 23,000, and forecasters expect it to swell more than 3 feet above the previous record crest by Tuesday while still rising.

All that water from Nichols and Conway is heading toward Georgetown County, where five different rivers dump into the Atlantic Ocean. Authorities planned to start handing out 15,000 sandbags Friday.