hydroelectric dams,
Although hydroelectric power is admittedly one of the cleanest and most environmentally-friendly sources of energy, it too has the capability to alter or damage its surroundings. Among the main problems that have been demonstrated by hydroelectric power is significant change in water quality. Because of the nature of hydroelectric systems, the water often takes on a higher temperature, loses oxygen content, experiences siltation, and gains in phosphorus and nitrogen content. 

Man has utilized the power of water for years. Much of the growth of early colonial American industry can be attributed to hydropower. Because fuel such as coal and wood were not readily available to inland cities, American settlers were forced to turn to other alternatives. Falling water was ideal for powering sawmills and grist mills.

As coal became a better-developed source of fuel, however, the importance of hydropower decreased. When canals began to be built off of the Mississippi River, inland cities became linked to mainstream commerce. This opened the flow of coal to most areas of America, dealing the final blow to hydropower in early America.

Water power really didn't stage a major comeback until the 20th century. The development of an electric generator helped increase hydropower's importance. In the mid-20th century, as Americans began to move out of the cities and into "suburbia," the demand for electricity increased, as did the role of hydroelectricity. Hydroelectric power plants were built near large cities to supplement power production.

Disadvantages

Smaller models depend on availability of fast flowing streams or rivers 
Run-of-the-River plants can impact the mobility of fish and other riverlife. NOTE: Building a fish ladder can lessen this negative aspect of hydroelectric power 
Hydropower had some help from the federal government in being established in America. The Tennessee Valley Authority (TVA) was implemented during the Great Depression of the 1930s. The program was designed to correct several problems in the Tennessee River Basin. These problems included frequent floods, erosion, and deforestation. The TVA provided for the building of several hydroelectric dams. Not only were the dams successful in controlling the flooding, they also provide electricity to the region. The TVA is an example of successful implementation of hydroelectric power.

There are many different ways in which the abundance of energy around us can be stored, converted, and amplified for our use. To help you understand the key energy sources that will play an important role in the world's future, we have designed a number of pages to familiarize you with some of the history, theory, economics, and problems of the various types of energy. 
The energy sources have been split into three categories: fossil fuels, renewable sources, and nuclear sources. The fossil fuels covered here are coal, petroleum, and natural gas. The renewable energy sources are solar, wind, hydroelectric, biomass, and geothermal power. The nuclear-powered sources are fission and fusion.

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