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.

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.

C. Flood Control

Dams also protect low-lying areas from floods (see Flood Control). Floods occur when more rain falls than the soil and vegetation can absorb. The excess water runs off the land in greater quantities than rivers, streams, ponds, and wetlands can contain. Such heavy rains, and also snowmelt, periodically cause rivers to overflow their banks, spilling onto the surrounding floodplain. Ensuing floods can damage property and endanger the lives of people and animals.

D. Navigation

Dams help make inland waterways accessible to ships and barges. By inundating shallow, rocky streambeds and controlling the release of water from reservoirs, dams make rivers deep enough for ships and barges to pass through without running aground.

When a dam obstructs a navigable river, engineers build a canal adjacent to the dam to permit ships and barges to bypass the dam. Canals may incorporate one or more locks, which contain mechanisms to control the water level. Ships and barges are raised or lowered with changes in the water level in the lock. One gate in the lock then opens, enabling a vessel to exit to a higher or lower section of the waterway. Locks prevent water from rushing uncontrolled through the canal.