Louisiana residents are being warned today: The Army Corps of Engineers will open the Morganza spillway along the Mississippi River by Sunday, flooding millions of acres of rural farmland and sparing big cities like Baton Rouge and New Orleans.
"This is a historic amount of water," Louisiana Gov. Bobby Jindal said.
"Some people may think, 'Well, the house is not underwater yet,'" Mississippi Gov. Haley Barbour said. "But they don't know the road is closed, may become closed. So if in doubt, people should get out. We want people to evacuated, not have to be rescued."
As much as 25 feet of water will spill out over 100 miles, displacing 2,500 people. In addition, 22,500 people and 11,000 structures in the backwater areas could be flooded.
If the gates remained closed and the levees along the Mississippi failed, Baton Rouge and New Orleans could both be flooded -- leaving a disaster worse than Katrina.
Inspectors are making daily checks of the levees that surround New Orleans.
"All indications are that the levees that have been inspected on a regular basis for some time, they're all holding and we are expecting them to do so," said New Orleans Mayor Mitch Landrieu.
Other low-lying areas are not faring as well. The Mississippi River has been breaking high-water records that have stood since the 1920s and '30s.
Barbour told residents to prepare for the worst, though he said the main levee was holding along the river.
"The river is going to crest Thursday," he said today. "We'll have water for a long time after that."
President Obama is expected to meet with families affected by flooding along the river when he travels to Memphis, Tenn., on Monday. Today, Republicans on the House Appropriations panel awarded $850 million to the Federal Emergency Management Agency for disaster payments.
The Coast Guard also is likely to close the river to barge traffic next week, costing the U.S. economy $295 million a day. It's just the latest in a costly year of extreme weather disasters.
Floods, Twisters: Expensive Destruction
The massive Mississippi floods -- a seven-state, 560-mile liquid trail -- are adding to the nation's laundry list of expensive destruction. Already, there have been five separate billion-dollar storms and floods this year.
In January, a monster Midwest blizzard paralyzed Chicago and then moved to the Northeast, causing $1 billion in damage.
In April, thunderstorms and tornados in the Midwest and Plains led to more than $2 million in damage. The following week, twisters touched down in Iowa and Wisconsin. The damage cost $2.25 billion.
In North Dakota and Minnesota, the notorious Red River overflowed its banks, leading to millions of dollars in damage.
Texas was hit with wildfires that destroyed 310 homes and endless acres of brush and tumbleweed at a cost of nearly $200 million.
Deadly cyclones reigned over Alabama and six other states in the Southeast, killing more than 300 people and leaving more than $1.5 billion in damage.
The Associated Press contributed to this story.