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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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.