Memphis, Tennessee (CNN) -- The swollen Mississippi River rolled south Wednesday as communities along its delta braced for flooding, and vast farms remained under threat as it left a trail of submerged homes.
The river crested at Memphis, Tennessee, a day earlier just a few inches short of a record set in 1937.
As it slowly headed south, flooding concerns turned to Louisiana and Mississippi, where it is expected to rise to levels unseen since 1927.
Louisiana Gov. Bobby Jindal said as many as 3 million acres could be affected by the flooding.
Some 500 National Guard members have been mobilized so far and 21 parishes have issued emergency declarations, according to the governor.
The river's crest is expected to begin arriving in Louisiana next week.
Floodwaters on rise in Mississippi, Louisiana
Flooding will be a major setback for southern Louisiana, which is still recovering from Hurricane Katrina, said Lynn Magnuson, a New Orleans resident.
"I went through Katrina," Magnuson said. "I would not wish flooding on anyone, and this city is the last place on Earth that needs any more high water."
U.S. Sen. Mary Landrieu, a Democrat from Louisiana, said a lot is at stake.
"After hurricanes Katrina, Rita, Gustav and Ike -- as well as the oil spill -- Louisiana can ill-afford another large-scale disaster," she said. "Billions of dollars in property is at stake, not to mention the threat to human life."
In neighboring Mississippi, some of the waters seeped into casinos as the river inched toward a 48-foot crest Tuesday evening.
About 600 people in the Tunica community of Cutoff have been driven from their homes, said Larry Liddell, a county spokesman.
"We're just watching and waiting," he said.
How the Mississippi River levees could fail
Downstream in Louisiana, the Corps said it was closing a major lock that allows for the transfer of barge traffic between the Mississippi and the Red River Basin.
The Corps opened 44 more gates to the Bonnet Carre spillway in Norco, Louisiana, on Tuesday, sending millions of gallons of water rushing into Lake Pontchartrain and, eventually, the Gulf of Mexico.
In addition to 28 gates opened on Monday, it may consider an additional 38 on Wednesday, according to John Young, the Jefferson Parish president.
As the swollen waters inch closer, anxious residents are demanding answers.
Some have posted on Facebook pages operated by the Corps, demanding answers about when certain spillways will be opened and what other areas are facing flooding.
In Arkansas, the farm bureau estimated damage to the state agriculture could top $500 million as more than a million acres of cropland are under water.
The river in Memphis measured 47.8 feet Tuesday night, according to the National Weather Service.
A crest is defined as the high point of the water during a flood before it begins to recede. Observers generally know that cresting is occurring when the gradual rise stops and the water level becomes stable.
What does cresting mean?
The Mississippi is the highest it's been at Memphis since 1937, when it crested at 48.7 feet -- 14.7 feet above flood stage. That flood killed 500 people and inundated 20 million acres of land, said Col. Vernie Reichling, the Corps of Engineers Memphis District commander.
So far, the levees protecting the area have only shown minor weaknesses, which workers have been able to control, he said.
President Barack Obama signed disaster declarations for Tennessee, which will help direct federal aid toward recovery efforts in areas hit by severe storms, flooding and tornadoes.
Once past Tennessee, Louisiana and Mississippi will be in peril as river levels could break records set in 1927, when flooding displaced 600,000 people and caused nearly $624 million in damage, according to the National Weather Service.
Forecasters don't expect anywhere near that kind of flooding largely because of the network of levees built after that disaster.
Flooding also continues to be a problem in southeastern Missouri and southern Illinois, though the Mississippi and Ohio rivers have already crested in those states.
