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The rise and fall of the water level in a river is called the flood wave. Its highest point, or crest, travels progressively downstream. In the upstream portions of a river the flood crest passes quickly. Further downstream the greater volume of water causes slower passage of the flood crest, resulting in floods of longer duration. In many regions, annual floods follow the thaws and rains of spring; flooding also may occur because of thawing ice jamming narrower and shallower parts of a river. In the Arctic regions, especially in the basins of northward flowing rivers, the floods are caused by the thawing of the southern portion of the basin before the ice blocking the lower course of the river melts. Less predictable are floods resulting from ocean waves, called storm surges, pushed onshore by an advancing hurricane, and from sudden torrential flows, called flash floods, following a brief, intense rainstorm or the bursting of a natural or man-made dam or levee. In addition to the duration and quantity of rainfall, the nature of the soil (permeability; state of saturation) of an area affects the frequency of floods.
Generally, flood control measures along a river are attempted at both its headwaters and its low-lying floodplains. Runoff can be detained in the headwaters by planting ground cover on the slopes, building terraces to increase soil infiltration and prevent soil erosion, and building small check dams or retaining ponds to reduce the flow of water. Flood control on the lower floodplains involves building levees to contain the flow and straightening or dredging the channel to improve flow characteristics. Among the chief flood-control projects in the United States are the flood control works along the Mississippi River, the installations of the Tennessee Valley Authority, the Glen Canyon and Hoover dams on the Colorado River, and the systems of dams in the Columbia River basin (including Grand Coulee Dam) and in the Missouri River basin.
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