Kids Research Express

Dam, structure that blocks the flow of a river, stream, or other waterway. Some dams divert the flow of river water into a pipeline, canal, or channel. Others raise the level of inland waterways to make them navigable by ships and barges. Many dams harness the energy of falling water to generate electric power. Dams also hold water for drinking and crop irrigation, and provide flood control. See also: Why People Build Dams Types of Dams Ecological Impact

Dams are classified by the type of material used in their construction and by their shape. Dams can be constructed from concrete, stone masonry, loose rock, earth, wood, metal, or a combination of these materials. Engineers build dams of different types, depending on the conditions of the riverbed, the geology of the surrounding terrain, the availability of construction materials, and the availability of workers. When more than one type of dam will suffice, engineers often opt to construct a type that they have built previously. A. Gravity Dams Gravity dams use only the force of gravity to resist water pressure—that is, they hold back the water by the sheer force of their weight pushing downward. To do this, gravity dams must consist of a mass so heavy that the water in a reservoir cannot push the dam downstream or tip it over. They are much thicker at the base than the top—a shape that reflects the distribution of the forces of the water against the dam. As water becomes deeper, it ex

Building a dam changes the ecology of the surrounding area. Among the most affected animals are fish that depend on free-flowing water to live. Some kinds of salmon, trout, and other fish species migrate downstream to spend part of their lives in the open ocean. As adults, they return upstream to lay their eggs in the gravel bottoms of the rivers where they were born. Large dams block the passage of such migratory fish. Some dams incorporate a fish pass to allow fish a chance to swim around the dam and reach upstream spawning grounds. Fish passes called fish ladders comprise a series of small pools arranged like stair steps. Each pool is slightly higher than the previous one. Fish ladders are based on the idea that a fish swimming upstream cannot leap over a dam that is more than about 5 meters high, but it can leap up a series of pools, each slightly higher than the one below it. Despite fish passes and other efforts to help fish bypass dams, the cumulative effect of multiple dams bui

People build dams to divert water out of rivers for use in other locations or to capture water and store it for later use. The volume of water flowing in any given river varies seasonally. In the spring and early summer, rivers typically swell with water from rainstorms and mountain snowmelt. In the drier months of late summer and autumn, many rivers slow to a trickle. Storage dams impound seasonal floodwater so it can be used during periods of little or no rainfall. The water that backs up against a storage dam forms an artificial lake, called a reservoir. Release of water from the reservoir can be controlled through systems of pipes or gates called outlet works. A. Irrigation and Drinking Water From ancient times to the present, people have built dams to capture water to irrigate crops in areas where rainfall does not provide enough ground moisture for plant growth. Simple irrigation systems often depend on small diversion dams that raise the height of a stream. Flowing water backs u