Flooding has long been recognized as vital part of the ecosystem. Floodwaters often carry sediment and nutrients along their path, nourishing the land wherever they are deposited. This builds valuable habitats for a variety of wildlife and vegetation, and rich alluvial soil for agricultural use. The earliest human civilizations arose on the flood plains of the Tirgis and Euphrates Rivers of Mesopotamia. For centuries, the regular flooding of the Nile River between July and September of each year deposited soils in Egypt's narrow Nile River Delta, allowing the cultivation of crops that made the growth of Egyptian civilization possible.
Worldwide, riverine flood plains cover more than 772,204 sq. mi. (2 million sq. km.) of land and coastal flood plains cover much more land. Most are considered environmentally threatened; in the United States and Europe, almost 90 percent of riverine flood plains are under cultivation, making them, in the words of one researcher, "functionally extinct." One of the main reasons for the threat is the prevalence of flood controls such as dams, levees, impoundments, and flood gates, which protect human life and property, but often destroy natural flood cycles.
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