Hydroelectric dam. Traffic crossing the dam of a hydroelectric power station. Electricity generated by hydroelectric stations is relatively cheap & has little impact on the environment other than on the river it is built on. The amount of power generated may be increased quickly simply by raising the flood gates. Hydroelectric power stations are often used during peak demand times. Photograph of the Hoover Dam on the Colorado River, USA. Built during 1931-36, this dam is 221 metres high and 379 metres wide at its crest. The reservoir behind it, Lake Mead, has an area of 694 square kilometres. The dam's turbines are capable of generating 1.5 million kilowatts of power.