People from the beginning of recorded history have constructed barriers across rivers and other water courses to store or divert water. The earliest of these dams were used to water farms. For example, the ancient Egyptians built earth dams that raised the river level and diverted water into canals to irrigate fields above the river. Behind the dam, waters pile up to form an artificial lake which sometime can be very long. The artificial lake backed up by a dam is called a reservoir.

Dams are built primarily for irrigation, water supply, flood control, electric power, recreation, and improvement of navigation. Many modern dams are multipurpose. Irrigation dams store water to equalize the water supply for crops throughout the year. Water supply dams collect water for domestic, industrial, and municipal uses for cities without suitable lakes or rivers nearby for a water supply. Flood control dams impound floodwaters of rivers and release them under control to the river below the dam. Hydroelectric power dams are built to generate electric power by directing water in penstocks through turbines, wheels with curved blades as spokes. The falling water spins the blades of the turbines connected to generators. Power dams are expected to generate power to repay the cost of construction. The output depends, first, upon the head of water, or height of stored water above the turbines. The higher the water the more weight and pressure bear upon the turbine blades. A second factor is the volume of water throughout the year. The minimum flow in dry months fixes the amount of firm power which customers can rely upon to receive regularly. Sometimes extra power, or run-of-stream power, generated in flood seasons can be sold, usually at lower rates.

Dams also provide benefits other than those mentioned above. Their reservoirs provide recreation, such as fishing and swimming. They become refuges for fish and birds. Dams conserve soil by preventing erosion. They slow down streams so that the water does not carry away soil. Dams can also create problems. Their reservoirs may cover towns or historic and scenic places. Dams may impair fishing. Another problem of dams is silting. Some rivers pick up clay and sand and deposit them behind the dam, thereby lessening its usefulness.

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