Rising Mississippi River floodwaters have threatened cities along America's longest river and prompted President Obama to declare a state of emergency in Mississippi, Tennessee, and Louisiana in recent days. As the crest pushes south, Memphis and the Mississippi Delta region brace for their biggest flood in nearly a century.

New Orleans, with still-fresh memories of hurricane Katrina’s storm surge, should be protected by its levees, but river communities across low-lying parts of Louisiana and Mississippi are watching the approaching crest with alarm.

In Louisiana, the U.S. Army Corps of Engineers is preparing to open two more spillways (this time, without explosives) to divert floodwaters before the crest reaches Baton Rouge and New Orleans. The Bonnet Carre Spillway, which opens every few years to pour excess river water into Lake Ponchartain, will open at 8 a.m. on Monday morning. The Morganza Spillway, north of Baton Rouge, will probably open – for the first time since 1973 – on Thursday, May 12.

Timeline of the Mississippi River flood crest

Jeff Graschel, a hydrologist with the National Weather Service in Slidell, La., said that the Mississippi River flood crest was moving through southeastern Missouri on Saturday, and will push south in coming weeks. It will reach Memphis on Tuesday, May 10 at 48 feet, putting it within inches of Memphis’s all-time high of 48.7 feet.

The crest will near the Gulf Coast in two weeks, hitting Baton Rouge on May 23 at 47.5 feet, and New Orleans on May 24, he said. Without opening the Morganza Spillway, the crest at New Orleans would be 19.5 feet, just six inches below the city’s 20-foot-high levee system.

“It would be less, maybe a couple of feet less, with opening the Morganza spillway, depending on how much was diverted,” Mr. Graschel said. Additional rainfall in the next two weeks could add to the flow, he added, but so far, rain isn’t in the forecast.

Opening the Morganza Spillway

Forecasts predict that the Mississippi could be flowing at 1.9 million cubic feet per second near the Morganza Spillway later this week. The “trigger” to open it is 1.5 million cubic feet per second. “The function of these spillways is to relieve pressure on the rest of the flood control system,” said corps spokeswoman Rachel Rodi on Saturday. “Not opening the Morganza would stress the rest of the river’s flood control system. We’ll open it as much as we have to, to keep it at 1.5 million.”

Opening the spillway will flood the Atchafalaya River Basin by between 5 and 25 feet, according to the Army Corps. Although the area sparsely populated, up to 20,000 residents will be forced to evacuate, and many will likely lose their homes.

St. Martin Parish resident Sally Carabine has spent the weekend packing her home, pets, and business – a horse transport company – to move to higher ground. Along with a dog and eight cats, she has over one hundred birds, including cockatiels and exotic love birds, housed in a backyard aviary.

“What do you do with all your things and all your animals when you just have to pick up and go?” asks Ms. Carabine. “With my birds, I might just have to open the door and let them all fly out.”

Bracing for the flood

In Memphis, some suburbs have already flooded, though the crest is still days away. As the crest heads south, over one million acres of Delta landscape may be inundated.

In Mississippi, where the anticipated flooding will break records dating to 1927, more than 2,000 households have already been evacuated.

In Louisiana, Gov. Bobby Jindal has activated the National Guard and ordered the evacuation of the Louisiana State Penitentiary at Angola, which sits in a bend of the Mississippi River. In Baton Rouge this weekend, city and parish workers are sandbagging low points on the river levee system, though low-lying communities further west and south are in much greater danger of flooding.

In parishes along the Gulf Coast, workers are reinforcing levees in anticipation of flooding, which could begin as early as Thursday and last for weeks. Terrebonne Parish president Michel Claudet declared a state of emergency last Tuesday. While coastal areas of Terrebonne Parish were heavily flooded by hurricane Katrina, the northern portion of the parish is rarely threatened, said Claudet. "We have an extensive levee system that protects against hurricane storm surges along our coast, but the northern end of the parish has few flood control structures," he explained.

“I expect this to be one of the more significant events in the history of the parish,” he said.