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Much of our folklore has explored the possibility of "escaping" on the river. Before the advent of the car, many a youth dreamed of traveling cross-country by water. Today there are 25,500 miles of navigable inland waterways in the United States; the Mississippi River, featured in the segment, accounts for approximately 9,000 miles of this system. Equally appealing to youth is building a pint-size version of a dam by piling up sticks and debris to alter the flow of rainwater that is running down street-side curbs toward sewer drains. Dams are fascinating and useful; ruins of the world's oldest dam in Egypt along the Nile date to 2,700 B.C. (It was 37 feet high and 348 feet long.) More recently, the United States has built more than 5,000 dams, Japan has built about 2,000, and India has more than 1,000. Universally, humans need water, but nature delivers water in imperfect amounts due to climate and weather. (Humans also contribute to problems of water availability; for example, deforestation can create desert-like climates in places where water was once abundant.) Some areas have too much water, some not enough, and some have both conditions at different times. Because of this irregularity, dams can be built to make water supplies more useful. Dams have many functions: diversion dams control the supply of water to prevent floods; storage dams reserve water for use during dry periods; power dams generate electricity; and navigation dams provide navigable waters. Some river areas may be unnavigable by commercial boats and barges because of shoals, rapids, waterfalls, or low water. The riverbed itself may change in elevation, which may prohibit reliable and economical navigation. Finally, some rivers have been dammed. A lock can help make navigation possible in each of these situations. The technology of locks looks complex, but the principle is simple: The river is an inclined plane whose water moves in and out of locks by gravity. Think of locks as a flight of "water stairs" going up and down a hill. Water is drained from the first lock (using gravity) until the water level is even with the second one. The downstream gate is opened to allow the vessel into the lower lock, and the process is repeated. The lifting and lowering of vessels, some weighing up to 60 tons, is done without a great use of energy.
