About 25,000 people and 11,000 structures could be in harm's way when the Morganza spillway is unlocked for the first time in 38 years. Sheriffs and National Guardsmen were warning people in a door-to-door sweep through the area, and shelters were ready to accept up to 4,800 evacuees, Gov. Bobby Jindal said.
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.
"Now's the time to evacuate," Jindal said. "Now's the time for our people to execute their plans. That water's coming."
Opening the spillway will release a torrent that could submerge about 3,000 square miles (7,770 square kilometers) under as much as 25 feet (7.5 meters) of water in some areas but 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.

"Protecting lives is the No. 1 priority," Army Corps of Engineers Maj. Gen. Michael Walsh said at a news conference aboard a vessel on the river at Vicksburg. A few hours later, the corps made the decision to open the key spillway.
Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as 20 feet (six meters) of water in a disaster that would have been much worse than Hurricane Katrina in 2005.
Instead, the water will flow 20 miles (30 kilometers) south into the Atchafalaya Basin. From there it will roll on to Morgan City, an oil-and-seafood hub and a community of 12,000, and eventually into the Gulf of Mexico, flooding swamps and croplands.
A sliver of land north of Morgan City, about 70 miles (115 kilometers) long and 20 miles (30 kilometers) wide, was expected to be inundated with 10- to 20-feet (3- to 6-meters) of water, according to Army Corps of Engineers estimates. It will take hours and days for the water to run south, and wasn't expected to reach Morgan City until around Tuesday. Still, the city has already taken steps to shore up its levee.
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 (520 square kilometers) of farmland and damaging or destroying about 100 homes — with damage that will probably exceed $100 million. It was done to take the pressure off the levees protecting the town of Cairo, Illinois, population 2,800, at a bottleneck where the Ohio and Mississippi rivers meet.


This intentional flood is more controlled, however, and residents are warned by the corps each year in written letters, reminding them of the possibility of opening the spillway, which is 4,000 feet (1,200 meters) long and has 125 gate bays.
The spillway, built in 1954, is part of a flood plan largely put into motion in the 1930s in the aftermath of the devastating 1927 flood that killed hundreds.
It is set to be opened when a flow rate of 1.5 million cubic feet (40,000 cubic meters) per second is reached and projected to rise. Just north of the spillway at Red River Landing, the river had reached that flow rate, according to the National Weather Service.
To put things in perspective, corps engineer Jerry Smith crunched some numbers and found that the amount of water flowing past Vicksburg, Mississippi, would fill the Superdome, where the National Football League's New Orleans Saints play, in 50 seconds.
This is the second spillway to be opened in Louisiana. About a week ago, the corps used cranes to remove some of the Bonnet Carre's wooden barriers, sending water into the massive Lake Ponchatrian and eventually the Gulf of Mexico.
That spillway, which the corps built about 30 miles (50 kilometers) upriver from New Orleans in response to the flood of 1927, was last opened in 2008. May 9 marked the 10th time it has been opened since the structure was completed in 1931. The spillways could be opened for weeks, or perhaps less, if the river flow starts to subside.
In Vicksburg, Miss., Warren County Sheriff Martin Pace said at least five neighborhoods have taken on water.
"We're patrolling subdivisions by boat," Pace said Friday.
U.S. Highway 61, a major north-south route has been cut off by water, affecting thousands of people, Pace said.
Meanwhile, farmers along the lower Mississippi had been expecting a big year with crop prices skyrocketing, but now many are facing ruin, with floodwaters swallowing up corn, cotton, rice and soybean fields.
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