A natural levee is formed when the movement of water pushes sediment to the side of rivers and creeks. This creates a slight elevation on the sides, making it taller than the river bed. Materials that make up a natural levee typically include sediment and silt.
Natural levees generally rise up in a parallel line to the river's natural flow, helping the river maintain its direction. Levees serve the function of keeping rivers from flooding structures and cities around it during a surge in the water levels. Levees can also be created artificially.
Artificial levees can be made using sand, soil or rocks piled on a level surface near a river. In areas with strong river currents or in those that are dangerously susceptible to flooding, levees can be made using wooden blocks, plastic or metal. They are often further reinforced with concrete.
Levees have been made artificially since the Indus Valley Civilization around 2500 B.C. The first artificial levees were created to protect land around the Indus River from flooding to create farmland. Levees also serve a defensive purpose, as they transform river banks into natural moats. Levees were also used during warfare to create floods to wash away invading enemies. One instance of this occurred in 1938 when Chinese leaders destroyed a Yellow River levee to halt the advance of the Japanese military.