Sharon Shaffer, age 12, of Helena, Mt., for her question:
WHERE WAS THE FIRST DAM BUILT?
A dam is a barrier that is constructed across a river or stream to impound water and raise its level. The first dam of which record exists was built about 4000 B.C. to divert the Nile River in Egypt in order to provide a site for the city of Memphis.
Many ancient earth dams, including a number built by the Babylonians, were part of elaborate irrigation systems that transformed unproductive regions into fertile plains capable of supporting large populations.
Because of the ravages of periodic flooding, very few dams more than 100 years old are still standing. The construction of virtually indestructible dams of appreciable height and storage capacity became possible after the development of Portland cement and mechanization of earth moving and materials handling equipment.
The most common reasons for building dams are to concentrate the natural fall of a river at a given site, thus making it possible to generate electricity; to direct water from rivers into canals and irrigation and water supply systems; to increase river depths for navigational purposes; and to control water flow during times of flood and drought.
Control and use of water by means of dams profoundly affect the economic aspects of vast areas. One of the first stages in the progress of developing countries usually involves control of water for power generation, agriculture and flood protection.
Dams are classified on the basis of structural form and materials used. The basic types are gravity, arch, buttress and embankment. The first three types usually are built of concrete. A single structure may include more than one type of dam.
Embankment dams are rockfill and earth dams, levees and dikes that are the structures most commonly used to impound water. Everything from clay to large stones can be used in the construction of embankment dams.
The base of an embankment dam is usually four to seven times wider than the height.
Embankment dams may consist almost entirely of impermeable material, such as clay, or may have a core of impermeable material bounded both upstream and downstream by zones of more permeable material, such as sandy gravel or rockfill. The core may extend well below the level of the main dam foundation in order to reduce seepage.
Tarbela Dam, completed in 1977 on the Indus River in Pakistan, is 485 feet high and 9,000 feet long. It contains about 165 million cubic yards of earth and rock in the main dam, the largest volume ever used in an embankment dam. The project, including hydroelectric power facilities, cost more than $1 billion.
The Oroville Dam on the Feather River in California is the highest embankment dam in the United States. It is 770 feet high.