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Dikes and Levees

Dikes and levees are embankments that are built to prevent flooding. Levees may be formed naturally or artificially. They are raised banks along rivers that prevent the water from overflowing and flooding surrounding areas. The word levee comes from the French word lever, which means to raise. When a river floods, it carries with it silt and other material from the river channel. The heaviest material is dumped on the banks of the river and the finest grains of silt are carried some distance from the river. The material dumped along the river bank builds up into a levee.

However, natural levees are sometimes not high enough or strong enough to hold back flood water. Engineers then build artificial levees from rock or concrete. Grass is usually planted on the slopes of artificial levees to stop rainwater from washing the earth away. The levees are often built a short way from the river's edge. This stops the river from undermining the embankment. Levees have been built in many river valleys throughout the world.

The first levees on the Mississippi River were built at New Orleans. By 1735, they extended for more than 40 miles, but were only three feet high. Today there are more than 3,500 miles of levees along the Mississippi. These have not always prevented flooding. In 1965, the Mississippi flooded in five states despite levees.

Dikes are a special kind of levee. The most famous dikes are in the Netherlands where much of the land lies below sea level. The dikes are walls that hold back the sea. The land would be flooded if the dikes were broken down. The dikes were first built to reclaim land from the sea. They enclosed an area under the sea. When the dikes were complete, the sea-water was pumped out. Within a few years, the land became fertile farmland. However, the sea has broken through the dikes on many occasions in Dutch history, flooding great areas of land.