How Does The Army Corps Handle Flooding?

  • MAY 20: An American flag reflects in Mississippi River floodwaters outside of a home in south Vicksburg, Miss.
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    MAY 20: An American flag reflects in Mississippi River floodwaters outside of a home in south Vicksburg, Miss.
    Rogelio V. Solis/AP
  • MAY 19: Floodwaters from the Yazoo River creep across fields of crops near Yazoo City, Miss.
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    MAY 19: Floodwaters from the Yazoo River creep across fields of crops near Yazoo City, Miss.
    Dave Martin/AP
  • MAY 19: A boat motors through Mississippi River floodwaters past a wall of flood-containment baskets, known as Hesco baskets, that is protecting the Riverside Medical Center in Vidalia, La.
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    MAY 19: A boat motors through Mississippi River floodwaters past a wall of flood-containment baskets, known as Hesco baskets, that is protecting the Riverside Medical Center in Vidalia, La.
    Gerald Herbert/AP
  • MAY 19: A corrections officer motors through floodwaters of the Mississippi River to pick up prison trustees, who are being used for flood-abatement work in Vidalia.
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    MAY 19: A corrections officer motors through floodwaters of the Mississippi River to pick up prison trustees, who are being used for flood-abatement work in Vidalia.
    Gerald Herbert/AP
  • MAY 17: Floodwaters from the Mississippi river creep across the field as farmers work in Natchez, Miss.
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    MAY 17: Floodwaters from the Mississippi river creep across the field as farmers work in Natchez, Miss.
    Dave MartinAP
  • MAY 16: Barbara Fontanille recovers a tire from the rising waters of the Atchafalaya River in Simmesport, La. Her family has no flood insurance and have relocated to a trailer provided by FEMA.
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    MAY 16: Barbara Fontanille recovers a tire from the rising waters of the Atchafalaya River in Simmesport, La. Her family has no flood insurance and have relocated to a trailer provided by FEMA.
    Mario Tama/Getty Images
  • MAY 15: The flooding in Vicksburg, Miss. has put many houses underwater. Only the roofs are visible on others, like this one.
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    MAY 15: The flooding in Vicksburg, Miss. has put many houses underwater. Only the roofs are visible on others, like this one.
    Carrie Kahn/NPR
  • MAY 14: People watch as water diverted from the Mississippi River spills through a bay in Morganza, La. A  floodgate was slowly raised for the first time in nearly four decades, unleashing a torrent of water from the Mississippi River.
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    MAY 14: People watch as water diverted from the Mississippi River spills through a bay in Morganza, La. A floodgate was slowly raised for the first time in nearly four decades, unleashing a torrent of water from the Mississippi River.
    Patrick Semansky/AP
  • MAY 12: A Black Hawk helicopter carrying Louisiana Gov. Bobby Jindal flies over the Atchafalaya Basin in Louisiana during a tour of areas that may be affected by flooding if the Morganza Spillway north of Baton Rouge is opened.
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    MAY 12: A Black Hawk helicopter carrying Louisiana Gov. Bobby Jindal flies over the Atchafalaya Basin in Louisiana during a tour of areas that may be affected by flooding if the Morganza Spillway north of Baton Rouge is opened.
    Patrick Semansky/AP
  • MAY 11: City workers load sandbags for re-enforcing a levee gate onto a boat in Vicksburg. Historic Vicksburg, the site of a pivotal Civil War battle, has been one of the hardest-hit cities.
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    MAY 11: City workers load sandbags for re-enforcing a levee gate onto a boat in Vicksburg. Historic Vicksburg, the site of a pivotal Civil War battle, has been one of the hardest-hit cities.
    Scott Olson/Getty Images
  • MAY 10: A flood wall protects the Pyramid Arena from the swollen Mississippi River after it crested at nearly 48 feet in Memphis, Tenn.
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    MAY 10: A flood wall protects the Pyramid Arena from the swollen Mississippi River after it crested at nearly 48 feet in Memphis, Tenn.
    Jeff Roberson/AP
  • MAY 9: A street sign in the Ford subdivision is nearly submerged  in Vicksburg.
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    MAY 9: A street sign in the Ford subdivision is nearly submerged in Vicksburg.
    Bryant Hawkins/The Vicksburg Evening Post/AP
  • MAY 8: Residents gather at the edge of the floodwaters in the West Junction neighborhood of Memphis.
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    MAY 8: Residents gather at the edge of the floodwaters in the West Junction neighborhood of Memphis.
    Scott Olson/Getty Images
  • MAY 8: Volunteers in Memphis fill sandbags to help hold back rising floodwater.
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    MAY 8: Volunteers in Memphis fill sandbags to help hold back rising floodwater.
    Scott Olson/Getty Images
  • MAY 4: Sally Nance helps her neighbors remove clothes from their home in Tiptonville, Tenn.
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    MAY 4: Sally Nance helps her neighbors remove clothes from their home in Tiptonville, Tenn.
    Scott Olson/Getty Images

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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 says 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, was opened to further relieve pressure on the levees. Campanella says one-fifth of the Mississippi's water volume can be diverted through Bonnet Carre; the water then flows to Lake Pontchartrain and on to the Gulf of Mexico. 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, says 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, says Colten; the levees were not designed to prevent every flood. Colten says 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 says: "It's a fool's errand to confine the river permanently."

Campanella sees "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 says, that decision comes at a cost. "Humans and water want to occupy the same place," he says, so compromises and concessions will always need to be made.

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