The mechanics

Water cannot create power unless it is flowing from a higher place to a lower place as in a river, a waterfall, or a dam. People use the effects of gravity (the attraction the earth exerts on an object) pulling the water downward when they harness water for power. For example, in the metric system of measurement, cubic meter of water weighs 1 metric ton. The pull of gravity then creates a pressure of 100 metric tons per square meter at the base of a body of water 100 meters tall. If this water were released from a nozzle at the bottom of its source, the stream of water would travel at a speed about 80 meters per second. The force of this stream striking the blades of water wheel would cause the wheel to turn, producing useful mechanical energy.

In the metric system, power is measured in watts. A kilowatt (1000 watts) is the power needed to lift a cubic meter of water through a distance of one meter in one second. The power of a waterfall in kilowatts is calculated by multiplying the flow of water in cubic meters per second by the height of the fall in meters. The potential (greatest possible) power of waterfall 100 meters high with a flow of 10 cubic meter per second would be 100 * 10, or 1,000, kilowatts.
            But mechanical system must be used to make useful energy out of the power of the waterfall, and no mechanical system can make use of all potential power. The power which the mechanical system develops is found by multiplying the potential power of the fall by the percentage of the potential power of the fall by the percentage of the potential power which the system uses.

Production

The potential water power of the entire world is about 2.25 billion kilowatts of electric power. This is a very general estimate, because the flow of many large rivers has not been measured. Of this enormous potential, only about 600 million kilowatts is developed.
            The United States has about a sixth of the world’s developed power. Canada, Australia, and Europe have most of the rest of the developed power. The potential of Asia, Africa, and Latin America is just beginning to be developed.
            The world’s largest hydroelectric power plants in operation include the Grand Coulee on the Columbia River in the United States and the Sayano Shushensk on the Yenisey River in Russia. Each of these plants has a capacity of produce about 6.25 million kilowatts. The Guri power plant on the Caroni River in Venezuela has the Capacity to produce 10 million kilowatts. The Itaipú power plant of Brazil and Paraguay on the Paraná River has a capacity of about 12.25 million kilowatts.