By Ashley Hayes, CNN

Water from the rising Mississippi River pours over the Bonnet Carre Spillway in Norco, Louisiana, on Tuesday.

STORY HIGHLIGHTS 

• Residents in low-lying areas have been put on alert
• There is concern about a plan to open the Morganza Spillway
• The spillway has not been opened since 1973

(CNN) — As the Mississippi River was cresting Tuesday near Memphis, Tennessee, residents downstream in low-lying Louisiana were eying the rising waters nervously — and worried about a plan to open a spillway, which is expected to trigger flooding in southern portions of the state.

“I’m very scared,” Kim Hartley of Morgan City, Louisiana, told CNN affiliate WVUE. “I’m concerned because I have a new home.”

Morgan City is one area that could be affected by the opening of the Morganza Spillway, an emergency spillway above Baton Rouge, Louisiana. Opening it would relieve pressure on the levee system and reduce the record-setting volume of water heading down the Mississippi, reducing the likelihood of flooding more populated areas such as New Orleans and Baton Rouge.

Col. Ed Flemming, New Orleans district commander of the U.S. Army Corps of Engineers, requested a partial opening of the spillway over the weekend, Louisiana Gov. Bobby Jindal said in a statement.

The corps has also released maps outlining areas that could be flooded by the opening of the spillway, which was last used in 1973 to redirect waters into Atchafalaya Basin.

“They expect the height of the water to reach between 5 and upwards of 25 feet above ground elevation, causing flooding through many southern portions of our state,” Jindal said. Areas expected to be impacted include Krotz Springs and Butte La Rose.

“We’ve been hearing about the possibility of flooding,” Dan Gober, who lives in the town of Gibson, about 15 miles east of Morgan City, told CNN affiliate WWL. “We’ve not been hearing any specifics as to how high the water might get.”

Gibson residents remember the last time the spillway was opened, saying it put 3 to 4 feet of water in their homes.

“It was high. Water was coming all over,” Harrison Short of Gibson told WWL. “It was bad.”

And it could be even worse this time, Terrebonne Parish President Michel Claudet said. What was farmland in 1973 is now residential land, he said. “At that time, we were not as populated.”

Terrebonne Parish has already declared a state of emergency, and its website was directing residents to sandbag distribution centers. Measures aimed at minimizing flooding were being implemented, Claudet said. Levees were being elevated and reinforced.

Claudet declined to speculate on how many residents could be affected, but said he believes the Corps of Engineers maps show a worst-case scenario. Residents are being told to remain vigilant and pay attention to updates, he said.

In Concordia Parish, across the river from Natchez, Mississippi, Sheriff Randy Maxwell told residents Monday to plan for the possibility of evacuations.

“We want everyone to prepare for that possibility, just like we’ve been advising for the past few weeks,” he said in a statement posted on the sheriff’s office website. “We surely want everyone to be prepared, just like South Louisiana residents prepare for hurricanes.”

A hospital has already been evacuated in Vidalia, in Concordia Parish, officials said.

In West Feliciana Parish, “our biggest concern is Angola State Prison,” said parish President Ken Dawson. The prison, officially the Louisiana State Penitentiary, is protected by a levee system, he said.

“A lot of those inmates have been moved out of the prison to other facilities, and that is going to continue until they get to a number they can house on higher ground with no problem,” he said.

Residents of some low-lying areas are threatened by rising water, he said, but those areas typically flood, and those people are “kind of, in a sense, used to the drill,” he said.

West Feliciana Parish officials have been told that if the Morganza Spillway is opened, it would actually help the parish, not hurt it. Some residents were nervous earlier because of some misinformation being spread, they said, but officials do not believe the flooding will spread into areas not normally known to flood.

CNN’s Ashley Hayes contributed to this report.