Hydroelectric power is electricity generated by harnessing the natural energy in moving water by using it to spin turbines in a hydroelectric dam. The idea of harnessing the power in moving water is thousands of years old. It is thought the Greeks were the first to use water mills (akin to windmills) to produce flower. The first hydroelectric generator in the United States was at Niagra Falls, built in 1880. Today there are nearly 3000 hydroelectric dams in the United States. As far as electrical generation is concerned Itaipu Dam, located between Brazil and Paraguay, is the largest hydroelectric dam in the world, followed closely by Grand Coulee Dam, the largest in the United States.
Of all of our current ways of producing energy, hydroelectric generation is by far the safest and cleanest as well as being completely replenishable from year to year. Do dams destroy habitat? Yes, and no. A dam does not create any pollution, however it radically alters a river and surrounding areas. However, change is not necessarily destruction. For instance, cities destroy habitat for some animals, but become habitat for us humans and other symbiotic parasites (e.g. squirrels, pigeons, rats, etc.).
This same way, the environment around a dam is not the same as it was before the dam was installed. However, the environment is not a wasteland as it may be after a core meltdown of your local nuclear power plant or even as it is with that black filth that is pumped into the air by coal burners. Although these dams have their down side they provide us with the least of evils for our need for power.
Worldwide, hydroelectric power plants generate 675,000 megawatts, that's 2.3 trillion kilowatt-hours of electricity every year, the equivalent of burning 3.6 billion barrels of oil.