A levee is simply an embankment that is built up to keep a river from overflowing. In New Orleans, the levees do dual duty -- on one side of the city, levees protect against floods on the Mississippi river, and on the other side, levees help keep Lake Pontchartrain at bay.

Levees have been with us for centuries. On the Mississippi river, levees have tried to control the river since settlers started arriving. Mark Twain notes in "Life on the Mississippi":

"One who knows the Mississippi will promptly aver... that ten thousand River Commissions, with the mines of the world at their back, cannot tame the lawless stream, cannot curb it or confine it, cannot say to it Go here or Go there, and make it obey; cannot save a shore that it has sentenced."

In parts of the Netherlands, where the land is extremely flat and either at or below sea level, levees are called dikes. The famous windmills of Holland, for example, pump water from behind the dikes back out to sea to keep the land dry. There are even parts of the Netherlands, called polders, where new dry land has been created along the sea by diking and draining it.

A levee is typically nothing more than a mound of dirt. The mound runs in a long strip, sometimes for many miles, along a river, lake or ocean. Most of the levees along the Mississippi are 10 to 20 feet (3 to 7 meters) tall. In Holland they can go as high as 10 meters (30 feet).

Levees control flooding. For example, along the Mississippi river, the levees are dry until the river floods in the spring. The levees allow the river to rise without flowing into adjacent land. In this application, the levees can be both good and bad.

The good part is that the levees allow people to live along the river without getting flooded out every year. They prevent property damage. The bad part is that the levees can break, or the river can flood over them. Originally the Mississippi would flood hundreds of square miles of land in the spring. Now the river stays in its channel. If a levee breaks, the river flows through the breach and can inundate a huge area.

The Levees of New Orleans

Levees along the Mississippi river in the New Orleans area have been under construction since the early 1700s. The Army Corps of Engineers took them over in the mid 1800s. New Orleans is now completely dependent on levees to protect the city from floods.

The problem that New Orleans faces is that much of the city lies below sea level. This was not always the case. As the city expanded, low-lying, wet and boggy areas were pumped dry to create new land. Much of this reclaimed land also sank as it dried out. The result is that parts of New Orleans lie as much as 10 feet (3 meters) below sea level. The entire city now depends on the levees, along with massive pumping stations, to keep the water out.

Another problem the New Orleans faces is that the levees themselves have been sinking. They can also weaken in a large flood and need repairs. Hundreds of millions of dollars have been spent maintaining the New Orleans levee system over the years. Recently, with money at the federal level being tight, repairs have lagged. This may be one reason for the Levee failure seen after Hurricane Katrina.