|Authorities have been forced to divert floodwater towards smaller communities to save larger cities [Al Jazeera]
Thousands of homes and swathes of farmland in the US state of Louisiana are at risk of being inundated under as much as 7.6 metres of water, after US army engineers opened a key floodgate on the inflated Mississippi River for the first time in nearly four decades.
The opening of the Morganza spillway on Saturday diverted water from reaching Baton Rouge and New Orleans, as well as numerous oil refineries and chemical plants along the banks of the Mississippi Delta.
The diversion means that levees further downstream will be less strained, reducing the risk of flooding in New Orleans that could have been even worse than that seen in 2005 when Hurricane Katrina struck the city.
Water levels at New Orleans' 6m levees are currently at about 5.1m, considered a flood stage.
As the gate to the spillway was opened, the river gushed through, at times spraying up to 1.8m into the air. Fish jumped or were hurled through the white foaming torrent, and within half an hour 100 acres (40 hectares) of what was previously dry land was submerged.
"We're using every flood control tool we have in the system," Maj-Gen Michael Walsh, the Mississippi Valley Division Commander for the US Army Corps of Engineers, said ahead of the gate's opening.
The Morganza spillway is part of a system of locks and levees that was built after massive flooding in 1927, which killed hundreds and left many more homeless. It took about 15 minutes to open the 8.5m gate on Saturday.
The opening of the Morganza marked the first time that three Mississippi flood-control systems have been used simultaneously.
Earlier in May, the US army intentionally blew holes in a Missouri levee to divert water from cities in that state, and also opened a spillway northwest of New Orleans. By Sunday, they had also opened all 350 bays on the 2.1km Bonnet Carre spillway.
The Mississippi, the longest river system in North America, has peaked at levels not seen in 70 years, fuelled by melting snow and heavy rain.
Towns brace for impact
In Krotz Springs, Louisiana, a town on the Atchafalaya River basin that is bracing for floodwaters to hit, phones at the police department rang non-stop as residents sought information on evacuation routes.
About 25,000 people and 11,000 structures could be affected by the flooding, and some residents of a threatened stretch of Cajun countryside - an area inhabited by French speakers and known for its fish camps - have already fled.
One or two additional gates will be opened by the US Army on Sunday, in order to give both residents and local wildlife an opportunity to flee.
Water will flow 32km south into the Atchafalaya basin, and from there into Morgan City, community of 12,000 people. Eventually, the waters will flow into the Gulf of Mexico.
Officials say they do not expect water to overflow Morgan City's 7.3m floodwalls, but they have filled sandbags to shore up the levees. The water is expected to reach the town on Tuesday.
"These levees will be under a lot of pressure for a long period of time," said Corps Colonel Ed Fleming.
Mitch Landrieu, the mayor of New Orleans, termed the fact that some towns were having to be sacrificed for the sake of his city "tragic".
"We believe the city of New Orleans is going to be safe ... [But] this is a very tragic situation, really, for everybody in America and, of course, the people that live along the Atchafalaya basin, as well as 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."
The crest of the Mississippi flood is still more than a week away from reaching the Morganza spillway, but officials say that when it arrives they expect that it will linger.
The spillway could remain open for weeks, depending on river flow levels.
"This is certainly going to be a marathon and not a sprint," Major General Walsh told a press conference on Saturday.