As the Mississippi River reaches its highest level in more than 70 years, threatening to inundate dozens of cities and towns, a hero has emerged: the region's $13 billion flood-control system.
Some 35 million acres of floodplains were eliminated as farms that feed Americans and the world moved in. The elimination of this natural release valve meant the potential for more catastrophic floods.
"This is history repeating itself," Andrew Falhund of the group American Rivers, told NBC News. "We've lost our floodplains and wetlands and people are at risk again. We can't let that happen."
The answer from government — federal and state — was not to uproot those farms, but to rely on an elaborate system to protect the four million people now living in the potential flood zone.
Managed by the Army Corps of Engineers, the system got started after a 1927 flood that killed hundreds.
The nation vowed to never again see Americans suffer in a flood of that kind, so following the disaster, Congress authorized the Army Corps of Engineers to build a 2,203-mile long levee system on the river.
The Corps now has an annual budget that hovers between $300-$400 million — some for maintenance and some to complete the system by 2030. It's about 95 percent built out, says Corps spokesman Bob Anderson.
The viability of that system was called into question after Hurricane Katrina, when when corps-built levees busted and water filled most of New Orleans, killing more than 1,600 people. But so far, the levees have been holding back the water.
"When you have a series of failures as you see with Katrina ... people are going to ask, 'What's happening?'" said Gerald Galloway, a University of Maryland civil engineer and former Army Corps officer. "The thing to do is to modernize and upgrade our infrastructure."
The nation's defense system against the Mississippi does not protect everyone: small towns and many farms get swamped every time a spillway is opened to protect more populated areas. That's likely to be the case soon along the Mississippi Delta, the flatlands that stretch about 200 miles from Memphis, Tenn., to Vicksburg, Miss.
And there's another hitch: the levees that keep the Mississippi River from flooding end up pushing water back up into tributaries, many of which aren't protected by the levee system. Towns there are pretty much on their own.
Still, for the major population areas along the lower half of the Mississippi, the system has done its job. Anderson says third-party estimates place the value of protected property over the years at $370 billion.
In addition to the thousands of miles of levees, here are the four key floodways, or spillways, as they are called:
- New Madrid Floodway. The Corps last week blew up a two-mile stretch of levee, diverting water that could have swamped the historic town of Cairo, Ill., and other communities by flooding 130,000 acres of Missouri farmland. It was only the second time the floodway was opened; the first was during the 1937 flood. Missouri had sued to block the opening this time, but lost in court.
- Bonnet Carre Spillway. In Louisiana, the Corps on Monday opened some of the spillway's gates, diverting some of the Mississippi into Lake Pontchartrain to ease pressure on New Orleans' levees. Built about 30 miles upriver from New Orleans in response to the 1927 flood, it was last opened in 2008. Monday marked the 10th time it has been opened. From the lake, the water flows east into the fertile fishing and oyster grounds of Lake Borgne and the Mississippi Sound, and ultimately the Gulf of Mexico.
- Morganza Spillway. The Corps has asked for permission to open some of the gates on this spillway, located 35 miles north of Baton Rouge, for the first time since 1973. Officials warned residents that even if it is opened, they can expect water 5 to 25 feet deep over parts of seven parishes. Some of Louisiana's most valuable farmland is expected to be inundated and towns like Houma and Morgan City would have to be evacuated. Already, communities are sandbagging as a precaution.
- West Atchafalaya Floodway. This last line of defense for New Orleans was built to take half of the Mississippi's highest flows and divert that to the Gulf. So far the Corps has not said it is considering opening this system.
In Memphis, Col. Vernie Reichling, the Army Corps commander for the region, declared victory there on Tuesday.
The protection around Tennessee's largest city is impressive: While the river crested at just under 48 feet on Tuesday, levees in the Memphis area are 58 feet high on average, and the floodwalls downtown are 54 feet.
"The levees are performing as designed I'm happy to report," he said on CBS's "The Early Show."
But Reichling noted the river isn't done yet. "I think we'll breathe a sigh of relief once this crest has passed and is in the Gulf of Mexico," he said.
NBC's Anne Thompson, msnbc.com's Miguel Llanos as well as The Associated Press and Reuters contributed to this report.