Karun-3 dam in Iran.

A dam is a barrier across flowing water that obstructs, directs, or slows the flow, often creating a reservoir, lake, or impoundment. In Australian and South African English, the word dam may refer to the reservoir as well as the barrier. Most dams have a section called a spillway or weir that allows water to flow out, either intermittently or continuously.


A dam may serve one or more purposes, such as to provide water for neighboring towns, farms, and industries, to produce hydroelectric power, to improve navigation, to control flooding, and to maintain wildlife habitats. On the downside, a dam may adversely affect the area's ecosystem and destabilize geological formations. The construction of a large dam may force the relocation of many local inhabitants, and structural failure of a dam can have catastrophic effects. The building and maintenance of a dam needs to take these factors into consideration.

A cross-section of a hydroelectric dam.

Etymology and history

The word dam can be traced back to Middle English,[1] and before that, from Middle Dutch, as seen in the names of many old cities in the Netherlands.[2]

Most of the first Dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, for Mesopotamia's weather affected the Tigris and Euphrates rivers and could be quite unpredictable. The earliest recorded dam is believed to have been on the Sadd Al-Kafara at Wadi Al-Garawi, which is located about 25 kilometers south of Cairo, and built around 2600 B.C.E.[3] It was destroyed by heavy rain shortly afterwards.[3]

The oldest standing dam in the world is believed to be the Grand Anicut, also known as the Kallanai, built on the Kaveri River (or Cauvery River) in the state of Tamil Nadu in southern India. It was built by the Chola king Karikalan, and dates back to the second century C.E.[4] Du Jiang Yan in China is the oldest surviving irrigation system that includes a dam to direct water flow. It was completed in 251 B.C.E.

The Kallanai is a massive dam of unhewn stone, over 300 meters long, 4.5 meters high, and 20 meters wide,[4] across the main stream of the Cauvery. The purpose of the dam was to divert the waters of the Cauvery across the fertile delta region for irrigation via canals. The dam is still in good condition and has served as a model for later engineers, including the Sir Arthur Cotton's nineteenth-century dam across the Kollidam, the major tributary of the Cauvery. The land area irrigated by the ancient irrigation network, of which the dam was the centerpiece, was 69,000 acres (280 square kilometers). By the early twentieth century, the irrigated area had been increased to about 1,000,000 acres (4,000 square kilometers).

Sunshu Ao, Prime Minister of the state of Chu of ancient China, was China's first known hydraulic engineer. He served Duke Zhuang of Chu during the reign of King Ding of Zhou (606 B.C.E.-586 B.C.E.), ruler of the Eastern Zhou Dynasty. His large earthen dam flooded a valley in modern-day northern Anhui province to create an enormous irrigation reservoir (62 miles in circumference), which still exists today.[5]

In the Netherlands, a low-lying country, dams were often applied to block rivers to regulate the water level and to prevent the sea from entering the marshlands. Those dams often marked the beginning of a town or city because it was easy to cross the river such locations, often giving rise to the respective place's names in Dutch. For instance, the Dutch capital Amsterdam (old name Amstelredam) started with a dam through the river Amstel in the late twelfth century, and Rotterdam started with a dam through the river Rotte, a minor tributary of the Nieuwe Maas. The central square of Amsterdam, believed to be the original place of the 800 year old dam, still carries the name Dam Square.

Types of dams

Dams can be formed by human agency or natural causes, including the intervention of wildlife such as beavers. Man-made dams are typically classified according to their size (height), intended purpose, or structure.

Classification by size

International standards define large dams as those higher than 15 meters and major dams as those over 150 meters in height.[6]

Classification by purpose

A dam may be constructed for one or more purposes, such as:

  • to provide water for irrigation;
  • to provide water supply for a town or city;
  • to improve navigation;
  • to create a reservoir of water for industrial uses;
  • to generate hydroelectric power;
  • to create recreational areas;
  • to maintain a habitat for fish and wildlife;
  • to control floods; and
  • to contain effluents from sites such as mines or factories.

Few dams serve all these purposes, but some serve more than one purpose.


An overflow dam is designed for water to flow over its top. A weir is a type of small overflow dam that can be used for flow measurement.




Classification by structure


Masonry dams

Arch dams

See also: Geotechnical engineering





Gravity dams




Embankment dams



Rock-fill dams


Earth-fill dams



Asphalt-concrete core

A third type of embankment dam is built with asphalt concrete core. The majority of such dams are built with rock and or gravel as main fill material. Almost 100 dams of this design have now been built world- wide since the first dam was completed in 1962. All dams built have an excellent performance record. This type of asphalt is a viscoelastic, plastic material that can adjust to the movements and deformations imposed on the embankment as a whole and to settlements in the foundation. The flexible properties of the asphalt make such dams especially suited in earthquake regions.




Timber dams

A timber crib dam in Michigan, photographed in 1978.





Steel dams

Red Ridge steel dam, b. 1905, Michigan

A steel dam is a type of dam briefly experimented with in around the turn of the nineteenth-twentieth century which uses steel plating (at an angle) and load bearing beams as the structure. Intended as permanent structures, steel dams were an (arguably failed) experiment to determine if a construction technique could be devised that was cheaper than masonry, concrete or earthworks, but sturdier than timber crib dams.

Beaver dams

Main article: Beaver#Dams


Construction elements

Power generation plant

Hydraulic turbine and electrical generator.
Main article: Hydroelectricity

As of 2005, hydroelectric power, mostly from dams, supplies some 19 percent of the world's electricity, and over 63 percent of renewable energy.[8] Much of this is generated by large dams, although China uses small-scale hydro generation in many locations and is responsible for about 50 percent of world use of this type of power.[8]



Spillway on Llyn Brianne dam, Wales soon after first fill




Dam creation

Common purposes

Function Example
Power generation Hydroelectric power is a major source of electricity in the world. many countries have rivers with adequate water flow, that can be dammed for power generation purposes. For example, the Itaipu on the Paraná River in South America generates 14 GW and supplied 93 percent of the energy consumed by Paraguay and 20 percent of that consumed by Brazil as of 2005.
Stabilize water flow / irrigation Dams are often used to control and stabilize water flow, often for agricultural purposes and irrigation.[9] Others such as the Berg Strait dam can help to stabilize or restore the water levels of inland lakes and seas, in this case the Aral Sea.[10]
Land reclamation Dams (often called dikes or levees in this context) are used to prevent ingress of water to an area that would otherwise be submerged, allowing its reclamation for human use.
Water diversion A diversion dam diverts all or part of a river flow from its natural course into an artificial course or canal. The redirected flow may be used for irrigation, passed through hydroelectric generators, channeled into a different river, or dammed to form a reservoir.

Siting (location)



  • permeability of the surrounding rock or soil
  • earthquake faults
  • landslides and slope stability
  • peak flood flows
  • reservoir silting
  • impacts on human habitations
  • compensation for land being flooded as well as population resettlement
  • removal of toxic materials and buildings from the proposed reservoir area

Impact assessment

The impact of a dam is assessed by several criteria:

  • the benefits to human society arising from the dam, such as for agriculture, water supply, damage prevention, and electric power;
  • the harm or benefits to nature and wildlife, especially fish and rare species;
  • the effect on the area's geology, whether changes in water flow and levels will increase or decrease geological stability; and
  • the disruption of human lives, such as by relocation of peoples and the loss of archaeological and cultural artifacts underwater.



Once completed, a well-designed, well-maintained hydroelectric power plant is a comparatively cheap and reliable source of electricity. It is a renewable energy source that can be readily regulated to store water as needed and generate high power levels on demand.

Impacts of dams

Environmental impact

Dams affect many ecological aspects of a river. For example, a dam slows a river and affects the ecological pattern established by the river through its rate of flow. Also, rivers tend to have fairly homogeneous temperatures, but reservoirs have layered temperatures: warm on the top and cold on the bottom. In addition, because it is water from the colder (lower) layer of the reservoir that is often released downstream, it may have a different dissolved oxygen content than regular river water. Organisms depending upon a regular cycle of temperatures may be unable to adapt; the balance of other fauna (especially plant life and microscopic fauna) may be affected by the change of oxygen content.

Older dams often lack a fish ladder, preventing many fish from moving upstream to their natural breeding grounds. This leads to failure of breeding cycles and blocks migration paths.[12] Even with the presence of a fish ladder, there may be a reduction in the number of fish reaching their upstream spawning grounds. In some areas, young fish ("smolt") are transported downstream by barge during parts of the year. Researchers are actively working on turbine and power-plant designs that could have lower impacts on aquatic life.

A large dam can cause the loss of entire ecospheres, including endangered and undiscovered species in the area, and replacement of the original environment by a new inland lake.

Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. For example, the daily cyclic flow variation caused by the Glen Canyon Dam contributed to sand bar erosion.

Depending upon the circumstances, a dam may either increase or decrease the net production of greenhouse gases. An increase can occur if the reservoir created by the dam itself acts as a source of substantial amounts of potent greenhouse gases (such as methane and carbon dioxide), by the decay of plant material in flooded areas in an anaerobic environment. According to a report by the World Commission on Dams, when a relatively large reservoir is built with no prior clearing of forest in the flooded area, greenhouse gas emissions from the reservoir could be higher than those of a conventional, oil-fired power plant.[13] On the other hand, a decrease of greenhouse gas emissions can occur if the dam is used in place of traditional power generation, because electricity produced from hydroelectric generation does not give rise to any flue gas emissions from fossil fuel combustion (including sulfur dioxide, nitric oxide, carbon monoxide, dust, and mercury from coal).

Social impacts

The impacts of a dam on human society are also significant. For example, the Three Gorges Dam on the Yangtze River in China, is more than five times the size of the Hoover Dam (USA) and will create a reservoir 600 km long, to be used for hydro-power generation. Its construction led to the loss of over a million people's homes and their mass relocation, the loss of many valuable archaeological and cultural sites, as well as significant ecological changes.[14]

Dam Failure

The reservoir emptying through the failed Teton Dam.
International special sign for works and installations containing dangerous forces.

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine monitoring of seepage from drains in and around larger dams is necessary to anticipate any problems and permit remedial action before structural failures occur. Most dams incorporate mechanisms to permit the reservoir level to be lowered or drained in the event of such problems. Another solution is rock grouting, that is, pressure-pumping portland cement slurry into weak, fractured rock.

During an armed conflict, a dam is considered an "installation containing dangerous forces," because destruction of the dam could have a massive impact on the civilian population and environment. As such, it is protected by the rules of International Humanitarian Law (IHL) and shall not be made the object of attack if that could result in severe losses among the civilian population. To facilitate the identification of a dam, a protective sign is displayed, consisting of three bright orange circles aligned along a single axis, as defined by the rules of IHL.

The main causes of dam failure include spillway design error (South Fork Dam), geological instability caused by changes to water levels during filling or poor surveying (Vajont Dam, Malpasset), poor maintenance, especially of outlet pipes (Lawn Lake Dam, Val di Stava Dam Collapse), extreme rainfall (Shakidor Dam), and human, computer, or design error (Buffalo Creek Flood, Dale Dike Reservoir, Taum Sauk pumped storage plant).

Prior to the above IHL ruling, a notable case of deliberate dam failure was the British Royal Air Force Dambusters raid on Germany during World War II (codenamed "Operation Chastise"). In that raid, three German dams were selected to be breached to have an impact on German infrastructure and manufacturing and power capabilities deriving from the Ruhr and Eder rivers. This raid later became the basis for several films.

See also


  1. Definition of "dam" bartleby.com. Retrieved October 7, 2007.
  2. Source: Tijdschrift voor Nederlandse Taal- en Letterkunde (Magazine for Dutch Language and Literature), 1947. The word dam was used by itself as far back as 1165, but one village, Obdam, was already mentioned in 1120. The word seems to be related to the Greek word taphos, meaning grave or grave hill. So the word should be understood as dike from dug out earth. The names of more than 40 places (with minor changes) from the Middle Dutch era (1150 - 1500 C.E.) such as Amsterdam (founded as 'Amstelredam' in the late twelfth century) and Rotterdam, also bear testimony to the use of the word in Middle Dutch at that time.
  3. 3.0 3.1 Mohamed Bazza.overview of the hystory of water resources and irrigation management in the near east region.fao.org. Retrieved October 7, 2007.
  4. 4.0 4.1 Bijker Wiebe. Dikes and Dams, Thick with Politics.ISIS. Retrieved October 7, 2007.
  5. Joseph Needham, Science and Civilization in China, Vol. 4, Part 3. (Taipei: Caves Books, Ltd. 1986)
  6. Methodology and Technical Notes- iucn.org. Retrieved October 7, 2007.
  7. Arch Dam Forces. PBS Online. Retrieved October 7, 2007.
  8. 8.0 8.1 Renewables Global Status Report 2006 Update, REN21, 2006. Retrieved October 7, 2007.
  9. The Impact of Agricultural Development on Aquatic Systems and its Effect on the Epidemiology of Schistosomes in Rhodesia. "Recently, agricultural development has concentrated on soil and water conservation and resulted in the construction of a multitude of dams of various capacities which tend to stabilize water flow in rivers and provide a significant amount of permanent and stable bodies of water." [1]. IUCN. Retrieved October 7, 2007.
  10. Kazakhstan Land and Water Development Division. 1998. "Construction of a dam (Berg Strait) to stabilize and increase the level of the northern part of the Aral Sea." Retrieved October 7, 2007.
  11. Blackwater Dam. US Army Corps of Engineers "The principal objective of the dam and reservoir is to protect downstream communities." Retrieved October 7, 2007.
  12. Batavians for a Healthy River - Dam Fact Sheet. Retrieved October 7, 2007.
  13. Duncan Graham-Rowe, Feb. 24, 2005, Hydroelectric power's dirty secret revealed New Scientist. Retrieved October 7, 2007.
  14. china-embassy,org. Three Gorges dam wall completed. May 20, 2006. Retrieved October 7, 2007.


  • Fell, Robin, Patrick MacGregor, David Stapledon, and Graeme Bell. 2005. Geotechnical Engineering of Dams. London, UK: Taylor & Francis. ISBN 978-0415364409.
  • Jackson, Donald C. 1996. Great American Bridges and Dams. New York: Wiley. ISBN 0471143855.
  • Needham, Joseph. Science and Civilization in China, Vol. 4, Part 3. Taipei: Caves Books, Ltd. 1986.
  • Siddiqui, Iqtidar. 2008. Dams and Reservoirs: Planning, Engineering and Maintenance. Oxford, UK: Oxford Univ Pr. ISBN 978-0195475852.
  • Stevens, Joseph E. 1990. Hoover Dam: An American Adventure. Norman: University of Oklahoma Press. ISBN 0806122838.

External links

All links retrieved November 17, 2017.


