MORGANZA, La. — Engineers prepared Saturday to slowly open emergency floodgates for the first time in nearly four decades, purposefully inundating farmlands and homes in Louisiana's Cajun country to drain the swelling Mississippi River.
Across the countryside, people fled to higher ground, shored up levees that held the last time the Morganza spillway was opened and built new walls of sand and dirt to hold back the flood they have known was coming for weeks. Sheriffs and National Guardsmen were warning people in a door-to-door sweep, and shelters were ready to accept up to 4,800 evacuees.
It will take about 10 minutes for one of the Morganza gates to open, then several hours before any of the water hits sparsely populated communities. The corps plans to open three gates Saturday afternoon in a painstaking process that gives residents and animals a chance to get out of the way.
"It's better for the people. It's better for the critters, and it's better for the structure," said corps spokeswoman Rachel Rodi said of the slow opening.
About 25,000 people and 11,000 structures could be in harm's way when the Morganza spillway is unlocked for the first time since 1973, but diverting the river water will help take the pressure off levees downstream. Easing the strain on the river walls helps make sure the river doesn't flood more populated cities like Baton Rouge and New Orleans, and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi.
In Krotz Springs, La., one of the towns in the Atchafalaya River basin bracing for floodwaters, Monita Reed, 56, recalled the last time the Morganza 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.
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 heading out. Reed's family packed her furniture, clothing and pictures in a rental truck and a relative's trailer.
"I'm just going to move and store my stuff. I'm going to stay here until they tell us to leave," Reed said. "Hopefully, we won't see much water, and then I can move back in."
Opening the spillway will release water that could submerge about 3,000 square miles, some places would be under as much as 25 feet of water in some areas.
"Protecting lives is the No. 1 priority," Army Corps of Engineers Maj. Gen. Michael Walsh said Friday 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.
Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as 20 feet of water in a disaster that would have been much worse than Hurricane Katrina in 2005.
Instead, the water will flow 20 miles south 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.
The Krotz Springs area was in a sliver of land about 70 miles long and 20 miles wide, north of Morgan City. The finger-shaped strip of land was expected to be inundated with 10- to 20-feet of water, according to Army Corps of Engineers estimates.
It will take days for the water to run south, and it wasn't expected to reach Morgan City until around Tuesday.
The corps employed a similar cities-first strategy earlier this month when it blew up a levee in Missouri — inundating an estimated 200 square miles 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 4,000 feet long and has 125 gate bays.
At the site of the spillway, a vertical crane was in position to hoist the gate panel and let water out one of the bays. On one side of the spillway, water was splashing over the gates. The other side was dry.
Typically, the site of the spillway is dry on both sides. But when the river rises to historic levels, like the ones seen over the past couple of weeks, it holds the Mississippi in place.
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. Just north of the spillway at Red River Landing, the river had reached that flow rate, according to the National Weather Service.
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 Ponchatrain and eventually the Gulf of Mexico.
That spillway, which the corps built about 30 miles upriver from New Orleans in response to the flood of 1927, was last opened in 2008. It has now been opened 10 times since the structure was completed in 1931. The spillways could be opened for weeks, or perhaps less, if the river flow starts to subside.
In Vicksburg, Miss., where five neighborhoods were underwater, a steady stream of onlookers posed for pictures on a river bluff overlooking a bridge that connects Louisiana and Mississippi. Some people posed for pictures next to a Civil War cannon while others carried Confederate battle flags being given away by a war re-enactor.
Vicksburg was the site of a pivotal Civil War battle and is home to thousands of soldier graves.
James Mims, 50, drove about an hour from Calhoun, La., with his wife, son and three grandchildren to snap a photo.
"It's history in the making, and we're seeing it happen," Mims said.