People living in a climate with a dry season, during which little or no rain falls, or in places where the rainfall is always unreliable, building a dam is an obvious way to store water. In other places, heavy seasonal rains or the melting of snow near the source of a river can cause sudden floods that destroy crops and wash away homes (Adler, 59). Again, a dam can hold back the floodwaters and release them a little at a time, so there is water for irrigation and other uses, and no flooding. Modern dams also generate electrical power. The first large dam to be built was the Hoover Dam on the Colorado River. It was completed in 1936 and stands 726 feet (221 m) high (Scudder, 134).
The first dams were probably made entirely from clay or other fine-grained soil that would be fairly impermeable when its particles were packed tightly together (Lowry, 116). The shape of the dam is also important. As the diagram below shows, the walls should have a shallow slope.
On the upstream side this spreads the force of the water over a larger surface area, thus protecting the structure from being battered until it fails. This strengthening is often increased by covering the surface with a layer of large, loose rocks, called riprap, to absorb some of the energy of the moving water (Adler, 124). On the downstream side, the surface must be protected from rain. If not, in time the rain can wash away enough material to cause failure (Scudder, 148). The downstream surface can be protected by growing grass or other plants on it and these are most easily planted and managed if the slope is not too steep (Scudder, 156). Some dams also have a drain, made from a layer of sand and gravel set into the ...