Locks and Dams

Dams are impoundments of rivers and streams. They block the free flow or water, or they may direct or retard the flow of water. Usually a dam impounds water behind it, creating a lake or reservoir. Dams have an outlet for the water that they impound. The spillway or weir is usually a trough or pipe or some outlet that allows the build up of water behind the dam to flow out at the rate that it is flowing in once the reservoir had been filled. The water may flow out continuously or be released intermittently.

Dams may be the results of acts of nature. Landslides or earthquakes that block the flow of streams or rivers have formed numerous lakes. Animals, especially beavers, also build dams on streams. Farmers or developers have often fought their impoundments; however, beaver engineering of dams increases the amount of water available for fish, birds, and other wildlife. Their dams also raise the local water table providing for a different ecology than if their dams were not there.

Human construction of dams began over 2,000 years ago in China when dams were built on tributaries of the Yangtze. The first dams were built from earth, timber, and stone. Modern dams may be simple earthen containment dams, or elaborate concrete and steel dams such as the Hoover Dam. Dams have been built for many purposes, including irrigation, water supply, hydroelectric power, recreation, habitat, flood control, or for containing industrial or mining effluent. Many dams are built to facilitate shipping. The level of water in a river or stream usually varies with seasonal rainfall. Dams deepen the water in a waterway and allow shipping to take place all year long, unless the water freezes in the winter.

In order to facilitate shipping or transportation on rivers or other waterways, dams have been built [Page 1064]to maintain a more or less constant water level. At places where rapids exist as a natural barrier, boats and ships have been in ancient times off loaded and then loaded below the rapids. Or they have been hauled overland to the next navigable area of water. Some dams have locks built into them or beside them. The lock is a structure that facilitates shipping on the dammed waterway. It allows cargo or passenger boats to easily navigate over a natural water barrier.

In the United States, rivers like the Tennessee have numerous dams that can affect plant and animal life.

Locks operate by shutting doors to a box that is then filled with water or emptied of water. If a vessel is going upstream, the lock receives it at a lower downstream level. The doors to the lock are closed and water is pumped into the lock to raise the vessel to the higher upstream level. The lock acts like an elevator to raise or lower vessels to the upstream or downstream level. Once the doors of the lock are opened the vessel can then go on its way.

Today many of the rivers and streams in the United States have been dammed. Rivers like the Tennessee have numerous dams with locks in them that permit barges to move vast quantities of goods. Dams can have positive benefits, including the regular flow of water provided by a system of dams that allows for irrigation, recreation, and shipping.

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