The Mississippi river has flooded low-lying areas of Memphis, but latest reports say it is receding after peaking at lower levels than predicted.
There are signs across Memphis, Tennessee, that water levels are starting to recede.
The Mississippi river peaked at 47.8 feet early on Tuesday.
Hundreds of people had fled to shelters as the river and its backed-up tributaries began to inundate homes, businesses and schools.
The warnings contrasted with a carnival-like atmosphere in downtown Memphis as people gaped at the river, which had widened to three miles in one spot, while its flow speed had more than doubled to 12 miles per hour.
“I’ve never seen anything like it. I was born and raised here and it’s pretty crazy to look at it,” said Ashlee Omar, who works in the Beale Street entertainment district of Memphis.
The volume of water passing by in one second was enough to fill a large stadium, US Army Corps of Engineers Colonel Vernie Reichling told reporters.
Up to 5,000 people may have to evacuate when the crest moves about 300 miles downstream to Vicksburg, Mississippi, on May 19 and Natchez two days later, officials said.
The US Army Corps of Engineers opened the Bonnet Carre spillway north of New Orleans on Monday to divert some water to Lake Pontchartrain. It had no impact on homes or businesses.
“We are not going to open it up full bore immediately,” the Army Corps’ Victor Landry said.
Preparations were also under way to open a second Louisiana spillway to ease flows at New Orleans and Baton Rouge as the state began to relocate inmates from the Angola state prison.
Downstream from Memphis, communities without levees north and south of Vicksburg, Mississippi, were already inundated, and other residents near the swollen banks of the largest river in North America eyed their flood protections uneasily.
Authorities warned that poisonous snakes and rats seeking shelter may lurk in closets.