Hydroelectricity or hydroelectric power is the electricity obtained by harnessing the power of water flowing down from a high level. It is a timeless and renewable resource.

Huge generators  convert the potential energy of falling or fast moving water into electrical energy. The potential energy of the water is first converted into mechanical energy and then into electrical energy.

Water, usually stored in dams, is led down through large pipes or tunnels to lower levels, and in the course of the fall, the water rotates turbines. The mechanical energy produced is converted to electricity by the generators connected to it. The structure that houses the turbines and generators is called the powerhouse.

Transformers change the alternating current produced by the generators into current of very high voltage for easy transmission through long distances.

Hydropower is one of the three principal sources of energy used to generate electricity, the other two being fossil fuels and nuclear fuels. Hydroelectricity has certain advantages over these other sources: it is continually renewable thanks to the recurring nature of the water cycle, and causes no pollution. Also, it is one of the cheapest sources of electrical energy.

The Itaipú Dam between Brazil and Paraguay, which  is 190 metres high and generates more than 12,600 MW,  is the largest hydropower dam in the world. (The 185-metre Three Gorges Dam, now being constructed in China on the upper Yangtze River, would take its place by 2009, producing 18,200 MW of  power ). The Grand Coulee Dam, located near Spokane, Washington, is the largest hydropower dam in the United States, producing 6,480 MW. The Rogun Dam, in Russia, the tallest in the world, is 335 metres high. The Oroville dam, located in California, the tallest in the United States, is 235 metres high. Idukki dam in India is Asia's first and largest archdam.

Layout of Hydroelectric Power Plants