About 25,000 people and 11,000 structures could be in harm's way when the gates on the Morganza spillway are unlocked for the first time in 38 years.
Opening the spillway will release a torrent that could submerge about 3,000 square miles under as much as 25 feet of water but would take the pressure off the downstream levees protecting New Orleans, Baton Rouge, and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi.
Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as 20 feet of water in a disaster that would have been much worse than Hurricane Katrina in 2005.
Instead, the water will flow 20 miles south into the Atchafalaya River. From there it will roll on to the Gulf of Mexico, flooding swamps and croplands. Morgan City, an oil-and-seafood hub and a community of 12,000, shored up levees as a precaution.
The Army Corps of Engineers said it will open the gates when the river's flow rate reaches a certain point, expected today. But some people living in the threatened stretch of countryside — an area known for small farms, fish camps, and a drawling French dialect — have already started fleeing for higher ground.
Sheriffs and National Guard members will warn people in a door-to-door sweep through the area, Governor Bobby Jindal said. Shelters are ready to accept up to 4,800 evacuees, the governor said.
"Now's the time to evacuate,'' Jindal said. "Now's the time for our people to execute their plans. That water's coming.''
The corps employed a similar cities-first strategy earlier this month when it blew up a levee in Missouri — inundating an estimated 200 square miles of farmland and damaging or destroying about 100 homes — to take the pressure off the levees protecting the town of Cairo, Ill., population 2,800.
With crop prices soaring, farmers along the lower Mississippi had been expecting a big year. But now many are facing ruin, with floodwaters swallowing up corn, cotton, rice, and soybean fields.
In far northeastern Louisiana, Tap Parker and about 50 other farmers filled and stacked massive sandbags along an old levee to no avail.
The Mississippi flowed over the top Thursday, and nearly 19 square miles of soybeans and corn, known in the industry as "green gold,'' was lost.
"This was supposed to be our good year. We had a chance to really catch up. Now we're scrambling to break even,'' said Parker, who has been farming since 1985.