Dam

I

Introduction

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.

The oldest known human-made dams were built more than 5,000 years ago in arid parts of the Middle East to divert river water to irrigate crops. Today there are more than 500,000 dams worldwide. The vast majority of these are small structures less than 3 m (10 ft) tall. Engineers regard dams that measure more than 15 m (50 ft) high as large dams. About 40,000 large dams exist in the world today.

II

Why People Build Dams

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 up against the dam until it overflows into a canal, ditch, or pipe that carries the water to fields.

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Large storage dams support sophisticated modern irrigation systems that have dramatically altered the landscape of arid regions throughout the world. For example, large storage dams in the American West have transformed millions of acres of arid desert into productive cropland. Hoover Dam, which stretches across the Colorado River near Las Vegas, Nevada, stores about twice the annual flow of the river in its reservoir, Lake Mead. This reservoir holds enough water to cover the state of Pennsylvania to a depth of one foot. Lake Mead helps provide a dependable water supply for more than 400,000 hectares (1 million acres) of farmland in southern California and southwestern Arizona, and 162,000 hectares (400,000 acres) in Mexico.

Dams also replenish the water supply of cities and towns. Colorado River water impounded by Hoover Dam in Lake Mead helps provide water for drinking and other uses to more than 16 million people in greater Los Angeles, California, and portions of Arizona and southern Nevada.

B

Hydroelectric Power

Hydroelectric dams generate electricity (see Waterpower). Hydroelectric dams harness the energy of water released from the reservoir to turn hydraulic turbines. The turbines convert the energy of the falling water into mechanical energy, which is used to power electric generators.

The Grand Coulee Dam, on the Columbia River in Washington State, continuously generates more than 6,500 megawatts of electricity. This power is distributed to industrial and residential consumers throughout the western United States. The Itaipú Dam, on the Panará River between the countries of Brazil and Paraguay, continuously generates more than 12,600 megawatts of electricity. It supplies nearly 80 percent of the electric power used in Paraguay and 25 percent of the electricity used in Brazil.

Dams designed to generate electricity deliver water to a building, called a powerhouse, which contains highly specialized power-generating equipment. Large pipes called penstocks carry water from the reservoir down into the powerhouse. Water exits a penstock through small openings, which concentrate the flow and direct it onto the blades of a large hydraulic turbine. The force exerted by the falling water rotates the blades, and this action drives the shaft of an electric generator. The shaft spins giant magnets in the generator, creating an electric current (see Induction). Power lines transmit the current to consumers within a regional power network (see Electric Power Systems).

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