Role of Human Activities: River Modification
In the western Gulf of Mexico, along the Pacific coast, and in parts of New England, artificial inland lakes and other freshwater impoundments also contribute to coastal land loss by regulating river discharge and sediment load delivered to downstream deltas and beaches. Dams used to store potable water supplies, control floods, and provide recreation have a doubly negative effect on land loss by trapping sediment and eliminating peak flood discharges. It is these highest discharges that are responsible for flushing the lower reaches of rivers and transporting most of the sediment to the coast. Proposed plans to build sediment-bypass structures at dams ignore the fact that this will only stockpile sediment below the dams and will be inefficient without unregulated floods to transport the material farther downstream.
The most extensive control of a river system in the United States is confinement of the Mississippi River and prevention of it switching into the Atchafalya River channel. Construction of high levees along the main channel and distributaries of the Mississippi River, which began in the early 1700s (Davis, 1990), has dramatically reduced the frequency and volume of sediment deposited on the levees and adjacent delta plains. Consequently, land loss on the delta plains has accelerated at an alarming rate and costly projects are currently being conducted to mitigate the wetland losses or to construct new wetland areas.
U.S. Department of the Interior, U.S. Geological Survey
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Last modified: 23:12:51 Sat 12 Jan 2013
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