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Encyclopedia of Global Resources part 31

Encyclopedia of Global Resources part 31 provides a wide variety of perspectives on both traditional and more recent views of Earth's resources. It serves as a bridge connecting the domains of resource exploitation, environmentalism, geology, and biology, and it explains their interrelationships in terms that students and other nonspecialists can understand. The articles in this set are extremely diverse, with articles covering soil, fisheries, forests, aluminum, the Industrial Revolution, the U.S. Department of the Interior, the hydrologic cycle, glass, and placer mineral deposits. . | 270 Dams Global Resources storing water. Historically dams are the oldest means of controlling the flow of water in a stream. The primary function of most dams is to smooth out or regulate flows downstream of the dam. Generally dams are permanent structures. In some cases however temporary structures may be constructed to divert flows as from a construction site. Cofferdams are used in this regard not to store water but to keep a construction area free of water. Such temporary structures are designed with a higher assumed risk than are permanent dams. Dams date in recorded history to about 2600 to 2900 b.c.e. Uses of Dams Dams are primarily used for four purposes conservation navigation flood control and generation of hydroelectric power. A fifth but somewhat less important use is recreation. Conser vation purposes include water supply including irrigation and low-flow augmentation to achieve wastewater dilution requirements . Flood-control objectives dictate that a dam s reservoir be as empty as possible so that any excess water from the watershed the area upstream of the dam that sheds water to it can be detained or retained in the reser voir to reduce any potential flood-related damage of property and or loss of life. A single dam can be used for all the foregoing purposes. In that case the reser voir is called a multipurpose reser -voir. Rarely can the cost of a major dam be justified for one purpose. Types of Dams Dams are classified according to the type of material used in their construction and or the structural principles applied in their design for structural integrity and stability . Generally dams are constructed with concrete or earthen materials readily available at the construction site. In some cases the nature of the construction site with regard to underlying rock formation climate topography and sometimes the width and load-carr ying capacity of the valley in which the dam is constructed significantly influences the type of dam selected. Common .