Flooding

For millennia, human populations have chosen to live near bodies of water. There are three main reasons for this practice: (1) the soil near the waters is very fertile and can be used for growing crops; (2) the bodies of water themselves are sources of drinking and irrigation water; and (3) water courses support transportation and facilitate commerce and trade.

While floods can have disastrous effects, they leave behind silt and other sediments making the land surrounding rivers and other bodies of water rich and fertile. The soil deposited by moving water is known as alluvial soil. At first, populations avoided settling directly on the low-lying land, called flood plains, surrounding the rivers and instead built their villages on terraces or bluffs close to but higher than the rivers. Examples of cities developing on such terraces are Washington, Paris, and Budapest. The advantages of building on terraces is that towns are relatively safe from floods because they are situated higher in elevation than the natural flood plain, but they are also close to fertile land so food is accessible. As populations grew, however, they needed the extra land near the rivers and, therefore, moved closer to the water.

In 1992, in the United States alone, there were almost 3,800 settlements containing 2,500 or more people, each located in an area likely to flood. Furthermore, according to another estimate, nearly 1.5 billion people worldwide still farm their crops in alluvial soil; this is almost one third of the world's population. Likewise, ever since the Mesopotamians established the "cradle of civilization" between the Tigris and Euphrates Rivers in the Middle East in about 3000 B.C., populations have been attracted to rivers for transportation and trade. Narrow stretches of rivers have always been especially attractive locations for people wanting to take advantage of the natural commerce along a trade route.