(CNN) -- Historic flooding forced the U.S. Coast Guard to shut down commercial traffic on part of the Mississippi River Friday as authorities downstream raced to build and reinforce levees, polish evacuation plans and warn residents to start thinking about getting out.
"When you see the Mississippi River and it's about two miles wide because it's lost its borders, it's sobering," said Tennessee Gov. Bill Haslam.
The Coast Guard closed a five-mile stretch of the river at Caruthersville, Missouri, on Friday to prevent waves generated by passing barges from damaging levees and flood gates along the river, Chief Warrant Officer Lionel Bryant said.
The closure could last as long as eight days, Bryant said.
While other stretches of the river remain open, the closure will block barges loaded with commodities headed for Gulf of Mexico ports. It could have ramifications across the country, Memphis, Tennessee, Mayor A.C. Wharton said.
"When you close the Mississippi River, it affects the economy nationwide," the mayor of the historic river town said.
Authorities in the Memphis area fanned out to low-lying neighborhoods Friday to warn residents of 1,089 homes that they are at risk from the coming floodwaters and to urge them to leave, according to city and county officials.
"We're trying to prevent as many rescue missions as we can and manage this through evacuation missions," said Bob Nations, director of the county's Office of Preparedness.
Memphis Police knocked on Pamela Holliday's front door Friday morning.
"The police were telling us if the floodwaters get any higher they will turn off electricity and we will have to move," she told CNN affiliate WMC.
The Mississippi River is expected to crest about 14 feet above flood stage at Memphis on Wednesday. Nations said authorities are planning as if the river will exceed that forecast.
"This is an act of nature, and nature plays funny tricks sometimes," he said.
Wharton said city officials have not ordered anyone to evacuate, but are prepared to do so should the need arise.
In Cotton Plant, Arkansas, which was under a mandatory evacuation order that also affected the towns of Gregory and McClelland, some residents were staying put.
Waters topped at least one levee in the area, prompting the evacuation order. The order affected about 1,000 residents in the three small towns.
"I saw a lot of people leaving and I just laughed," Cotton Plant resident Cleodis Smith told CNN affiliate KATV. "I just said they're going to be coming right back because the water's not getting here."
Floods prompted authorities to close 22 miles of Interstate 40 between Hazen and Brinkley in eastern Arkansas early Thursday. The eastbound stretch of the interstate was also closed Thursday night, state transportation officials said, and both remained closed Friday.
The Mississippi Department of Transportation advised motorists that the Interstate 40 situation in Arkansas will bring increased traffic on U.S 82. between Interstate 55 and the Greenville Bridge.
The flooding in the Mississippi and Ohio river valleys has already chased people from their homes in Missouri, Illinois, Indiana, Tennessee, Kentucky and Arkansas. Now it has Mississippi and Louisiana under the gun.
In Mississippi, flooding was impacting casinos in Tunica, threatening an anticipated $87 million hit to the local economy for the month of May, CNN affiliate WREG reported.
Farmers face losing their crops, said Andy Prosser, a spokesman for the Mississippi Department of Agriculture and Commerce.
"Once water goes over the top of corn, it's gone," he said.
In Louisiana, the state Department of Corrections began preparing to evacuate inmates from Angola State Prison, suspended some construction work and put more National Guard troops on the job as the flood drew closer.
In Vidalia, Louisiana, authorities had completed the evacuation of a hospital there and were working feverishly to build fortifications around water supplies and other infrastructure, Gov. Bobby Jindal said.
He said forecasters have told him parts of the state could be dealing with high water levels until July.
"You'll have a crest that could last seven to 10 days but you could have elevated water levels last for weeks after that," he said.
The Army Corps of Engineers said that it would open a spillway 28 miles upstream from New Orleans to reduce the volume and velocity of the Mississippi River as the record level of water races south to the sea.
The spillway can accommodate about 1.87 million gallons of water per second, diverting it to the Gulf of Mexico by way of Lake Pontchartrain and sparing the low-lying city of New Orleans from high waters. The spillway is scheduled to open Monday, the Corps said.
A second spillway may open later in the week, Jindal said.
If the Morganza Spillway is opened, Morgan City, Louisiana, could face record flooding, said Ken Graham of the National Weather Service. The record flood there is 10.53 feet set in 1973 -- the last time the spillway was opened.
Graham said his modeling shows as much as 12 feet of water inundating the area this time around.
Jindal said he had requested that President Barack Obama make a disaster declaration for the state because of "predicted and imminent record flooding."
Earlier this week, the Corps of Engineers intentionally breached a levee south of where the Ohio River joins the Mississippi River to help alleviate water pressure on levees throughout the region. The Corps said the breach would help lower river levels and prevent widespread flooding in communities such as Cairo, Illinois, which sits at the confluence of the two rivers.
That city remained under a mandatory evacuation order Friday. Six other southern Illinois communities were under voluntary evacuation recommendations.
"We don't want to get into a situation where people go back to their homes, and then we have to make emergency rescues if something changes," said Patti Thompson, spokeswoman for the Illinois Emergency Management Agency.