LEVEE

any low ridge or earthen embankment built along the edges of a stream or river channel to prevent flooding of the adjacent land. Artificial levees are typically needed to control the flow of rivers meandering through broad, flat floodplains. Levees are usually embankments of dirt built wide enough so that they will not collapse or be eroded when saturated with moisture from rivers running at unusually high levels. Grass or some other matlike vegetation is planted on the top of the levee's bank so that its erosion will be kept to a minimum. Levees protecting inhabited river valley areas against inundations during floods were among the earliest engineering works. In ancient Egypt a series of levees was built along the left bank of the Nile River for more than 600 miles (966 km), from Aswan to the Mediterranean. The cooperative and coordinated enterprise involved in building such long, massive embankments must have been a strong incentive for the development of an organized society and a unified government in ancient Egypt, as well as in ancient Mesopotamia and China, which engaged in similar hydraulic engineering projects. One of the largest modern systems of levees is that built along the Mississippi River and its tributaries and backwaters in the broad alluvial valley extending southward from Cape Girardeau, Mo., to the Mississippi delta, a distance of about 1,000 miles (1,600 km) by river channel. These levees, begun by French settlers in Louisiana in the early 18th century, were in 1735 about 3 feet (0.9 m) high and had been constructed along the river's banks from 30 miles (48 km) north of New Orleans to 12 miles (19 km) south of that city. The system was extended until by the mid-1980s it included more than 3,500 miles (5,600 km) of levees having an average height of about 24 feet (7 m), with some levees reaching 50 feet (15 m) in height. Some silt-laden streams, as their flow slows, may deposit sediment in their bed between their enclosing levees and thus build their channels up higher than the surrounding floodplains. Such streams commonly breach the levees, flowing out onto lower ground and causing catastrophic floods. The lower portions of the Huang Ho in China are noted for this type of behaviour. The lower reaches of the Mississippi River also are poised on such mid-valley ridges.