Up to 7,770 sq kms are expected to flood as a result of opening a key sluice along the Mississippi river. [Reuters]

US army engineers have opened a key spillway in the southern US state of Louisiana in an effort to relieve pressure on the bulging Mississippi River.

The move will see thousands of homes and farms in Louisiana's Cajun country inundated over several hours, but it could help avert a potentially bigger disaster in the cities of Baton Rouge and New Orleans.

About 25,000 people and 11,000 structures were put in harm's way when the gates on the Morganza spillway were unlocked for the first time in 38 years on Saturday. 

"Protecting lives is the number one priority,'' engineers officer Michael Walsh said during a flyover of the Mississippi flooding, before the decision was made to open the spillway.

Once opened, the sluice is expected to release a torrent that could submerge about 7,770 sq kms under as much as 7.6m of water, but that should 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 up to 6m of water in a disaster that would have been worse than Hurricane Katrina in 2005. 

Instead, the water will flow 32km south into the Atchafalaya River. From there it will roll onto 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.

Residents speaking to Al Jazeera said they were told "to pack up as if they were moving and never coming back".

Crops ruined

Meanwhile, crop prices are soaring as rice and soybean fields all face ruin. 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. 

The increase is attributed, in part, to worldwide demand, crop-damaging weather elsewhere and rising production of ethanol.

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

The coast guard warns if the river around New Orleans rises further, it may place restrictions on shipping as well.

Source: Al Jazeera and agencies