People have also died in earthquakes caused by the great weight of water in large reservoirs. A magnitude 6.3 earthquake caused by Koyna Dam in India in 1967 killed around 180 people. Nearly one-fifth of the world's electricity is generated by dams. Dams also provide flood control, supply water to cities, and can assist river navigation. Many dams are multipurpose, providing two or more of the above benefits.
Hydroelectricity is cheap to produce - once the dams are built.
The problem is the huge costs of building dams and the long time it takes to build them. Actual costs for hydropower dams are also almost always far higher than estimated costs - on average around 30 percent higher. Dam designers are often very optimistic about how much power their dams will produce and often fail to account for the impacts of droughts, meaning that dams often produce less power than promised. Itaipu generates around 20 percent less electricity than predicted. When these high costs, delays and risks of low river flows are factored into calculations of the costs of electricity it can be seen that hydropower is now an expensive form of power generation.