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The energy supplied by flowing water has been used by humans since shortly before the start of the Christian era: the [https://en.wikipedia.org/wiki/Water_wheel waterwheel], the [https://en.wikipedia.org/wiki/Watermill geared water mill], and the [https://en.wikipedia.org/wiki/Watermill Vitruvian mill] received increased use as civilization developed in the Mediterranean region.

The energy supplied by flowing water has been used by humans since shortly before the start of the Christian [[era]]: the [[Wikipedia:Water_wheel| waterwheel]], the [[Wikipedia:Watermill|geared water mill]], and the [[Wikipedia:Watermill#History|Vitruvian mill]] received increased use as civilization developed in the Mediterranean region.

[[file:Uglich hydroelectric power plant, 2010.jpg|thumb|400px|Uglich hydroelectric power plant, Russia. Courtesy [https://commons.wikimedia.org/wiki/File:Uglich_hydroelectric_power_plant,_2010.jpg Wikimedia Commons].]]

[[file:Uglich hydroelectric power plant, 2010.jpg|thumb|400px|Uglich hydroelectric power plant, Russia. Courtesy [https://commons.wikimedia.org/wiki/File:Uglich_hydroelectric_power_plant,_2010.jpg Wikimedia Commons].]]

During the [http://www.history.com/topics/middle-ages Middle Ages], the use of the geared water mill extended throughout Western Europe, where it was used to grind grain, to saw wood and marble, and to crush metallic ores. Water mills became the base for the beginning of industrial development. Flowing water as a source of energy extended to many other regions of the world in subsequent years.

During the [http://www.history.com/topics/middle-ages Middle Ages], the use of the geared water mill extended throughout Western Europe, where it was used to grind grain, to saw wood and marble, and to crush metallic ores. Water mills became the base for the beginning of industrial development. Flowing water as a source of energy extended to many other regions of the world in subsequent years.

[[file:St54Figure46.JPG|thumb|400px|{{figure number|1}}World hydroelectric power generation and percent of the world's total electricity generation.<ref name+Salvador_2005>Salvador, Amos, Energy-A Historical Perspective and 21st Century Forecast: AAPG Studies in Geology 54, 208 p.</ref>]]

[[file:St54Figure46.JPG|thumb|400px|{{figure number|1}}World hydroelectric power generation and percent of the world's total electricity generation.<ref name+Salvador_2005>Salvador, A., Energy-A Historical Perspective and 21st Century Forecast: AAPG Studies in Geology 54, 208 p.</ref>]]

However, it was not until the flow of water was used in the generation of electricity that water started to make a major contribution to the world's supply of energy. Hydroelectric power is, as of 2005, the second largest source of electricity in the world (18%) after [[coal]], which supplies about 38%, but ahead of [[nuclear power]], [[natural gas]], and [[oil]], which contribute 17, 16, and 10%, respectively, of the total generated electricity.

However, it was not until the flow of water was used in the generation of electricity that water started to make a major contribution to the world's supply of energy. Hydroelectric power is, as of 2005, the second largest source of electricity in the world (18%) after [[coal]], which supplies about 38%, but ahead of [[nuclear power]], [[natural gas]], and [[oil as an energy source|oil]], which contribute 17, 16, and 10%, respectively, of the total generated electricity.

The world's first hydroelectric power plant was built at Godalming, in England, in 1881. Hydroelectric power developed rapidly during the 20th century, even in countries with other abundant sources of energy.

The world's first hydroelectric power plant was built at Godalming, in England, in 1881. Hydroelectric power developed rapidly during the 20th century, even in countries with other abundant sources of energy.

Hydroelectric power was at first believed to be an attractive source of energy, potentially widely distributed around the world, environmentally clean, inexhaustible, efficient, and economically competitive, especially attractive to countries with limited [[fossil fuel]] resources. It has been found that this was not always the case.

Hydroelectric power was at first believed to be an attractive source of energy, potentially widely distributed around the world, environmentally clean, inexhaustible, efficient, and economically competitive, especially attractive to countries with limited [https://www.udemy.com/blog/types-of-fossil-fuels/ fossil fuel] resources. It has been found that this was not always the case.

Although dams, not only those built for the generation of electricity but also those designed for flood control, irrigation, water supply, or a combination of these purposes, have delivered significant benefits, they have often been the cause of disastrous social upheavals and major negative environmental impacts, and their construction has serious geographic limitations.

Although dams, not only those built for the generation of electricity but also those designed for flood control, irrigation, water supply, or a combination of these purposes, have delivered significant benefits, they have often been the cause of disastrous social upheavals and major negative environmental impacts, and their construction has serious geographic limitations.

* Dams alter and divert river flow affecting access to water, transforming landscapes, and resulting in significant irreversible impact on the environment both upstream and downstream from the dams. They often have a negative impact on rivers, watersheds, and aquatic ecosystems.

* Dams alter and divert river flow affecting access to water, transforming landscapes, and resulting in significant irreversible impact on the environment both upstream and downstream from the dams. They often have a negative impact on rivers, watersheds, and aquatic ecosystems.

* Evaporation of the water in the reservoirs upstream from the dams becomes a problem in some cases, particularly in arid regions where fresh water is a priceless commodity, needed for agriculture and human consumption.

* Evaporation of the water in the reservoirs upstream from the dams becomes a problem in some cases, particularly in arid regions where fresh water is a priceless commodity, needed for agriculture and human consumption.

* Hydroelectric plants are not necessarily inexhaustible sources of electricity. They have a limited operating life. The reservoirs behind the dams become silted up in many cases with the consequent long-term loss of water storage capacity. The sediment caught at the dam can sometimes cause the erosion of the turbines if it reaches the power intakes. The retention of sediment behind the dam can also be the cause of serious problems downstream, where the reduced influx of sediment often results in important changes of the river valley, sometimes all the way to its delta. The Aswan High Dam in Egypt and the Hoover Dam in southwestern United States are good examples of such problems. In Egypt, for thousands of years, farmers along the Nile Valley counted on the yearly floods to bring organically rich silt that fertilized their intensely cultivated plots. Now, the silt piles up behind the Aswan Dam.

* Hydroelectric plants are not necessarily inexhaustible sources of electricity. They have a limited operating life. The reservoirs behind the dams become silted up in many cases with the consequent long-term loss of water storage capacity. The sediment caught at the dam can sometimes cause the erosion of the turbines if it reaches the power intakes. The retention of sediment behind the dam can also be the cause of serious problems downstream, where the reduced influx of sediment often results in important changes of the river valley, sometimes all the way to its [[Deltaic environments|delta]]. The Aswan High Dam in Egypt and the Hoover Dam in southwestern United States are good examples of such problems. In Egypt, for thousands of years, farmers along the Nile Valley counted on the yearly floods to bring organically rich silt that fertilized their intensely cultivated plots. Now, the silt piles up behind the Aswan Dam.

* Finally, the development of large-scale hydroelectric power requires sites that allow the building of dams that provide a high head (height of water fall), located in a river with high water flow, with large water-storage capacity, and, preferably, near centers of electricity consumption. These dams are sometimes called ldquolarge damsrdquo and are defined as having a height of 15 m (50 ft) or more from the foundation or, if only 5–15 m (16–50 ft) in height, having a reservoir volume of more than 3 million m³ (177 million ft³). Sites favorable for the building of large dams are limited in number and unevenly distributed throughout the world. They are not found, for instance, in arid regions with low rainfall and, therefore, with no permanent streams, or in regions of low relief. Although most of the world's sites for the building of hydroelectric power plants have been identified, only a relatively small proportion have been developed, mostly in the industrialized countries. At least seven European countries (Austria, France, Italy, Norway, Spain, Sweden, and Switzerland) generate substantial percentages of their electricity in hydroelectric plants.

* Finally, the development of large-scale hydroelectric power requires sites that allow the building of dams that provide a high head (height of water fall), located in a river with high water flow, with large water-storage capacity, and, preferably, near centers of electricity consumption. These dams are sometimes called "large dams" and are defined as having a height of 15 m (50 ft) or more from the foundation or, if only 5–15 m (16–50 ft) in height, having a reservoir volume of more than 3 million m³ (177 million ft³). Sites favorable for the building of large dams are limited in number and unevenly distributed throughout the world. They are not found, for instance, in arid regions with low rainfall and, therefore, with no permanent streams, or in regions of low relief. Although most of the world's sites for the building of hydroelectric power plants have been identified, only a relatively small proportion have been developed, mostly in the industrialized countries. At least seven European countries (Austria, France, Italy, Norway, Spain, Sweden, and Switzerland) generate substantial percentages of their electricity in hydroelectric plants.

The building of large dams requires large initial capital investment that is not always rewarded with favorable economic profitability. The reported returns on the investments made in hydroelectric power plants have increasingly been questioned.

The building of large dams requires large initial capital investment that is not always rewarded with favorable economic profitability. The reported returns on the investments made in hydroelectric power plants have increasingly been questioned.

Nevertheless, hydroelectric power is still important in some countries, especially in those that are short of other sources of electricity but have large rivers originating in mountainous regions.  

Nevertheless, hydroelectric power is still important in some countries, especially in those that are short of other sources of electricity but have large rivers originating in mountainous regions.  

(For a discerning although somewhat supportive review of the pros and cons of hydropower, see the World Energy Council issue of the ''Survey of Energy Resources'' [http://www.worldenergy.org/publications/2001/world-energy-resources-2001/ 2001].)

World Energy Council Survey of Energy Resources, [http://www.worldenergy.org/publications/2001/world-energy-resources-2001/ 2001]. ''Discerning although somewhat supportive review of the pros and cons of hydropower.''

==See also==

==See also==