Louisiana deputies were warning residents Monday to head for higher ground to avoid water gushing from the Mississippi River after a floodgate was opened this weekend for the first time in nearly four decades.

The massive Morganza spillway is reducing stress on levees protecting Baton Rouge and New Orleans, but it is diverting the water to other rivers, bayous and wetlands.

Most residents heeded the warnings to get out, even in places where there hadn’t been so much as a trickle, hopeful that the flooding engineered to protect heavily populated cities would be merciful to their way of life.

The floodwaters could reach depths of 20 feet in the coming weeks, though levels were nowhere close to that yet in the towns about 50 miles west of Baton Rouge.

Meanwhile, President Obama planned to fly to Memphis, Tenn., on Monday to meet with families affected when the river flooded there as well as local officials, first responders and volunteers.

It will be at least a week before the Mississippi — which has broken river-level records that had held since the 1920s in some places — crests at the Morganza spillway. Officials opened two of its 125 massive gates on Saturday and another two Sunday. More gates were likely to be opened later Monday.

Louisiana Town To Be Among The First Hit

U.S. Army Corps of Engineers maps indicated that the area around Krotz Springs, La., would be among the first to see rising water levels from the Morganza, according to Lisa Vidrine, director of St. Landry Parish’s Department of Emergency Preparedness.

“The water we are expecting to see is backwater — water that’s from the basin area and that’s going to come around the levee system that stops south of St. Landry parish,” Vidrine said. “And that’s going to spread out through the basin and start backing up to St. Landry.”

To help prevent backflooding in Krotz Springs, the National Guard is working with parish and state officials to build an emergency levee. Backloaders are filling wire Hesco baskets with soil and recycled asphalt to hold back the water.

“We’re probably, if I had to guess, 60 percent completed on our section,” said Sgt. Lloyd Martin of the Louisiana National Guard. “The top grade on the entire project will be at a level of 28 feet, 6 inches.”

That’s the height above sea level, tall enough to protect nearly 250 homes and a nearby refinery. It’s all part of the massive flood control fight involving thousands of federal, state and parish officials in Louisiana.

‘If The Levee Breaks, Then We’re In For A World Of Hurt’

On Sunday, officials in St. Landry Parish ordered a mandatory evacuation for some residents around Krotz Springs.

At one home just south of town, a trailer with a crane loaded a self-storage unit filled with nearly all of Alan Snyder and his wife’s worldly possessions. But Snyder said he wasn’t sure all the precautions were necessary.

The last time the Morganza floodway was opened, in 1973, Snyder said the area where he built his home never flooded.

“I’m not real scared like I say, but to protect me and my wife — I got a wife now you know and I wouldn’t do nothing to jeopardize the levee breaking or something,” he said. “If the levee breaks, then we’re in for a world of hurt.”

Despite the mandatory evacuation order, Krotz Springs town clerk Suzanne Bellau said it was unlikely the sheriff’s office would force people to leave. For most, the worst part was wondering what may happen.

“It’s the unknown, that’s the problem,” Bellau told The Associated Press. “Is it going to come into their homes or not? And the people who are leaving, what are they coming back to?”

About 11 miles north of Krotz Springs, in the town of Melville, Mary Ryder and family members loaded up a trailer over the weekend with as many belongings as could fit to drive over the levee to stay with relatives on the other side of town. Ryder lives in a mandatory evacuation area, where water is starting to creep into backyards, and she worried about what might happen if a broader evacuation is ordered.

“They say we have to leave town. We have nowhere to go,” she told the AP. “What are we going to do? I have no idea. We need help up here.”

Spillway May Stay Open For Weeks

Those who live in the Atchafalaya Basin and have to evacuate their homes may be away for some time. Corps officials have said the Morganza floodway is likely to remain open for at least three weeks.

Army Corps of Engineers Maj. Gen. Michael Walsh said this weekend that the battle in Louisiana is just getting started. The river here is still rising and isn’t expected to crest until later in the week.

In Mississippi, the corps said the Mississippi River is expected to crest in the city of Greenville on Tuesday at 64.5 feet, a half-foot lower and a day later than originally expected.

In Greenville, Peter Nimrod, chief engineer of the Board of Mississippi Levee Commissioners, said anticipation is high among local residents as are hopes that the historic event passes without too much damage.

Downriver, the projected crest remained at 57.5 feet in Vicksburg on Thursday. In Natchez, the corps says the crest is now project at 63 feet on Saturday, also down a half-foot.

Emergency officials said the river is slowly falling at Helena, Ark., and Tunica, Miss., and mainline levees are holding.

NPR’s Greg Allen reported from Baton Rouge, La., for this story, which contains material from The Associated Press. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.