Army engineers said they will open a key spillway along the bulging Mississippi River as early as Saturday and inundate thousands of homes and farms in Louisiana's Cajun country to avert a potentially bigger disaster in Baton Rouge and New Orleans.

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 7,770 sq. kilometres under as much as 7.6 metres of water 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.


A bulldozer drives alongside a line of sand-filled baskets on top of a levee in Morgan City, La., on Friday. (Patrick Semansky/Associated Press )

Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as six metres of water in a disaster that would have been much worse than Hurricane Katrina in 2005.

Instead, the water will flow 32 kilometres 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.

Shelters being prepared

The corps said it will open the gates when the river's flow rate reaches a certain point, expected Saturday. 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 Guardsmen will warn people in a door-to-door sweep through the area, Gov. 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 Army Corps of Engineers employed a similar cities-first strategy earlier this month when it blew up a levee in Missouri — inundating an estimated 518 sq. kilometres of farmland and damaging or destroying about 100 homes — to take the pressure off the levees protecting the town of Cairo, Illinois, 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, where 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 49 sq. kilometres 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.

Cotton prices are up 86 per cent from a year ago, and corn — which is feed for livestock, a major ingredient in cereals and soft drinks, and the raw material used to produce ethanol — is up 80 per cent. Soybeans have risen 39 per cent. The increase is attributed, in part, to worldwide demand, crop-damaging weather elsewhere and rising production of ethanol.

Long-term effects unknown

While the Mississippi River flooding has not had any immediate impact on prices in the supermarket, the long-term effects are still unknown. A full damage assessment can't be made until the water has receded in many places.

Some of the estimates have been dire, though.

More than 3,885 sq. kilometres of farmland in Arkansas, which produces about half of the nation's rice, have been swamped over the past few weeks.

Many farmers have crop insurance, but it won't be enough to cover their losses. And it won't even come close to what they could have expected with a bumper crop.

The river's rise may also force the closing of the river to shipping, from Baton Rouge to the mouth of the Mississippi, as early as next week.