Dams are one of humanity's oldest innovations. People may have borrowed the idea from the beavers, but human ingenuity has taken dam-building to incredible heights. These structures serve many functions: flood control, navigation, water supply, power generation and even recreation.
Essentially, dams are edifices, usually built of concrete, on a river to back up the water on one side. Depending on its purpose, it may have locks and a series of gates, called spillways, or it may have only a powerhouse and turbines. A good example of ones that serve all the aforementioned functions are those in the Tennessee Valley Authority system in the Southeastern United States. Many of the TVA dams have become vital parts of the river's ecosystem.
One hundred years ago, the Tennessee River was wild and narrow, running its 600 or so miles (around 965 km) from east Tennessee, down through northern Alabama, and then turning north, back into western Tennessee, before it emptied into the Ohio River in Paducah, Kentucky. The river flooded every spring, and the treacherous shoals in northwest Alabama made navigation hazardous. With the advent of the Tennessee Valley Authority, and the building of the dams on the river, the Tennessee became one of the country's largest shipping routes.
Dams that assist in navigation have locks and spillways, which also aid in flood control. A lock is a gated chamber built on one side of the structure. Using gravity, the lock can be filled from the water above the dam, and emptied into the water below it. Boats can then navigate through the locks and continue on their journeys.
Spillways are gates built into the dams. They open and close, allowing water through, in order to lower the lake level above the structure. This aids in navigation because it helps keep a constant water depth in the main river channel. It also helps with flood control, since too much water from one portion of the river can be moved to other parts of the river that are not in danger of flooding.
Power generation is another primary purpose of dams. Water flows through the turbines, which turn the generators to produce power. The kinetic energy needed to turn the turbines comes from the force of the water falling into the turbines and turning their blades to power the generators. Hydroelectric power is much cheaper and more environmentally sound than many other generation methods.
Recreation on a river also profits from these structures. Again, since they keep the water level consistent, boaters and fishermen can count on safe water levels for their activities. The environment may also benefit, since lake levels can be lowered in the winter, for example, exposing mud flats and other land that helps support waterfowl wintering in the area. Dams also help keep water supplies steady, which is crucial in drought periods.