Floodgates were opened today on rural Louisiana to prevent the swollen Mississippi River from flooding major cities.
Only one of 125 bays of the Morganza spillway was opened.
If the spillway hadn't been opened, Baton Rouge and New Orleans could have gone under water.
U.S. authorities on Saturday opened up a major floodgate to ease pressure from the swollen Mississippi River, hoping to save urban centers from historic flooding as rising waters swept south.
One bay of 125 available bays in Louisiana's Morganza Spillway was opened to stop flood waters from washing into major cities, the Army Corps of Engineers said, aiming to ease the Mississippi's flow as it heads for the Gulf of Mexico.
"Baton Rouge and New Orleans will flood if we don't open that spillway," retired general Russel Honore, best known for leading the military response to the 2005 Hurricane Katrina disaster, warned on CNN.
Opening the spillway means waters will gush over thousand of acres (hectares) of farmland and rural towns, prompting warnings of flash floods from forecasters and urgent evacuation orders in Louisiana and Mississippi.
The American Red Cross is already readying shelters for thousands of expected evacuees.
But Louisiana Governor Bobby Jindal said people had time to prepare and pack, adding that the Corps had told him the spillway would be opened Saturday, according to the New Orleans Times Picayune newspaper.
"Now is the time to take action. Don't delay. Don't hope something will change," said Jindal, quoted by the New Orleans Times Picayune, adding it would be a slow release of water.
The river, cresting upstream in Arkansas, is set to eclipse the high water records set in the epochal floods of 1927.
Near its height, the Mississippi town of Vicksburg is expecting a forecasted 57.5-foot (17.5-meter) crest on May 19, topping the 56.2-foot historic crest set 84 years ago this month, National Weather Service data said.
The Mississippi is the third-longest river in North America and its watershed is the fourth-largest in the world, according to the US National Park Service.
According to flood projections from the Corps, a flood as high as 15 feet (4.57 meters) was set to bear down on the small Louisiana community of Butte La Rose when the spillway is opened.
Despairing resident Pierre Watermeyer told CNN, simply: "It's over with, it's over with."
"It's worse than we thought," resident Kelli Trimm told the news network as the town scrambled to gather belongings and flee.
"It's going to take everything, everything we've got. It's scary. It's going to take out our whole community," she said.
If the Morganza Spillway were not opened in time, the Corps warned earlier this week, flooding reaching as high as 25 feet (7.6 meters) could be expected to soak New Orleans.
The worst floods to hit the central United States in more than 70 years have already swallowed up thousands of homes, farms and roads in Illinois, Missouri, Kentucky, Tennessee and Mississippi.
Heavy rains last month filled rivers and creeks already swollen from the melting of a thick winter snow pack, and which are now backing up because the Mississippi is so full.
The American Red Cross said back-to-back disasters over the last two months has prompted it to launch 23 separate relief operations backed by over 7,700 relief workers in 18 states, from North Dakota to the southern coast, and along the eastern seaboard.