Dams are built across flowing bodies of water and tend to serve two purposes.
Some dams have virtually no water reserve storage capacity.
The picture shows the various components of a hydroelectricity generating dam. Dams are the most largest man-made structures on the planet. The Itaipu Dam in South America stretches for nearly 8 km across the long Parana river. 11 million cubic metres of concrete were used to build the dam - enough to build four or five new cities . At full power, its generator outputs over 12 MW of electricity.