Heavy storms and snowmelt in April saturated the Mississippi River, causing historic flooding across the central United States. The river is expected to crest on Monday in Memphis, Tenn., and move south, toward New Orleans. The Army Corps of Engineers is trying to mitigate damage by diverting water to flood plains and spillways. This Q&A explains how the Mississippi River’s levee system works to contain flooding.

What’s a levee, and what purpose does it serve?

A levee is a protective embankment that prevents a body of water from overflowing. Artificial levees attempt to keep the river in place with a high earthen embankment along the river. The river builds natural levees on its own, by depositing sediment laterally along its banks, and the areas closer to the river are raised higher over time.

Why were levees built along the Mississippi River?

After the Civil War, Congress authorized the U.S. Army Corps of Engineers to develop a levee system to improve navigation and commerce along the Mississippi River. Tulane University geographer Richard Campanella said the levee system locked the river “in a straitjacket.” Rather than excavating the bottom of the river, which would make it easier to navigate, the levee system deposited sediment on the bottom of the river and raised it. In 1927, catastrophic floods overwhelmed hundreds of levees. The following year, the Corps revised its policy to include opening floodways and spillways to accommodate significant flooding.

Where does the excess water go?

The Corps activated its decades-old flood plan last week when it blasted three openings in levees on the Birds Point floodway in Missouri. It helped protect communities downstream and reduced the level of the river by a couple of feet. But the water also inundated 130,000 acres of Missouri farmland. On Monday morning, the Bonnet Carre Spillway, upriver from New Orleans, Louisiana, was opened to further relieve pressure on the levees. The water then flows to Lake Pontchartrain and on to the Gulf of Mexico. Campanella said one fifth of the Mississippi’s water volume can be diverted through Bonnet Carre. The Corps is also considering opening the Morganza Spillway, just above Baton Rouge. The Morganza Spillway can divert up to 50 percent of the Mississippi’s water volume.

What are the risks and benefits of flood control?

Craig Colten, a Professor of Geography and Anthropology at Louisiana State University, said the levee system creates a false sense of security, especially among farmers and landowners living in the flood plains. “People who use protected flood plains forget the risk they face. They come to expect complete protection, and that was never offered” by the levee system. He said the levees were not designed to prevent every flood. Colten said the entire system is really meant to protect major cities, refineries, and pipelines along the Gulf coast. It would make more sense to take the flood plain out of active use, he said. “It’s a fool’s errand to confine the river permanently.”

Campanella saw “difficult choices rather than a straightforward solution” with the plans. The benefits of cultivating rich soil in a vast flood plain can significantly improve crop yields. But, Campanella said, that decision comes at a cost. “Humans and water want to occupy the same place,” he said, so compromises and concessions will always need to be made. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.