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Hydroelectricity is electricity generated by hydropower, i.e., the production of power through use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide ( ) than fossil fuel powered energy plants. Worldwide, an installed capacity of 777 GWe supplied 2998 TWh of hydroelectricity in 2006. This was approximately 20% of the world's electricity, and accounted for about 88% of electricity from renewable sources.

Electricity generation

Hydroelectric dam in cross section

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. In this case the energy extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is proportional to the head. To obtain very high head, water for a hydraulic turbine may be run through a large pipe called a penstock.

Pumped storage hydroelectricity produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. Pumped storage schemes currently provide the only commercially important means of large-scale grid energy storage and improve the daily capacity factor of the generation system. Hydroelectric plants with no reservoir capacity are called run-of-the-river plants, since it is not then possible to store water. A tidal power plant makes use of the daily rise and fall of water due to tides; such sources are highly predictable, and if conditions permit construction of reservoirs, can also be dispatchable to generate power during high demand periods.

Less common types of hydro schemes use water's kinetic energy or undammed sources such as undershot waterwheels.

Calculating the amount of available power

A simple formula for approximating electric power production at a hydroelectric plant is: P = \rho hrgk ,where P is Power in watts, \rho is the density of water (~1000 kg/m3), h is height in meters, r is flow rate in cubic meters per second, g is acceleration due to gravity of 9.8 m/s2, and k is a coefficient of efficiency ranging from 0 to 1. Efficiency is often higher (that is, closer to 1) with larger and more modern turbines.

Annual electric energy production depends on the available water supply. In some installations the water flow rate can vary by a factor of 10:1 over the course of a year.

Industrial hydroelectric plants

While many hydroelectric projects supply public electricity networks, some are created to serve specific industrial enterprises. Dedicated hydroelectric projects are often built to provide the substantial amounts of electricity needed for aluminium electrolytic plants, for example. In the Scottish Highlands of United Kingdommarker, there are examples at Kinlochlevenmarker and Lochaber, constructed during the early years of the 20th century. The Grand Coulee Dammarker, long the world's largest, switched to support Alcoa aluminum in Bellingham, Washingtonmarker, United Statesmarker for American World War II airplanes before it was allowed to provide irrigation and power to citizens (in addition to aluminum power) after the war. In Surinamemarker, the Brokopondo Reservoirmarker was constructed to provide electricity for the Alcoa aluminium industry. New Zealand'smarker Manapouri Power Stationmarker was constructed to supply electricity to the aluminium smelter at Tiwai Point. As of 2007 the Kárahnjúkar Hydropower Projectmarker in Icelandmarker remains controversial.

Small-scale hydro-electric plants

Although large hydroelectric installations generate most of the world's hydroelectricity, some situations require small hydro plants. These are defined as plants producing up to 10 megawatts, or projects up to 30 megawatts in North America. A small hydro plant may be connected to a distribution grid or may provide power only to an isolated community or a single home. Small hydro projects generally do not require the protracted economic, engineering and environmental studies associated with large projects, and often can be completed much more quickly. A small hydro development may be installed along with a project for flood control, irrigation or other purposes, providing extra revenue for project costs. In areas that formerly used waterwheels for milling and other purposes, often the site can be redeveloped for electric power production, possibly eliminating the new environmental impact of any demolition operation. Small hydro can be further divided into mini-hydro, units around 1 MW in size, and micro hydro with units as large as 100 kW down to a couple of kW rating.

Small hydro schemes are particularly popular in Chinamarker, which has over 50% of world small hydro capacity.

Small hydro units in the range 1 MW to about 30 MW are often available from multiple manufacturers using standardized "water to wire" packages; a single contractor can provide all the major mechanical and electrical equipment (turbine, generator, controls, switchgear), selecting from several standard designs to fit the site conditions. Micro hydro projects use a diverse range of equipment; in the smaller sizes industrial centrifugal pumps can be used as turbines, with comparatively low purchase cost compared to purpose-built turbines.



The major advantage of hydroelectricity is elimination of the cost of fuel. The cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed.

Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago. Operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation.

Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation. It has been calculated that the sale of electricity from the Three Gorges Dammarker will cover the construction costs after 5 to 8 years of full generation.

Greenhouse gas emissions

Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide (a greenhouse gas). While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation. One measurement of greenhouse gas related and other externality comparison between energy sources can be found in the ExternE project by the Paul Scherrer Institut and the University of Stuttgartmarker which was funded by the European Commissionmarker. According to this project, hydroelectricity produces the least amount of greenhouse gases and externality of any energy source. Coming in second place was wind, third was nuclear energy, and fourth was solar photovoltaic. The extremely positive greenhouse gas impact of hydroelectricity is found especially in temperate climates. The above study was for local energy in Europe; presumably similar conditions prevail in North America and Northern Asia, which all see a regular, natural freeze/thaw cycle (with associated seasonal plant decay and regrowth).

Related activities

Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.


Failure Hazard

Dam failures have been some of the largest man-made disasters in history.Also, good design and construction are not an adequate guarantee of safety.Dams are tempting industrial targets for wartime attack, sabotage and terrorism.

For example, the Banqiao Dammarker failure in Southern China resulted in the deaths of 171,000 people and left millions homeless.Also, the creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dammarker in Italy, where almost 2000 people died, in 1963.

Smaller dams and micro hydro facilities create less risk, but can form continuing hazardseven after they have been decommissioned.For example, the Kelly Barnes small hydroelectric dam failed in 1967, causing 39 deaths with the Toccoa Flood,ten years after its power plant was decommissioned in 1957.

Large Power Outages caused by dam failures

Large dams, whilst generally reliable can suffer castrohpic failure to the dam itself, or the connections and substations, leading to extremely large and sudden loss of output, which can plunge an entire network off line, for hours or even months depending on the damage. Hence whilst these are regarded as "firm" or "despatchable" sources, in reality duplication or back up has to be provided. Examples are:

These are very large losses of power; for comparison, the average UK power demand is around 37 GW.

Limited Service Life

Almost all rivers convey silt. Dams on those rivers will retain silt in their catchments, because by slowing the water, and reducing turbulence, the silt will fall to the bottom.Siltation reduces a dam's water storage so that water from a wet season cannot be stored for use in a dry season.Often at or slightly after that point, the dam becomes uneconomic.Near the end of the siltation, the basins of dams fill to the top of the lowest spillway,and even storage from a storm to the end of dry weather will fail.Some especially poor dams can fail from siltation in as little as 20 years.

Larger dams are not immune. For example, the Three Gorges Dam in China has an estimated life that may be as short as 70 years.

Dams' useful lives can be extended with sediment bypassing, special weirs,and forestation projects to reduce a watershed's silt production,but at some point most dams become uneconomic to operate.

Environmental damage

Hydroelectric projects can be disruptive to surrounding aquatic ecosystems both upstream and downstream of the plant site. For instance, studies have shown that dams along the Atlanticmarker and Pacificmarker coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed. Salmon spawn are also harmed on their migration to sea when they must pass through turbines. This has led to some areas transporting smolt downstream by barge during parts of the year. In some cases dams have been demolished (for example the Marmot Dammarker demolished in 2007) because of impact on fish. Turbine and power-plant designs that are easier on aquatic life are an active area of research. Mitigation measures such as fish ladders may be required at new projects or as a condition of re-licensing of existing projects.

Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Since turbine gates are often opened intermittently, rapid or even daily fluctuations in river flow are observed. For example, in the Grand Canyonmarker, the daily cyclic flow variation caused by Glen Canyon Dammarker was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from pre-construction conditions. Depending on the location, water exiting from turbines is typically much warmer than the pre-dam water, which can change aquatic faunal populations, including endangered species, and prevent natural freezing processes from occurring. Some hydroelectric projects also use canals to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. Examples include the Tekapomarker and Pukaki Riversmarker in New Zealandmarker.

Greenhouse gas emissions

Lower positive impacts are found in the tropical regions, as it has been noted that the reservoirs of power plants in tropical regions may produce substantial amounts of methane and carbon dioxide. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant. Although these emissions represent carbon already in the biosphere, not fossil deposits that had been sequestered from the carbon cycle, there is a greater amount of methane due to anaerobic decay, causing greater damage than would otherwise have occurred had the forest decayed naturally.

In boreal reservoirs of Canada and Northern Europe, however, greenhouse gas emissions are typically only 2% to 8% of any kind of conventional fossil-fuel thermal generation. A new class of underwater logging operation that targets drowned forests can mitigate the effect of forest decay.

In 2007, International Rivers accused hydropower firms for cheating with fake carbon credits under the Clean Development Mechanism (CDM), for hydropower projects already finished or under construction at the moment they applied to join the CDM. These carbon credits – of hydropower projects under the CDM in developing countries – can be sold to companies and governments in rich countries, in order to comply with the Kyoto protocol.

Population relocation

Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost. Such problems have arisen at the Three Gorges Dam project in China, the Clyde Dammarker in New Zealand and the Ilısu Dammarker in Southeastern Turkey.

Affected by flow shortage

Changes in the amount of river flow will correlate with the amount of energy produced by a dam. Because of global warming, the volume of glaciers has decreased, such as the North Cascades glaciers, which have lost a third of their volume since 1950, resulting in stream flows that have decreased by as much as 34%. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power.

Comparison with other methods of power generation

Hydroelectricity eliminates the flue gas emissions from fossil fuel combustion, including pollutants such as sulfur dioxide, nitric oxide, carbon monoxide, dust, and mercury in the coal. Hydroelectricity also avoids the hazards of coal mining and the indirect health effects of coal emissions.Compared to nuclear power, hydroelectricity generates no nuclear waste, has none of the dangers associated with uranium mining, nor nuclear leaks. Unlike uranium, hydroelectricity is also a renewable energy source.

Compared to wind farms, hydroelectricity power plants have a more predictable load factor. If the project has a storage reservoir, it can be dispatched to generate power when needed. Hydroelectric plants can be easily regulated to follow variations in power demand.

Unlike fossil-fueled combustion turbines, construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment. Hydrological data up to 50 years or more is usually required to determine the best sites and operating regimes for a large hydroelectric plant. Unlike plants operated by fuel, such as fossil or nuclear energy, the number of sites that can be economically developed for hydroelectric production is limited; in many areas the most cost effective sites have already been exploited. New hydro sites tend to be far from population centers and require extensive transmission lines. Hydroelectric generation depends on rainfall in the watershed, and may be significantly reduced in years of low rainfall or snowmelt. Long-term energy yield may be affected by climate change. Utilities that primarily use hydroelectric power may spend additional capital to build extra capacity to ensure sufficient power is available in low water years.

In parts of Canada (the provinces of British Columbiamarker, Manitobamarker, Ontariomarker, Quebecmarker, Newfoundland and Labradormarker) hydroelectricity is used so extensively that the word "hydro" is often used to refer to any electricity delivered by a power utility. The government-run power utilities in these provinces are called BC Hydro, Manitoba Hydro, Hydro One (formerly "Ontario Hydro"), Hydro-Québec and Newfoundland and Labrador Hydro respectively. Hydro-Québec is the world's largest hydroelectric generating company, with a total installed capacity (2007) of 35,647 MW, including 33,305 MW of hydroelectric generation.

Countries with the most hydro-electric capacity

The ranking of hydro-electric capacity is either by actual annual energy production or by installed capacity power rating. A hydro-electric plant rarely operates at its full power rating over a full year; the ratio between annual average power and installed capacity rating is the capacity factor. The installed capacity is the sum of all generator nameplate power ratings. Sources came from BP Statistical Review - Full Report 2009

The top six dams, in descending order of their annual electricity generation, are: the Three Gorges Dammarker in Chinamarker, the Itaipu Dammarker on the border of Paraguaymarker and Brazilmarker, the Guri Dam in Venezuelamarker, the Tucurui dammarker in Brazilmarker, the Sayano-Shushenskaya Dam in Russiamarker and the Krasnoyarsk hydroelectric dammarker, also in Russiamarker (see List of the largest hydroelectric power stations).

Brazil, Canada, Norway, Switzerland and Venezuela are the only countries in the world where the majority of the internal electric energy production is from hydroelectric power, while Paraguaymarker not only produces 100% its electricity from hydroelectric dams, but exports 90% of its production to Brazil and to the Argentine. Norwaymarker produces 98–99% of its electricity from hydroelectric sources.

Country Annual Hydroelectric
Energy Production(TWh)
Capacity (GW)
Percent of
all electricity
(2008) 585.2 171.52 0.37 17.18
369.5 88.974 0.59 61.12
363.8 69.080 0.56 85.56

250.6 79.511 0.42 5.74
167.0 45.000 0.42 17.64
140.5 27.528 0.49 98.25
115.6 33.600 0.43 15.80
86.8 - - 67.17
69.2 27.229 0.37 7.21
65.5 16.209 0.46 44.34
(2006) 64.0 - -
63.4 25.335 0.25 11.23

Old hydro-electric power stations


  • A small hydroelectric station, generating 650 kW, opened at Waratah, Tasmaniamarker in 1885.
  • Duck Reachmarker, Launcestonmarker, Tasmaniamarker. Completed 1895. The first publicly owned hydro-electric plant in the Southern Hemisphere. Supplied power to the city of Launceston for street lighting.
  • The Snowy Mountains Scheme has turbines all along the tunnel so it, in some perspectives it is also another hydroelectric station. However, it only operates during peak hours of the day and mostly during the evening and early night. It supplies electricity to all over the regions of New South Walesmarker.


  • Decew Falls 1, St. Catharinesmarker, Ontariomarker completed 25 August 1898. Owned by Ontario Power Generation. Four units are still operational. Recognized as an IEEE Milestone in Electrical Engineering & Computing by the IEEE Executive Committee in 2002.

  • The oldest continuously-operated hydroelectric generator in Canada is located in St. Stephen, New Brunswickmarker. Part of the construction of the Milltown Cotton Mill, this rope-driven generator originally powered the electric lights for the mill when it opened in 1882, and in 1888 started providing power to homes in the town. NB Power now owns and operates this as part of the Milltown Dam hydroelectric station.


  • Chivilingo was the first hydroelectric plant in Chilemarker and the second in South America. With first power produced in 1897, it has two Pelton wheel turbines each turning a 215 kW generator. It was installed to provide power to mines and the city of Lota, Chilemarker.

United States

  • Appletonmarker, Wisconsinmarker completed 1882, A waterwheel on the Fox river supplied the first commercial hydroelectric power for lighting to two paper mills and a house, two years after Thomas Edison demonstrated incandescent lighting to the public. Within a matter of weeks of this installation, a power plant was also put into commercial service at Minneapolismarker.

  • Niagara Fallsmarker, New York. Operation began locally in 1895 and power was transmitted to Buffalo, New York, in 1896.

  • Claverack Creek, in Stottville, New Yorkmarker, believed to be the oldest hydro power site in the United States. The turbine, a Morgan Smith, was constructed in 1869 and installed 2 years later. It is one of the earliest water wheel installations in the United States to generate electricity. It is owned today by Edison Hydro.

  • The oldest continuously-operated commercial hydroelectric plant in the United States is built on the Hudson River at Mechanicville, New Yorkmarker. The seven 750 kW units at this station initially supplied power at a frequency of 38 Hz, but later were increased in speed to 40 Hz. It went into commercial service 22 July 1898. It is now being restored to its original condition and remains in commercial operation.

Major schemes under construction

Only projects with generating capacity greater than or equal to 2,000 MW are listed.
Name Maximum Capacity Country Construction started Scheduled completion Comments
Xiluodu Dammarker 12,600 MW Chinamarker December 26, 2005 2015 Construction once stopped due to lack of environmental impact study.
Siang Upper HE Project 11,000 MW Indiamarker April, 2009 2024 Multi-phase construction over a period of 15 years. Construction was delayed due to dispute with China.
Xiangjiaba Dammarker 6,400 MW Chinamarker November 26, 2006 2015
Longtan Dammarker 6,300 MW Chinamarker July 1, 2001 December 2009
Nuozhadu Dam 5,850 MW Chinamarker 2006 2017
Jinping 2 Hydropower Station 4,800 MW Chinamarker January 30, 2007 2014 To build this dam, 23 families and 129 local residents need to be moved. It works with Jinping 1 Hydropower Stationmarker as a group.
Laxiwa Dammarker 4,200 MW Chinamarker April 18, 2006 2010
Xiaowan Dammarker 4,200 MW Chinamarker January 1, 2002 December 2012
Jinping 1 Hydropower Stationmarker 3,600 MW Chinamarker November 11, 2005 2014
Pubugou Dam 3,300 MW Chinamarker March 30, 2004 2010
Goupitan Dammarker 3,000 MW Chinamarker November 8, 2003 2011
Guanyinyan Dam 3,000 MW Chinamarker 2008 2015 Construction of the roads and spillway started.
Lianghekou Dam 3,000 MW Chinamarker 2009 2015
Boguchan Dam 3,000 MW Russiamarker 1980 2012
Chapetón 3,000 MW Argentinamarker
Dagangshan 2,600 MW Chinamarker August 15, 2008 2014
Jinanqiao Dam 2,400 MW Chinamarker December 2006 2010
Guandi Dam 2,400 MW Chinamarker Novermber 11 2007 2012
Liyuan Dam 2,400 MW Chinamarker 2008
Tocoma Dam Bolívar Statemarker 2,160 MW Venezuelamarker 2004 2014 This new power plant would be the last development in the Low Caroni Basin, bringing the total to six power plants on the same river, including the 10,000MW Guri Dam.
Ludila Dam 2,100 MW Chinamarker 2007 2015 Construction halt due to lack of the evnironmental assessment.
Bureya Dammarker 2,010 MW Russiamarker 1978 2009
Shuangjiangkou Dam 2,000 MW Chinamarker December, 2007 The dam will be 314 m high.
Ahai Dam 2,000 MW Chinamarker July 27, 2006
Subansiri Lower Dam 2,000 MW Indiamarker 2005 2012

Proposed major hydroelectric projects

Only projects with generating capacity greater than or equal to 2,000 MW are listed.

Name Maximum Capacity Country Construction starts Scheduled completion Comments
Red Sea dam 50,000 MW Africa/Middle East Unknown Unknown Still in planning, would be largest dam in the world
Grand Ingamarker 40,000 MW Democratic Republic of the Congomarker 2010 Unknown
Baihetan Dam 13,050 MW Chinamarker 2009 2015 Still in planning
Wudongde Dam 7,500 MW Chinamarker 2009 2015 Still in planning
Rampart Dammarker 4,500 MW United Statesmarker Canceled
Maji Dam 4,200 MW Chinamarker 2008 2013
Songta Dam 4,200 MW Chinamarker 2008 2013
Liangjiaren Dam 4,000 MW Chinamarker 2009 2015 Still in planning
Jirau Dam 3,300 MW Brazilmarker 2007 2012
Pati Dam 3,300 MW Argentinamarker
Santo Antônio Dam 3,150 MW Brazilmarker 2007 2012
Dibang 3,000 MW Indiamarker
Lower Churchill 2,800 MW Canadamarker 2009 2014
HidroAysén 2,750 MW Chilemarker 2020
Lenggu Dam 2,718 MW Chinamarker 2015
Changheba Dam 2,200 MW Chinamarker 2009 2015
Subansiri Upper HE Project 2,500 MW Indiamarker 2012 Unknown
Banduo 1 Dam 2,000 MW Chinamarker 2009


United States

In the United States, a study is required before constructing a hydroelectric project. In 2008, a study could cost up to $50,000 for a run of a stream. Both federal and state licenses were required. A license typically cost between $150,000 and $1 million. A project earns money from the sale of energy, the sale of capacity, and the sale of renewable energy credits.

See also






  1. Energy Information Administration international statistics database
  2. Renewables Global Status Report 2006 Update, REN21, published 2007, accessed 2007-05-16; see Table 4, p. 20.
  3. Summer of International dissent against Heavy Industry, Saving Iceland, published 2007, accessed 2007-05-17
  4. Hydropower – A Way of Becoming Independent of Fossil Energy?
  5. Beyond Three Gorges in China
  6. Stay Clear, Stay Safe, Ontario Power Generation
  7. See the references for the articles in the list of Dam failures. Duplicating them here is wasteful.
  8. Toccoa Flood USGS Historical Site, retrieved 02sep2009
  9. Extreme Reservoir Siltation: A Case Study Retrieved 02sep2009
  10. Three Gorges Project. Part IV: Siltation of TGP Retrieved 02sep2009
  11. Siltation in Small Dams Retrieved 02sep2009
  12. Sedimentation Problems with Dams
  13. WCD Findal Report, Retrieved 7/10/2009
  14. Hydroelectric power's dirty secret revealed
  15. Inhabitat » “Rediscovered” Wood & The Triton Sawfish
  16. Briefing of World Commission on Dams
  17. .
  19. Hydro-Québec. 2007 Annual Report. Montreal, April 2008.
  20. Consumption TWh'!A1
  21. [1]
  22. Carl Sulzberger, The Chivilingo Plant- Early Hydropower in Chile, in IEEE Power & Energy, Volume 6, No. 4 July/August 2008, ISSN 1540-7977, pg. 60
  23. The Historic Mechanicville Hydroelectric Station Part 1: The Early Days, IEEE Industry Applications Magazine, Jan/Feb. 2007


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