US Army engineers began opening a major Louisiana floodgate on Saturday to ease pressure from the swollen Mississippi River, in a bid to save cities by sacrificing small towns and farmland that now face historic flooding.

The US Army Corps of Engineers opened a single bay at the key Morganza Spillway to allow a relatively small amount of the river through, avoiding what Louisiana Governor Bobby Jindal described as a "wall of water" slamming thousands of homes and farmland in the state's rural south.

The effort, intended to spare the cities of New Orleans and Baton Rouge, marked the first time the spillway has been opened since 1973, and only the second time in the structure's almost 60-year history.

"This is certainly going to be a marathon and not a sprint," Major General Michael Walsh told a press conference earlier Saturday before the spillway was opened.

Opening the Morganza Spillway completely would divert some 600,000 cubic feet of water every second -- about six times the daytime volume of Niagara Falls.

But in order to prevent a massive wave from being unleashed, the one bay opened Saturday was allowing out 10,000 cubic feet per second. One or two more bays were set to be opened Sunday.

The Bonnet Carre spillway between Baton Rouge and New Orleans was opened to full capacity Saturday, officials said meanwhile, diverting 250,000 cubic feet per second into Lake Pontchartrain.

Jindal said officials expected around three million acres (1.2 million hectares) to be flooded by the diversion into the Gulf of Mexico. Some 25,000 residents were set to be affected by the controlled flood.

The 20-foot (six-meter) levees protecting New Orleans are now holding back the river at 17 feet, considered a flood stage. If the Morganza Spillway was not open, river levels were predicted to reach 19.5 feet.

New Orleans Mayor Mitch Landrieu said he was aware of the tremendous sacrifices residents of rural communities were making to save more populous areas of the state.

"We believe the city of New Orleans is going to be safe," he said, but added: "This is a very tragic situation, really, for everybody in America and, of course, the people that live along the Atchafalaya basin, as well in Morgan City. So our hearts go out to them.

"It doesn't make us feel any good that (by) protecting New Orleans, other folks are going to get hurt."

Cindy Prejean of Gibson, Louisiana, some 70 miles (112 kilometers) southwest of New Orleans, said she was expecting five feet (1.5 meters) of water at her house.

"What gives them the right to flood us? I understand it, but there are so many communities, and so many farmers and so many businesses," she told AFP.

"Everybody pray for us," she asked.

According to flood projections, a flood as high as 15 feet (4.5 meters) was to bear down on some communities at its height after the spillway is opened.

So far, the major New Orleans port is continuing normal operations, while oil and gas companies have about 2,200 wells in the region at risk of flooding.

The trigger for the opening was when 1.5 million cubic feet (42,00 cubic meters) of water per second was flowing down the Mississippi at Red River Landing -- a point reached on Friday.

With the Morganza Spillway now partially opened, Colonel Ed Fleming, commander of the Army Corps' New Orleans district, said it would take about three days for the flood to reach Morgan City, further south in the floodzone.

The rising river, swollen by heavy rains last month and the melting of a thick winter snow pack, has been set to eclipse the high water records set in the epochal floods of 1927.

Near its height, the Mississippi town of Vicksburg was expected to see a 57.5-foot (17.5-meter) crest on May 19, topping the 56.2-foot historic crest set 84 years ago this month, according to National Weather Service.

The Mississippi is the third-longest river in North America and its watershed is the fourth-largest in the world.

The worst floods to hit the central US in more than 70 years have already swallowed up thousands of homes, farms and roads in Illinois, Missouri, Kentucky, Tennessee, Arkansas and Mississippi, and the mighty river is expected to remain above flood stage along hundreds of miles (kilometers) for many days.

The American Red Cross (ARC) has responded to back-to-back disasters over the past two months, with tornadoes, severe storms and wildfires impacting the region alongside the floods.

The events have prompted the ARC to launch 23 separate relief operations backed by over 7,700 relief workers in 18 US states.