Floodwaters that rose as swiftly as 8ft an hour left a trail of death as they tore through an Arkansas campsite packed with holidaymakers, carrying away tents and overturning their vehicles as they slept.
At least 16 people, including a number of children, were killed and dozens more are missing and feared dead.
Heavy rains caused the normally quiet Caddo and Little Missouri rivers to climb out of their banks.
Floodwaters barrelled into the Albert Pike Recreation Area, a 54-unit site in the Ouachita National Forest. The raging torrent poured through the valley with such force that it peeled asphalt off roads and bark off trees. Cabins dotting the river banks were severely damaged and mobile homes lay on their sides. Two dozen people were taken to hospital and authorities rescued 60 others.
After the water receded, anguished relatives pleaded with emergency workers for help in finding more than 40 missing loved ones. At one point, Arkansas governor Mike Beebe said the death toll had climbed to 20, but his office later revised that figure to 16. But that toll could easily rise. Forecasters warned of the approaching danger during the night, but campers could easily have missed those advisories because the area was isolated.
By early evening, state police had identified 14 of the 16 bodies recovered, but they did not name them. Mr Beebe said damage at the campground was comparable to that caused by a strong tornado. The force of the water carried one body eight miles downstream. Authorities prepared for a long effort to find other bodies that may have been washed away.
"This is not a one- or two-day thing," said Gary Fox, a retired emergency medical technician who was helping identify the dead and compile lists of those unaccounted for. "This is going to be a week or two or three-week recovery."
The heavily-wooded region offers a mix of campsites, hunting grounds and private homes. Denise Gaines, of Baton Rouge, Louisiana, escaped after she was woken in her riverfront cabin by a noise that sounded like fluttering wings. She saw water rushing under the cabin door. "I thought it must have been an angel that woke me up," she said. She woke up the six others in her cabin and started packing her things.
Ms Gaines said she had been through this before with Hurricane Gustav. "We could feel the cabin shaking," said her fiance, Adam Fontenot.
After the cabin filled with chest-deep water, the group clung to a tree and each other outside for more than an hour. But then the water dropped quickly, several feet in just a few minutes. As the water receded, the devastation emerged: Cars were piled on top of each other and bodies were in the water.