US South braces for Mississippi floodgate opening
US engineers are preparing to flood up to three million acres in southern Louisiana in a bid to protect large cities along the Mississippi River.
The Army Corps of Engineers said it would open floodgates to divert water from the river at about 1500 local time (2000 GMT).
Opening the Morganza Spillway would ease pressure on levees protecting the cities of Baton Rouge and New Orleans.
The Mississippi River has risen this year to levels not seen in decades.
Fed by rainwater and the spring thaw, the river and its tributaries have caused massive flooding upstream, and officials have said the flooding in Louisiana is the worst since 1927.
As many as 25,000 people are preparing to leave when the floodgates are opened.
'Shoulder to shoulder'
US Army Corps of Engineers spokesman Col Ed Fleming said one bay would be opened at Morganza at 1500 local time, allowing 10,000 cubic ft of water per second to pass.
He said the opening would be slow to "make sure folks have the understanding that water is coming their way and they evacuate according to their local procedures".
Wildlife also needed time to get to higher ground, he said.
Col Fleming said the main water crest was not expected at the spillway until 24 May and would last for 10-14 days, so that "no doubt that structure has the potential to be opened for the better part of three weeks".
Maj Gen Michael Walsh added: "The crest is still up in Arkansas. It's a marathon, not a sprint - there is huge pressure on the system as we work the water through. The protection of lives is the number one thing we're looking for."
Col Fleming said: "We are here with the communities fighting these floods shoulder to shoulder."
The trigger for the spillway opening was when 1.5m cubic feet (42,500 cubic metres) of water per second was flowing down the Mississippi River at Red River Landing, just north of the Morganza Spillway.
That flow rate had already been reached, the National Weather Service said, according to Associated Press news agency.
Opening the spillway will channel water out of the flooded Mississippi River and into the Atchafalaya River basin, a low-lying area of central Louisiana, to avoid flooding Baton Rouge, New Orleans and other cities.
The Corps warned that if the spillway was not opened, New Orleans could be flooded by about 20 feet (six metres) of water.
Instead, water will flow south into the Atchafalaya Basin, flooding homes and farms in the state's Cajun country under an expected 10-20 feet of water.
Over several days, the water should run south to Morgan City and then into the Gulf of Mexico.
Workers are rushing to reinforce the levees around Morgan City.
Much of the water will end up in swamplands, bayous and backwater lakes but several thousand homes are at risk of flooding.
Col Fleming said he was optimistic for Morgan City, as the walls are 20ft and the crest is expected at 12ft.
"My message to our people is they don't need to be delaying," Louisiana Governor Bobby Jindal said on Thursday. "Move their valuables. Think about where they would go."
Residents of the town of Butte La Rose, directly in the path of the spillway's water, said they had been told to pack for a long absence.
"They told us to move as though we were moving - period - not coming back, not to so much as leave a toothpick behind," said one woman.
Farmers in the region are expecting to lose their entire crops in a year of high prices for farm produce.
The Morganza Spillway, 45 miles (72km) north-west of Baton Rouge, was last opened in 1973.
The flooding is approaching records set 84 years ago when hundreds of people in the region died.
The US government has said farmers whose land has been flooded will be reimbursed for destroyed crops.