(CNN) -- With the opening of a major spillway all but certain in Louisiana, residents in the flood zone made preparations for the consequences of the decision.
In order to lower anticipated cresting levels along the rising Mississippi River and divert water from Baton Rouge and New Orleans, the U.S. Army Corps of Engineers could open the Morganza Spillway as early as Saturday.
The move would flood parts of low-lying south-central Louisiana.
"Right now, a lot of people are real nervous about it. It's sad," said Larry Doiron, a resident of Stephenville, a town that could face flooding because of the spillway's opening. "We need to have protection so that they don't flood us."
He said his subdivision would likely be fine, as it was built at a relatively high elevation, but that his neighbors were building additional levees and putting out sandbags.
Morgan City, which sits on the banks of the Atchafalaya River, could also see rising water if the spillway is opened.
"Really, we're just waiting," said Evie Bertaut, who has lived in Morgan City for 50 years.
Officials believe that the levees will protect the city from flooding, but some are taking preliminary precautions, she said. At Sacred Heart Church, where Bertaut works, people spent the day moving important documents such a baptismal, marriage and financial records to the second floor.
"Most people are getting their photographs together, things that you can't replace in case you have to go," she said.
The Mississippi River Commission has directed the Corps to operate the crucial spillway once river flows reach a certain trigger: 1.5 million cubic feet per second. Projections indicate the tipping point could be hit as early as Saturday evening, Jindal has said.
Opening the spillway would lower anticipated cresting levels along the lower Mississippi River and divert water from Baton Rouge and New Orleans but would flood much of low-lying south-central Louisiana. Seven parishes are expected to be affected by the opening -- Pointe Coupee, St. Landry, St. Martin, Iberia, Iberville, St. Mary and Terrebonne -- according to the Corps.
Earlier this week, Louisiana Gov. Bobby Jindal predicted that if the Morganza Spillway were to open, 3,900 people and 2,600 structures would be impacted by the high water.
The Mississippi River Commission has advised a "slow opening," and the flood would spread gradually over several days, the Corps said.
The Morganza Spillway has not been opened since 1973.
"When we reach the trigger of 1.5 million cubic feet per second then we will in fact open that floodway and, again, take the pressure off the levee systems that are downstream from that particular structure," Col. Ed Fleming, the Corps of Engineers' district commander in New Orleans, told CNN's John King.
Louisiana state and local officials braced for the possibility of major flooding in the Atchafalaya River Basin if, or when, federal authorities open the spillway north of Baton Rouge. They advised residents to expect road closings.
Meanwhile, in the Arkansas town of Helena, the river crested at 56.5 feet -- 12.5 feet above flood stage, according to the National Weather Service.
The river's slow pace has given emergency responders more time to prepare, forecasters said. But while the slow-moving water gives residents extra time to get ready, it also means that land could remain under water for some time.
Jindal urged southeastern Louisiana residents to evacuate.
"Now is the time to take action," he said.
The U.S. Coast Guard said floodwaters could close the Mississippi River to ships at the New Orleans port as early as Monday morning.
To help New Orleans, the Army Corps of Engineers said Friday that it will open 52 more bays at the Bonnet Carre Spillway just north of the city, diverting water into Lake Pontchartrain. That will mean a total of 264 bays will be open in the 350-bay spillway.
The National Weather Service said that as of Friday morning, the river was at 16.8 feet in New Orleans, just a fraction below flood stage. It is expected to crest May 23 at more than 19 feet. The New Orleans levees are built to withstand 20 feet, according to the weather service.
Upriver in Mississippi, the Mississippi Department of Wildlife, Fisheries, and Parks has closed all boating activity in counties affected by the recent flooding including: Adams, Bolivar, Claiborne, Coahoma, DeSoto, Jefferson, Humphreys, Issaquena, Sharkey, Tunica, Warren, Washington, Wilkinson and Yazoo.
In Vicksburg, Mississippi, Police Chief Walter Armstrong said 600 residents had been evacuated as of Thursday night. The river was expected to crest at 57.5 feet. Flood stage at Vicksburg, the level at which the river may begin flowing over its banks, is 43 feet.
Armstrong said he expected higher water Friday, with more homes affected. More than two dozen roads were closed and about 45 businesses will be closed by Friday.
Homes that were built between the levee and the Mississippi River were the first affected.
"We estimate that every home built on the river side of the levee from Memphis all the way to the Louisiana line is flooded," said Mike Womack, executive director of the Mississippi Emergency Management Agency.
Residents near Vicksburg counted on a levee for protection. In addition to the mainline levee along the river, starting near Vicksburg and extending northeast for more than 20 miles, a so-called backwater levee offers shelter.
The backwater levee is designed to keep water from backing into the Yazoo River delta and is designed lower than the mainline levee so that water can flow over it. That level is expected to be reached Monday, said Charlie Tindall, attorney for the Board of Mississippi Levee Commissioners.
The backwater levee was being "armored" by a heavy plastic coating to prevent it from washing out, he said.
"There's a chance it might not make it through an over-topping event," said Mississippi Levee Board Chief Engineer Peter Nimod about the backwater levee. "There is a risk. We don't think it will break, but we want people to be prepared."
"It's hard to plan for a multi-100 year event," he said.
Nonetheless, 1.4 million acres in Mississippi, including 602,000 acres where crops are growing, could flood, said Rickey Grey of the state's Department of Agriculture.
Across the South and lower Midwest, floodwaters have covered about 3 million acres of farmland, eroding for many farmers what could have been a profitable year for corn, wheat, rice and cotton, officials said.CNN's Mariano Castillo, Mia Aquino and Erica Henry contributed to this report.
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