Water from the inflated Mississippi River gushed through a floodgate on Saturday for the first time in nearly four decades and headed toward thousands of homes and farmland in Louisiana's French-speaking countryside, threatening to slowly submerge the land under water up to 7.6 meters deep.

As the gate was raised, the river poured out like a waterfall. Fish jumped or were hurled through the white froth and within 30 minutes, 40 hectares of what was dry land was under about 0.3 meters of water.

The opening of the Morganza spillway diverted water from Baton Rouge and New Orleans, and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi. Shifting the water away from the cities eased the strain on levees and thwarted flooding in New Orleans that could have been much worse than Hurricane Katrina.

"We're using every flood control tool we have in the system," Army Corps of Engineers Major General Michael Walsh said during a news conference on the dry side of the spillway, before the bay was opened.

The Morganza spillway is part of a system of locks and levees built following the great flood of 1927, which killed hundreds and left many more without homes. When the Morganza opened on Saturday, it was the first time three flood-control systems have been unlocked at the same time along the Mississippi River, a sign of just how historic the current flooding has been.

Earlier this month, the corps intentionally blew holes into a levee in Missouri to employ a similar cities-first strategy, and it also opened a spillway northwest of New Orleans.

Snowmelt and heavy rain swelled the Mississippi, and the river has peaked at levels not seen in 70 years.

In Krotz Springs, Louisiana, one of the towns in the Atchafalaya River basin bracing for floodwaters, phones at the local police department rang nonstop as residents sought information on road closings and evacuation routes.

Like so many other residents downstream of the Morganza, Monita Reed, 56, recalled the last time it was opened in 1973.

"We could sit in our yard and hear the water," she said as workers constructed a makeshift levee of sandbags and soil-filled mesh boxes in hopes of protecting the 240 homes in her subdivision.

About 25,000 people and 11,000 structures could be affected by the oncoming water, and some people living in the threatened stretch of Cajun countryside - an area colonized by French speakers that is known for fish camps - have already fled.

It took about 15 minutes for the one 8.5-meter gate to be raised in the middle of the spillway. Several hours will pass before any of the water hits sparsely populated communities, but residents nearby have been told to go.

The corps planned to open one or two more gates on Sunday in a painstaking process that gives residents and animals a chance to stay dry.