For centuries, mankind has been harnessing the power of rivers to move waterwheels for various purposes. With the advent of electrical power, it was found the rivers were also very useful for generating electricity. Hydroelectric power plants derive power from rivers by constructing dams to ensure a dependable supply of water. The overflow is then released through special chambers where the moving water turns giant turbines, generating electricity. When planned and engineered properly, this system provides an excellent power source with very limited environmental impact. The Hoover Dam, constructed on the Colorado River near Las Vegas, was one of the earliest hydroelectric power plants.