water power, mechanical energy derived from falling or flowing water, e.g., rivers, streams, and the overflow of dams. The wooden water wheel, long utilized for driving machinery in flour mills and factories, was largely supplanted by the steam engine in the early 19th cent. In modern practice, water flowing from a higher level to a lower level (as from a dam or waterfall) is used to activate a turbine that drives an electric generator, a process called hydroelectric power generation. The amount of power furnished is proportional to the rate of flow of the water and the vertical distance through which it falls. In a pumped-storage plant, water is pumped upward to a high-level reservoir during periods of low electricity demand by using the excess electricity available. During periods of high demand the facility produces electricity by using the water that flows down from the reservoir. The availability of water power along a fall line, which is a boundary between an upland region and a coastal plain, influenced the location of many cities in the E United States. Similarly, Canada, Norway, Sweden, and Switzerland, which have mountainous regions subject to heavy rainfall near industrialized areas, have highly developed hydroelectric programs. Asia, South America, and Africa have the greatest potential for further water power development, the nations of Europe and North America having developed their resources to the greatest extent. In the late 1990s the countries that produced the most hydroelectric power—about 51 percent of the world total—were the United States, Canada, Brazil, China, and Russia. For information on some important hydroelectric power projects, see Churchill Falls; Niagara Falls; Columbia basin project; Colorado River storage project; Saint Lawrence Seaway; Tennessee Valley Authority; Dniprohes.