Many rivers do not have a constant and regular flow of water. During rainy seasons, flood occurs and in other times, the level of water is less.
This would hamper the work of power stations and industries. A dam is actually, a reservoir of water to meet emergencies. The dams and embankments are used from the ancient times.
The sea water reservoirs provide water in the dry season and also for irrigation. Some notable dams in the world are the ‘Aswan Dam’ on the river Nile, the ‘Boulder Dam’ on the Colorado and the ‘Kariba Dam’ on the Zambesi.
Below Dam info from Wikipedia (Read More):-
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees (also known as dikes) are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are often used in conjunction with dams to provide clean electricity for millions of consumers.
Dams can be formed by human agency, natural causes, or even by the intervention of wildlife such as beavers. Man-made dams are typically classified according to their size (height), intended purpose or structure.
http://en.wikipedia.org/wiki/Dam
