Army engineers began to slowly open the gates of an emergency spillway along the rising Mississippi River on Saturday, a move that will divert floodwaters from Baton Rouge and New Orleans, yet also inundate homes and farms in parts of Louisiana's populated Cajun country.

"We'll take approximately 10,000 cubic feet per second off the top of the Mississippi River," said Col. Ed Fleming of the U.S. Army Corps of Engineers.

Fleming says a slow opening allows everyone, including animals, to be evacuated from the area that's being flooded: "Everybody from the first 24 hours of seeing water have been evacuated from the floodway."

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 at about 3 p.m. local time. 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 gates will release a torrent that could submerge about 7,700-square-kilometres of land under as much as eight metres of water in some areas. The move will take the pressure off the downstream levees protecting numerous oil refineries and chemical plants along the lower reaches of the Mississippi.

"Protecting lives is the No. 1 priority," U.S. 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 and inundate thousands of homes and farms in Louisiana's Cajun country to avert a potentially bigger disaster in Baton Rouge and New Orleans.

Levees under pressure for weeks

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

Instead, the water will flow 12 kilometres 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 parcel of land north of Morgan City, about 112 kilometres long and 32 kilometres wide, was expected to be inundated with three to six metres of water, according to corps estimates. It will take hours and days for the water to run south, and the flow isn'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 519 square kilometres 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.

The disaster was averted in Cairo, 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 about 1,200 metres long and has 125 gate bays.

Spillway was response to devastating flood

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 per second is reached and projected to rise.

To put things in perspective, corps engineer Jerry Smith crunched some numbers and found that the amount of water flowing past Vicksburg, Miss., would fill the Superdome, where the NFL'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 Pontchartrain and eventually the Gulf of Mexico.

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.