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{{short description|Barrier that stops or restricts the flow of surface or underground streams}}
{{About|structures for water impoundment}}
{{pp|small=yes}}
{{Use dmy dates|date=March 2020}}
[[File:Überlauf Edertalsperre vom Uhrenkopf.jpg|thumb|upright=1.75|The [[Edersee Dam]] in [[Hesse]], Germany]]
A '''dam''' is a barrier that stops or restricts the flow of [[surface water]] or underground streams. [[Reservoir]]s created by dams not only suppress floods but also provide water for activities such as [[irrigation]], [[drinking water|human consumption]], [[Industrial water|industrial use]], [[aquaculture]], and [[navigability]]. [[Hydropower]] is often used in conjunction with dams to generate electricity. A dam can also be used to collect or store water which can be evenly distributed between locations. Dams generally serve the primary purpose of retaining water, while other structures such as [[floodgate]]s or [[levee]]s (also known as [[Dike (construction)|dikes]]) are used to manage or prevent water flow into specific land regions.

The word ''dam'' can be traced back to [[Middle English]],<ref>{{cite web|url=http://www.bartleby.com/61/45/D0014500.html|title=Bartleby.com: Great Books Online – Quotes, Poems, Novels, Classics and hundreds more|work=bartleby.com|access-date=9 November 2015|url-status=dead|archive-url=https://web.archive.org/web/20090408164742/http://www.bartleby.com/61/45/D0014500.html|archive-date=8 April 2009|df=dmy}}http://www.bartleby.com/</ref> and before that, from [[Middle Dutch]], as seen in the names of many old cities,<ref>Source: ''Tijdschrift voor Nederlandse Taal- en Letterkunde'' (''Magazine for Dutch Language and Literature''), 1947</ref>  such as [[Amsterdam]] and [[Rotterdam]].

Ancient dams were built in Mesopotamia and the Middle East for water control. The earliest known dam is the [[Jawa Dam (Jordan)|Jawa Dam]] in [[Jordan]], dating to 3,000 BC. Egyptians also built dams, such as Sadd-el-Kafara Dam for flood control. In modern-day India, Dholavira had an intricate water-management system with 16 reservoirs and dams. The Great Dam of Marib in Yemen, built between 1750 and 1700 BC, was an engineering wonder, and Eflatun Pinar, a Hittite dam and spring temple in Turkey, dates to the 15th and 13th centuries BC. The [[Kallanai Dam]] in South India, built in the 2nd century AD, is one of the oldest water regulating structures still in use.

Roman engineers built dams with advanced techniques and materials, such as hydraulic mortar and Roman concrete, which allowed for larger structures. They introduced reservoir dams, arch-gravity dams, arch dams, buttress dams, and multiple arch buttress dams. In Iran, bridge dams were used for hydropower and water-raising mechanisms.

During the Middle Ages, dams were built in the Netherlands to regulate water levels and prevent sea intrusion. In the 19th century, large-scale arch dams were constructed around the British Empire, marking advances in dam engineering techniques. The era of large dams began with the construction of the Aswan Low Dam in Egypt in 1902. The Hoover Dam, a massive concrete arch-gravity dam, was built between 1931 and 1936 on the Colorado River. By 1997, there were an estimated 800,000 dams worldwide, with some 40,000 of them over 15 meters high.

==History==
===Ancient dams===
Early dam building took place in [[Mesopotamia]] and the [[Middle East]]. Dams were used to control water levels, for Mesopotamia's weather affected the [[Tigris]] and [[Euphrates]] Rivers.

The earliest known dam is the [[Jawa Dam (Jordan)|Jawa Dam]] in [[Jordan]], {{convert|100|km}} northeast of the capital [[Amman]]. This gravity dam featured an originally {{convert|9|m|ft|adj=mid|-high}} and {{convert|1|m|ft|adj=mid|-wide|abbr=on}} stone wall, supported by a {{convert|50|m|ft|adj=mid|-wide|abbr=on}} earthen rampart. The structure is dated to 3000 BC.<ref>Günther Garbrecht: "Wasserspeicher (Talsperren) in der Antike", ''Antike Welt'', 2nd special edition: ''Antiker Wasserbau'' (1986), pp.51–64 (52)</ref><ref>S.W. Helms: "Jawa Excavations 1975. Third Preliminary Report", Levant 1977</ref> However, the oldest continuously operational dam is [[Lake Homs Dam]], built in [[Syria]] between 1319-1304 BC.<ref>{{Cite web |title=The world's oldest dams still in use |url=https://www.water-technology.net/features/feature-the-worlds-oldest-dams-still-in-use/ |access-date=2023-12-18 |website=www.water-technology.net}}</ref>

The [[Ancient Egypt]]ian [[Sadd-el-Kafara Dam]] at Wadi Al-Garawi, about {{convert|25|km|abbr=on}} south of [[Cairo]], was {{convert|102|m|abbr=on}} long at its base and {{convert|87|m|abbr=on}} wide. The structure was built around 2800<ref name="Günther Garbrecht 52f.">Günther Garbrecht: "Wasserspeicher (Talsperren) in der Antike", ''Antike Welt'', 2nd special edition: ''Antiker Wasserbau'' (1986), pp.51–64 (52f.)</ref> or 2600 BC<ref name="Bazzasowr">{{cite web|url=http://www.fao.org/world/Regional/RNE/morelinks/Publications/English/HYSTORY-OF-WATER-RESOURCES.pdf |title=overview of the hystory &#91;sic&#93; of water resources and irrigation management in the near east region |access-date=1 August 2007 |author=Mohamed Bazza |date=28–30 October 2006 |publisher=Food and Agriculture Organization of the United Nations |url-status=dead |archive-url=https://web.archive.org/web/20070808082928/http://www.fao.org/world/Regional/RNE/morelinks/Publications/English/HYSTORY-OF-WATER-RESOURCES.pdf |archive-date=8 August 2007 }}http://www.fao.org/docrep/005/y4357e/y4357e14.htm</ref> as a [[diversion dam]] for flood control, but was destroyed by heavy rain during construction or shortly afterwards.<ref name="Günther Garbrecht 52f." /><ref name="Bazzasowr" /> During the [[Twelfth Dynasty of Egypt|Twelfth Dynasty]] in the 19th century BC, the Pharaohs Senosert III, [[Amenemhat III]], and [[Amenemhat IV]] dug a canal {{convert|16|km|abbr=on}} long linking the [[Fayum Depression]] to the [[Nile]] in Middle Egypt. Two dams called Ha-Uar running east–west were built to retain water during the annual flood and then release it to surrounding lands. The lake called ''Mer-wer'' or [[Lake Moeris]] covered {{convert|1700|sqkm|sqmi|abbr=on}} and is known today as Birket Qarun.<ref>{{Cite web|url=https://www.brown.edu/Departments/Joukowsky_Institute/courses/afterpharaohs2010/13390.html|title=Lake Moeris|publisher=Brown University |access-date=2018-08-14}}</ref>

By the mid-late third millennium BC, an intricate water-management system in [[Dholavira]] in modern-day [[India]] was built. The system included 16 reservoirs, dams and various channels for collecting water and storing it.<ref>{{cite web|url=http://himalmag.com/component/content/article/44/1062-The-reservoirs-of-Dholavira.html|title=The reservoirs of Dholavira|date=December 2008|publisher=The Southasia Trust|access-date=27 February 2011|url-status=dead|archive-url=https://web.archive.org/web/20110711170319/http://himalmag.com/component/content/article/44/1062-The-reservoirs-of-Dholavira.html|archive-date=11 July 2011|df=dmy}}http://old.himalmag.com/component/content/article/1062-the-reservoirs-of-dholavira.html {{Webarchive|url=https://web.archive.org/web/20160821053324/http://old.himalmag.com/component/content/article/1062-the-reservoirs-of-dholavira.html |date=21 August 2016 }}</ref>

One of the engineering wonders of the ancient world was the [[Marib Dam|Great Dam of Marib]] in [[Yemen]]. Initiated sometime between 1750 and 1700 BC, it was made of packed earth – triangular in cross-section, {{convert|580|m|abbr=on}} in length and originally {{convert|4|m|abbr=on}} high – running between two groups of rocks on either side, to which it was linked by substantial stonework. Repairs were carried out during various periods, most importantly around 750 BC, and 250 years later the dam height was increased to {{convert|7|m|abbr=on}}. After the end of the [[Sabaeans|Kingdom of Saba]], the dam fell under the control of the [[Himyarite Kingdom|Ḥimyarites]] (c. 115 BC) who undertook further improvements, creating a structure {{convert|14|m|abbr=on}} high, with five spillways, two masonry-reinforced sluices, a settling pond, and a {{convert|1000|m|abbr=on|adj=on}} canal to a distribution tank. These works were not finished until 325 AD when the dam permitted the irrigation of {{Convert|25000|acre|sqkm}}.

[[Eflatun Pınar]] is a [[Hittites|Hittite]] dam and spring temple near [[Konya]], Turkey. It is thought to date from the Hittite empire between the 15th and 13th centuries BC.

The [[Kallanai]] is constructed of unhewn stone, over {{convert|300|m|abbr=on}} long, {{convert|4.5|m|abbr=on}} high and {{convert|20|m|abbr=on}} wide, across the main stream of the [[Kaveri]] River in [[Tamil Nadu]], [[South India]]. The basic structure dates to the 2nd century AD<ref>{{cite web|url=http://lawlibrary.unm.edu/nrj/48/3/03_agoramoorthy_indian.pdf |title=The Check-Dam Route to Mitigate India's Water Shortages |author1=Govindasamy Agoramoorthy |author2=Sunitha Chaudhary |author3=Minna J. Hsu |publisher=Law library&nbsp;– University of New Mexico |access-date=8 November 2011 |url-status=dead |archive-url=https://web.archive.org/web/20130720143611/http://lawlibrary.unm.edu/nrj/48/3/03_agoramoorthy_indian.pdf |archive-date=20 July 2013 }}</ref> and is considered one of the oldest water diversion or water regulating structures still in use.<ref name="kallanai_oldest">{{cite web|last=Kalyanaraman|first=S |url=http://www.hindunet.org/saraswati/traditionwater.pdf |title=Water management: Historical maritime, riverine tradition of Bharat|date=18 March 2003|access-date=23 January 2021|archive-url=https://web.archive.org/web/20070206130842/http://www.hindunet.org/saraswati/traditionwater.pdf |archive-date=6 February 2007 |url-status=dead |df=dmy }}</ref> The purpose of the dam was to divert the waters of the Kaveri across the fertile delta region for irrigation via canals.<ref name="kallanai_googlebook">{{cite book |last=Singh |first=Vijay P. |author2=Ram Narayan Yadava |title=Water Resources System Operation: Proceedings of the International Conference on Water and Environment |publisher=Allied Publishers |year=2003 |url=https://books.google.com/books?id=Bge-0XX6ip8C&q=kallanai&pg=PA508 |isbn=978-81-7764-548-4 |page=508 |access-date=2015-11-09}}</ref>

[[Dujiangyan irrigation system|Du Jiang Yan]] is the oldest surviving [[irrigation]] system in China that included a dam that directed waterflow. It was finished in 251 BC. A large earthen dam, made by [[Sunshu Ao]], the [[prime minister]] of [[Chu (state)]], flooded a valley in modern-day northern [[Anhui]] Province that created an enormous irrigation reservoir ({{convert|62|mi|order=flip|abbr=on}} in circumference), a reservoir that is still present today.<ref name="needham volume 4 part 3 271">Needham, Joseph (1986). ''Science and Civilization in China: Volume 4, Part 3''. [[Taipei]]: Caves Books, Ltd.</ref>

===Roman engineering===
[[File:Roman Cornalvo dam, Extremadura, Spain. Pic 01.jpg|thumb|The [[Cornalvo Dam|Roman dam at Cornalvo]] in [[Spain]] has been in use for almost two millennia.]]

[[List of Roman dams and reservoirs|Roman dam]] construction was characterized by "the Romans' ability to plan and organize engineering construction on a grand scale."<ref name="Smith 1971, 49">{{harvnb|Smith|1971|p=49}}</ref> Roman planners introduced the then-novel concept of large [[reservoir]] dams which could secure a permanent [[water supply]] for urban settlements over the dry season.<ref>{{harvnb|Smith|1971|p=49}}; {{harvnb|Hodge|1992|pp=79f.}}</ref> Their pioneering use of water-proof hydraulic [[mortar (masonry)|mortar]] and particularly [[Roman concrete]] allowed for much larger dam structures than previously built,<ref name="Smith 1971, 49" /> such as the [[Lake Homs Dam]], possibly the largest water barrier to that date,<ref name="Smith 1971, 42">{{harvnb|Smith|1971|p=42}}</ref> and the [[Harbaqa Dam]], both in [[Syria (Roman province)|Roman Syria]]. The highest Roman dam was the [[Subiaco Dam]] near [[Rome]]; its record height of {{convert|50|m|abbr=on}} remained unsurpassed until its accidental destruction in 1305.<ref name="Hodge 1992, 87">{{harvnb|Hodge|1992|p=87}}</ref>

Roman engineers made routine use of ancient standard designs like embankment dams and masonry gravity dams.<ref>{{harvnb|Hodge|2000|pp=331f.}}</ref> Apart from that, they displayed a high degree of inventiveness, introducing most of the other basic dam designs which had been unknown until then. These include [[arch-gravity dam]]s,<ref>{{harvnb|Hodge|2000|p=332}}; {{harvnb|James|Chanson|2002}}</ref> [[arch dam]]s,<ref name="Earliest known arch dam">{{harvnb|Smith|1971|pp=33–35}}; {{harvnb|Schnitter|1978|pp=31f.}}; {{harvnb|Schnitter|1987a|p=12}}; {{harvnb|Schnitter|1987c|p=80}}; {{harvnb|Hodge|2000|p=332, fn. 2}}</ref> [[buttress dam]]s<ref>{{harvnb|Schnitter|1987b|pp=59–62}}</ref> and [[Arch dam#Variations|multiple arch buttress dams]],<ref name="Earliest known multiple arch buttress dam">{{harvnb|Schnitter|1978|p=29}}; {{harvnb|Schnitter|1987b|pp=60, table 1, 62}}; {{harvnb|James|Chanson|2002}}; {{harvnb|Arenillas|Castillo|2003}}</ref> all of which were known and employed by the 2nd century AD (see [[List of Roman dams]]). Roman workforces also were the first to build dam bridges, such as the [[Bridge of Valerian]] in Iran.<ref>{{harvnb|Vogel|1987|p=50}}</ref>

[[File:Sushtar Bridge.jpg|thumb|left|Remains of the [[Band-e Kaisar]] dam, built by the Romans in the 3rd century AD]]

In [[Iran]], bridge dams such as the [[Band-e Kaisar]] were used to provide [[hydropower]] through [[water wheel]]s, which often powered water-raising mechanisms. One of the first was the Roman-built dam bridge in [[Dezful]],<ref>{{harvnb|Hartung|Kuros|1987|pp=232, 238, fig. 13; 249}}</ref> which could raise water 50 [[cubit]]s (c. 23 m) to supply the town. Also [[diversion dam]]s were known.<ref name="Hill-Engineering">[[Donald Routledge Hill]] (1996), "Engineering", p. 759, in {{cite book |last1=Rashed |first1=Roshdi |last2=Morelon  |first2=Régis |year=1996 |title=Encyclopedia of the History of Arabic Science |publisher=[[Routledge]] |isbn=978-0-415-12410-2 |pages=751–795 |title-link=Encyclopedia of the History of Arabic Science }}</ref> [[Mill (grinding)|Milling]] dams were introduced which the [[Muslim Agricultural Revolution|Muslim engineers]] called the ''Pul-i-Bulaiti''. The first was built at Shustar on the River [[Karun]], Iran, and many of these were later built in other parts of the [[Islamic world]].<ref name="Hill-Engineering" /> Water was conducted from the back of the dam through a large pipe to drive a water wheel and [[watermill]].<ref name="Lucas">Adam Lucas (2006), ''Wind, Water, Work: Ancient and Medieval Milling Technology'', p. 62. Brill, {{ISBN|90-04-14649-0}}.</ref> In the 10th century, [[Al-Muqaddasi]] described several dams in Persia. He reported that one in [[Ahvaz|Ahwaz]] was more than {{convert|3000|ft|order=flip|abbr=on}} long,<ref name="Hill-56-8" /> and that it had many water-wheels raising the water into [[aqueduct (water supply)|aqueducts]] through which it flowed into reservoirs of the city.<ref name="Hill-31">{{cite book |title=A history of engineering in classical and medieval times |author=Donald Routledge Hill |publisher=Routledge |year=1996 |isbn=978-0-415-15291-4 |page=31}}</ref> Another one, the Band-i-Amir Dam, provided irrigation for 300 villages.<ref name="Hill-56-8">{{cite book |title=A history of engineering in classical and medieval times |author=Donald Routledge Hill |publisher=[[Routledge]] |year=1996 |isbn=978-0-415-15291-4 |pages=56–8 |author-link=Donald Routledge Hill}}</ref>

===Middle Ages===
In the [[Netherlands]], a low-lying country, dams were often built to block rivers to regulate the water level and to prevent the sea from entering the marshlands. Such dams often marked the beginning of a town or city because it was easy to cross the river at such a place, and often influenced Dutch place names. The present Dutch capital, [[Amsterdam]] (old name ''Amstelredam''), started with a dam on the river [[Amstel]] in the late 12th century, and [[Rotterdam]] began with a dam on the river [[Rotte (river)|Rotte]], a minor tributary of the [[Nieuwe Maas]]. The central square of Amsterdam, covering the original site of the 800-year-old dam, still carries the name ''[[Dam Square]]''.

===Industrial revolution===
[[File:Engraving of Rideau Canal locks.jpg|thumb|upright|240x240px|An engraving of the [[Rideau Canal]] locks at [[Bytown]]]]
The Romans were the first to build [[arch dam]]s, where the [[reaction force]]s from the abutment stabilizes the structure from the external [[hydrostatic pressure]], but it was only in the 19th century that the engineering skills and construction materials available were capable of building the first large-scale arch dams.

Three pioneering arch dams were built around the [[British Empire]] in the early 19th century. Henry Russel of the [[Royal Engineers]] oversaw the construction of the [[Mir Alam Tank|Mir Alam dam]] in 1804 to supply water to the city of [[Hyderabad]] (it is still in use today). It had a height of {{convert|12|m|abbr=on}} and consisted of 21 arches of variable span.<ref>{{cite web|url=http://www.simscience.org/cracks/advanced/butt_hist1.html |title=Key Developments in the History of Buttress Dams |url-status=dead |archive-url=https://web.archive.org/web/20120321190213/http://www.simscience.org/cracks/advanced/butt_hist1.html |archive-date=21 March 2012 |df=dmy }}</ref>

In the 1820s and 30s, Lieutenant-Colonel [[John By]] supervised the construction of the [[Rideau Canal]] in [[Canada]] near modern-day [[Ottawa]] and built a series of curved masonry dams as part of the waterway system. In particular, the [[Jones Falls Dam]], built by [[John Redpath]], was completed in 1832 as the largest dam in [[North America]] and an engineering marvel. In order to keep the water in control during construction, two [[sluice]]s, artificial channels for conducting water, were kept open in the dam. The first was near the base of the dam on its east side. A second sluice was put in on the west side of the dam, about {{convert|20|ft|abbr=on}} above the base. To make the switch from the lower to upper sluice, the outlet of Sand Lake was blocked off.<ref>{{cite web |url=http://www.mysteriesofcanada.com/Canada/john_redpath.htm |title=John Redpath, the Whispering Dam, and Sugar |date=2014-10-31}}</ref>

[[File:Lake Parramatta,New South Wales.jpg|left|thumb|Masonry arch wall, [[Parramatta]], [[New South Wales]], the first engineered dam built in Australia]]
Hunts Creek near the city of [[Parramatta]], [[Australia]], was dammed in the 1850s, to cater to the demand for water from the growing population of the city. The masonry [[arch dam]] wall was designed by Lieutenant Percy Simpson who was influenced by the advances in dam engineering techniques made by the [[Royal Engineers]] in [[British India|India]]. The dam cost £17,000 and was completed in 1856 as the first engineered dam built in Australia, and the second arch dam in the world built to mathematical specifications.<ref>{{cite web |url=http://www.traianvs.net/textos/archdams_en.htm |title=Historical Development of Arch Dams}}</ref>

The first such dam was opened two years earlier in [[France]]. It was the first French arch dam of the [[Industrial Revolution|industrial era]], and it was built by François Zola in the municipality of [[Aix-en-Provence]] to improve the supply of water after the [[Second cholera pandemic (1829–51)|1832 cholera outbreak]] devastated the area. After [[Louis Philippe I|royal approval]] was granted in 1844, the dam was constructed over the following decade. Its construction was carried out on the basis of the mathematical results of scientific stress analysis.

The 75-miles dam near [[Warwick, Queensland|Warwick]], Australia, was possibly the world's first concrete arch dam. Designed by [[Henry Charles Stanley]] in 1880 with an overflow spillway and a special water outlet, it was eventually heightened to {{convert|10|m|abbr=on}}.

In the latter half of the nineteenth century, significant advances in the scientific theory of masonry dam design were made. This transformed dam design from an art based on empirical methodology to a profession based on a rigorously applied scientific theoretical framework. This new emphasis was centered around the engineering faculties of universities in France and in the United Kingdom. [[William John Macquorn Rankine]] at the [[University of Glasgow]] pioneered the theoretical understanding of dam structures in his 1857 paper ''On the Stability of Loose Earth''. [[Rankine theory]] provided a good understanding of the principles behind dam design.<ref>Rankine, W. (1857) "On the stability of loose earth". ''Philosophical Transactions of the Royal Society of London'', Vol. 147.</ref> In France, J. Augustin Tortene de Sazilly explained the mechanics of vertically faced masonry gravity dams, and Zola's dam was the first to be built on the basis of these principles.<ref>{{cite encyclopedia |url=https://www.britannica.com/EBchecked/topic/150337/dam/72085/The-19th-century |title=dam |encyclopedia=Encyclopædia Britannica|date=13 August 2023 }}</ref>

===Modern era===
[[File:Ansel Adams - National Archives 79-AAB-01.jpg|thumb|The [[Hoover Dam]] by [[Ansel Adams]], 1942]]

The era of large dams was initiated with the construction of the [[Aswan Low Dam]] in Egypt in 1902, a gravity [[masonry dam|masonry]] [[buttress dam]] on the [[Nile River]]. Following their 1882 [[Battle of Tel el-Kebir|invasion and occupation of Egypt]], the British began construction in 1898. The project was designed by Sir [[William Willcocks]] and involved several eminent engineers of the time, including Sir [[Benjamin Baker (engineer)|Benjamin Baker]] and Sir [[Sir John Aird, 1st Baronet|John Aird]], whose firm, [[John Aird & Co.]], was the main contractor.<ref>{{cite web|url=http://www.collectstocks.com/egyptbond.html |title=Egyptian Irrigation Bond 1898 – Aswan Dam on Nile River |work=Scripophily |access-date=9 November 2015 |url-status=dead |archive-url=https://web.archive.org/web/20050513224447/http://www.collectstocks.com/egyptbond.html |archive-date=13 May 2005 }}</ref><ref>{{Citation |last=Roberts |first=Chalmers |date=December 1902 |title=Subduing the Nile |url=https://books.google.com/books?id=DoDNAAAAMAAJ&pg=PA2861 |journal=[[World's Work|The World's Work: A History of Our Time]] |volume=V |pages=2861–2870 |access-date=2009-07-10}}</ref> Capital and financing were furnished by [[Ernest Cassel]].<ref>[http://jewishencyclopedia.com/view.jsp?artid=142&letter=F&search=Finance Finance], ''[[Jewish Encyclopedia]]'', c.1906</ref> When initially constructed between 1899 and 1902, nothing of its scale had ever before been attempted;<ref>Frederic Courtland Penfield, [[s:Century Magazine/Volume 57/Issue 4/Harnessing the Nile|"Harnessing the Nile"]], ''[[The Century Magazine]]'', Vol. 57, No. 4 (February 1899)</ref> on completion, it was the largest masonry dam in the world.<ref>{{cite web|title=The First Aswan Dam |url=http://www.umich.edu/~kelseydb/Exhibits/AncientNubia/PhotoIntro.html |archive-url=https://web.archive.org/web/19970615204902/http://www.umich.edu/~kelseydb/Exhibits/AncientNubia/PhotoIntro.html |url-status=dead |archive-date=15 June 1997 |publisher=University of Michigan |access-date=2 January 2011 }}</ref>

The [[Hoover Dam]] is a massive concrete [[arch-gravity dam]], constructed in the [[Black Canyon of the Colorado|Black Canyon]] of the [[Colorado River]], on the border between the US states of [[Arizona]] and [[Nevada]] between 1931 and 1936 during the [[Great Depression]]. In 1928, Congress authorized the project to build a dam that would control floods, provide irrigation water and produce [[hydroelectric power]]. The winning bid to build the dam was submitted by a consortium called [[Six Companies, Inc.]] Such a large concrete structure had never been built before, and some of the techniques were unproven. The torrid summer weather and the lack of facilities near the site also presented difficulties. Nevertheless, Six Companies turned over the dam to the federal government on 1 March 1936, more than two years ahead of schedule.<ref>{{cite book |last1=Interior |first1=United States Dept of the |title=The Hoover Dam Documents |date=1948 |publisher=U.S. Government Printing Office |page=79 |url=https://books.google.com/books?id=jTNAAAAAIAAJ&pg=PA79 |language=en}}</ref>

By 1997, there were an estimated 800,000 dams worldwide, some 40,000 of them over {{convert|15|m|abbr=on}} high.<ref>{{Cite journal |pmc=1470397 |last1=Joyce |first1=S. |title=Is it worth a dam? |date=October 1997 |journal=Environmental Health Perspectives |volume=105 |issue=10 |pages=1050–1055 |pmid=9349830 |doi=10.1289/ehp.971051050}}</ref> In 2014, scholars from the [[University of Oxford]] published a study of the cost of large dams – based on the largest existing dataset – documenting significant cost overruns for a majority of dams and questioning whether benefits typically offset costs for such dams.<ref>{{Cite journal |author1=Atif Ansar |author2=Bent Flyvbjerg |author3=Alexander Budzier |author4=Daniel Lunn |date=June 2014|title=Should we build more large dams? The actual costs of hydropower megaproject development|ssrn=2406852|journal=Energy Policy |volume=69 |pages=43–56|doi=10.1016/j.enpol.2013.10.069|arxiv=1409.0002 |s2cid=55722535 }}</ref>

== Types==
{{more citations needed section|date=May 2014}}
Dams can be formed by human agency, natural causes, or even by the intervention of wildlife such as [[beaver]]s. Man-made dams are typically classified according to their size (height), intended purpose or structure.

=== By structure ===
Based on structure and material used, dams are classified as easily created without materials, [[arch-gravity dam]]s, [[embankment dam]]s or [[masonry dam]]s, with several subtypes.

==== Arch dams ====
{{main|Arch dam}}

[[File:Gordon Dam.jpg|thumb|upright|[[Gordon Dam]], [[Tasmania]], is an [[arch dam]].]]

In the arch dam, stability is obtained by a combination of arch and gravity action. If the upstream face is vertical the entire weight of the dam must be carried to the foundation by gravity, while the distribution of the normal [[Fluid pressure|hydrostatic pressure]] between vertical [[cantilever]] and [[arch action]] will depend upon the [[stiffness]] of the dam in a vertical and horizontal direction. When the upstream face is sloped the distribution is more complicated. The [[Normal (geometry)|normal]] component of the weight of the arch ring may be taken by the arch action, while the normal hydrostatic pressure will be distributed as described above. For this type of dam, firm reliable supports at the [[abutment]]s (either [[buttress]] or [[canyon]] side wall) are more important. The most desirable place for an arch dam is a narrow canyon with steep side walls composed of sound rock.<ref name="pbsarch">{{cite web |url=https://www.pbs.org/wgbh/buildingbig/dam/basics.html#arch |title=Arch Dam Forces |publisher=[[PBS]] |access-date=7 January 2007}}</ref> The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the character of the rock should be carefully inspected.

[[File:Barrage Daniel-Johnson2 edited.jpg|thumb|left|[[Daniel-Johnson Dam]], [[Quebec]], is a multiple-arch buttress dam.]]

Two types of single-arch dams are in use, namely the constant-angle and the constant-radius dam. The constant-radius type employs the same face radius at all elevations of the dam, which means that as the channel grows narrower towards the bottom of the dam the central angle subtended by the face of the dam becomes smaller. [[Jones Falls Dam]], in Canada, is a constant radius dam. In a constant-angle dam, also known as a variable radius dam, this subtended angle is kept constant and the variation in distance between the abutments at various levels is taken care of by varying the radii. Constant-radius dams are much less common than constant-angle dams. [[Parker Dam]] on the Colorado River is a constant-angle arch dam.

A similar type is the double-curvature or thin-shell dam. [[Wild Horse Reservoir|Wildhorse Dam]] near [[Mountain City, Nevada]], in the United States is an example of the type. This method of construction minimizes the amount of concrete necessary for construction but transmits large loads to the foundation and abutments. The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream.

The multiple-arch dam consists of a number of single-arch dams with concrete buttresses as the supporting abutments, as for example the [[Daniel-Johnson Dam]], Québec, Canada. The multiple-arch dam does not require as many buttresses as the hollow gravity type but requires a good rock foundation because the buttress loads are heavy.

==== Gravity dams ====
{{main|Gravity dam}}

[[File:Grand Coulee Dam spillway.jpg|thumb|The [[Grand Coulee Dam]] is an example of a solid gravity dam.]]

In a gravity dam, the force that holds the dam in place against the push from the water is Earth's gravity pulling down on the mass of the dam.<ref>British Dam Society http://www.britishdams.org/about_dams/gravity.htm {{Webarchive|url=https://web.archive.org/web/20110831091428/http://www.britishdams.org/about_dams/gravity.htm |date=31 August 2011 }}</ref> The water presses laterally (downstream) on the dam, tending to overturn the dam by rotating about its toe (a point at the bottom downstream side of the dam). The dam's weight counteracts that force, tending to rotate the dam the other way about its toe. The designer ensures that the dam is heavy enough that the dam's weight wins that contest. In engineering terms, that is true whenever the [[Parallelogram law|resultant]] of the forces of gravity acting on the dam and water pressure on the dam acts in a line that passes upstream of the toe of the dam.{{citation needed|date=March 2019}} The designer tries to shape the dam so if one were to consider the part of the dam above any particular height to be a whole dam itself, that dam also would be held in place by gravity, i.e., there is no tension in the upstream face of the dam holding the top of the dam down. The designer does this because it is usually more practical to make a dam of material essentially just piled up than to make the material stick together against vertical tension.{{citation needed|date=March 2019}} The shape that prevents tension in the upstream face also eliminates a balancing compression stress in the downstream face, providing additional economy.

For this type of dam, it is essential to have an impervious foundation with high bearing strength. Permeable foundations have a greater likelihood of generating uplift pressures under the dam. Uplift pressures are hydrostatic pressures caused by the water pressure of the reservoir pushing up against the bottom of the dam. If large enough uplift pressures are generated there is a risk of destabilizing the concrete gravity dam.<ref>{{cite web |title=Gravity Dams |url=https://www.ferc.gov/sites/default/files/2020-04/chap3.pdf}}</ref>{{citation needed|date=March 2019}}

On a suitable site, a gravity dam can prove to be a better alternative to other types of dams. When built on a solid foundation, the gravity dam probably represents the best-developed example of dam building. Since the fear of [[flood]] is a strong motivator in many regions, gravity dams are built in some instances where an arch dam would have been more economical.

Gravity dams are classified as "solid" or "hollow" and are generally made of either concrete or masonry. The solid form is the more widely used of the two, though the hollow dam is frequently more economical to construct. [[Grand Coulee Dam]] is a solid gravity dam and [[Braddock Locks & Dam]] is a hollow gravity dam.{{citation needed|date=March 2019}}

==== Arch-gravity dams ====
[[File:Hoover Dam (aerial view) - 30 April 2009.jpg|thumb|The [[Hoover Dam]] is an example of an arch-gravity dam.]]
{{main|Arch-gravity dam}}

A gravity dam can be combined with an arch dam into an [[arch-gravity dam]] for areas with massive amounts of water flow but less material available for a pure gravity dam. The inward compression of the dam by the water reduces the lateral (horizontal) force acting on the dam. Thus, the gravitational force required by the dam is lessened, i.e., the dam does not need to be so massive. This enables thinner dams and saves resources.

==== Barrages ====
[[File:Koshi.jpg|thumb|The [[Koshi Barrage]] of [[Nepal]]]]
{{main|Barrage (dam)|l1=Barrage dams}}

A barrage dam is a special kind of dam that consists of a line of large gates that can be opened or closed to control the amount of water passing the dam. The gates are set between flanking piers which are responsible for supporting the water load, and are often used to control and stabilize water flow for irrigation systems. An example of this type of dam is the now-decommissioned [[Red Bluff Diversion Dam]] on the [[Sacramento River]] near [[Red Bluff, California]].

Barrages that are built at the mouths of rivers or lagoons to prevent [[tide|tidal incursions]] or use the tidal flow for [[tidal power]] are known as [[tidal barrage]]s.<ref name="urlDams and Development: An Overview" />

==== Embankment dams ====
[[File:Chatuge Dam is an earthen embankment dam in North Carolina, United States.jpg|thumb|[[Chatuge Dam]] is an earthen embankment dam in [[North Carolina]]]]
{{main|Embankment dam}}
Embankment dams are made of [[Soil compaction|compacted]] earth, and are of two main types: rock-fill and earth-fill. Like concrete gravity dams, embankment dams rely on their weight to hold back the force of water.

==== Fixed-crest dams ====
{{See also|Low head dam}}

A fixed-crest dam is a concrete barrier across a river.<ref>{{cite web|url=http://wesa.fm/post/us-army-corps-engineers-wants-you-enjoy-rivers-safely#stream/0 |title=The U.S. Army Corps of Engineers Wants You To Enjoy The Rivers, Safely |publisher=90.5 WESA |date=23 June 2017 |access-date=2018-07-18}}</ref> Fixed-crest dams are designed to maintain depth in the channel for navigation.<ref>{{cite web|url=https://www.lrp.usace.army.mil/Media/News-Releases/Article/1218267/army-corps-waterways-partners-focus-on-fixed-crest-dam-safety/ |title=Army Corps, waterways partners focus on fixed-crest dam safety > Pittsburgh District > News Releases |publisher=Lrp.usace.army.mil |date=2017-06-19 |access-date=2018-07-18}}</ref> They pose risks to boaters who may travel over them, as they are hard to spot from the water and create induced currents that are difficult to escape.<ref>{{cite web|author=Bob Bauder |url=https://triblive.com/local/allegheny/13875841-74/family-of-kayaker-swept-over-dashields-dam-sues-us-army-corps-of |title=Family of kayaker swept over Dashields Dam sues U.S. Army Corps of Engineers |publisher=TribLIVE |date=2017-05-20 |access-date=2018-07-18}}</ref>

=== By size ===
There is variability, both worldwide and within individual countries, such as in the United States, in how dams of different sizes are categorized. Dam size influences construction, repair, and [[Dam removal|removal]] costs and affects the dams' potential range and magnitude of environmental disturbances.<ref>{{Cite conference |last1=Carter |first1=Edward F. |last2=Hosko |first2=Mary Ann |last3=Austin |first3=Roger |date=1997 |title=Guidelines for Retirement of Dams and Hydroelectric Facilities |conference=Waterpower '97 |url=https://cedb.asce.org/CEDBsearch/record.jsp?dockey=0106463 |publisher=ASCE |pages=1248–1256}}</ref>

====Large dams====
The [[International Commission on Large Dams]] (ICOLD) defines a "large dam" as "A dam with a height of {{convert|15|m|abbr=on}} or greater from lowest foundation to crest or a dam between {{convert|5|m|abbr=on}} metres and 15 metres impounding more than {{convert|3|e6m3|acre-ft|lk=out}}".<ref>{{cite web |title=Definition of a Large Dam |url=https://www.icold-cigb.org/GB/dams/definition_of_a_large_dam.asp |publisher=International Commission on Large Dams |access-date=23 January 2021}}</ref> "Major dams" are over {{convert|150|m|abbr=on}} in height.<ref>{{cite web|url=http://www.iucn.org/themes/wani/eatlas/html/technotes.html |quote=A large dam is defined by the industry as one higher than 15 meters high and a major dam as higher than 150.5 meters. |work=Watersheds of the World |title=Methodology and Technical Notes |access-date=1 August 2007 |archive-url=https://archive.today/20070704103642/http://www.iucn.org/themes/wani/eatlas/html/technotes.html |archive-date=4 July 2007 |url-status=dead |df=dmy }}</ref> The ''Report of the World Commission on Dams'' also includes in the "large" category, dams which are between {{convert|5|and|15|m|abbr=on}} high with a reservoir capacity of more than {{convert|3|e6m3|acre-ft|lk=out}}.<ref name="urlDams and Development: An Overview">{{cite web|url=http://www.dams.org/report/wcd_overview.htm |title=Dams and Development: An Overview |date=16 November 2000 |quote=Box 1. What is a large dam? |access-date=24 October 2010 |url-status=dead |archive-url=https://web.archive.org/web/20101028045913/http://www.dams.org/report/wcd_overview.htm |archive-date=28 October 2010 }}</ref> [[Hydropower]] dams can be classified as either "high-head" (greater than 30 m in height) or "low-head" (less than 30 m in height).<ref name="Poff Hart 2002">{{Cite journal|last1=Poff|first1=N. Leroy|last2=Hart|first2=David D.|date=2002-08-01|title=How Dams Vary and Why It Matters for the Emerging Science of Dam Removal;... |journal=BioScience|language=en|volume=52|issue=8|pages=659–668|doi=10.1641/0006-3568(2002)052[0659:HDVAWI]2.0.CO;2|issn=0006-3568|doi-access=free}}</ref>

{{as of|2021}}, ICOLD's World Register of Dams contains 58,700 large dam records.<ref name="UNU-2021">{{cite book |last1=Perera |first1=Duminda |display-authors=et al.|title=Ageing Water Storage Infrastructure: An Emerging Global Risk |date=2021 |publisher=United Nations University Institute for Water, Environment and Health |location=Hamilton, Canada |isbn=978-92-808-6105-1 |url=https://inweh.unu.edu/wp-content/uploads/2021/01/Ageing-Water-Storage-Infrastructure-An-Emerging-Global-Risk.pdf |access-date=23 January 2021 |format=Report Series, Issue 11}}</ref>{{RP|6}} The tallest dam in the world is the {{convert|305|m|ft|adj=mid|-high|abbr=on}} [[Jinping-I Dam]] in [[China]].<ref>{{cite web |title=The Jinping-I Double Curvature Arch Dam sets new world record |url=http://en.powerchina.cn/2016-12/28/content_27870606.htm |website=en.powerchina.cn |access-date=16 November 2018 |archive-date=17 November 2018 |archive-url=https://web.archive.org/web/20181117022403/http://en.powerchina.cn/2016-12/28/content_27870606.htm |url-status=dead }}</ref>

==== Small dams ====

[[File:Dam in Europe at Autumn as viewed from FPV drone.webm|thumb|Dam in Europe at Autumn as viewed from FPV drone.]]

As with large dams, small dams have multiple uses, such as, but not limited to, [[hydropower]] production, flood protection, and water storage. Small dams can be particularly useful on farms to capture runoff for later use, for example, during the dry season.<ref>{{Cite journal |last1=Nathan |first1=R. |last2=Lowe |first2=L. |date=2012-01-01 |title=The Hydrologic Impacts of Farm Dams |url=https://www.tandfonline.com/doi/abs/10.7158/13241583.2012.11465405 |journal=Australasian Journal of Water Resources |volume=16 |issue=1 |pages=75–83 |doi=10.7158/13241583.2012.11465405 |doi-broken-date=31 January 2024 |issn=1324-1583}} <!-- as of 3 July 2023, doi is still broken --></ref> Small scale dams have the potential to generate benefits without displacing people as well,<ref>{{Cite web|title=Why small-scale hydroelectric plants benefit local communities|date=3 September 2015 |url=https://www.weforum.org/agenda/2015/09/why-small-scale-hydroelectric-plants-benefit-local-communities/|publisher=World Economic Forum|language=en|access-date=2020-05-11}}</ref> and small, decentralised hydroelectric dams can aid rural development in developing countries.<ref>{{Cite journal|last=Faruqui|first=N. I.|title=Small Hydro for Rural Development|date=1994|journal=Canadian Water Resources Journal|language=en|volume=19|issue=3|pages=227–235|doi=10.4296/cwrj1903227|issn=0701-1784|doi-access=free}}</ref> In the United States alone, there are approximately 2,000,000 or more "small" dams that are not included in the [[United States Army Corps of Engineers|Army Corps of Engineers]] [[National Inventory of Dams|National Inventory of dams]].<ref>{{Cite book|last=Graf|first=WL|title=Sustaining Our Water Resources|publisher=National Academy Press|year=1993|location=Washington DC|pages=11–42|chapter=Landscapes, commodities, and ecosystems: The relationship between policy and science for American rivers}}</ref> Records of small dams are kept by state regulatory agencies and therefore information about small dams is dispersed and uneven in geographic coverage.<ref name="Poff Hart 2002" />

Countries worldwide consider small hydropower plants (SHPs) important for their energy strategies, and there has been a notable increase in interest in SHPs.<ref name="Couto Olden 2018">{{Cite journal|last1=Couto|first1=Thiago BA|last2=Olden|first2=Julian D.|date=2018|title=Global proliferation of small hydropower plants – science and policy|journal=Frontiers in Ecology and the Environment|language=en|volume=16|issue=2|pages=91–100|doi=10.1002/fee.1746|issn=1540-9309|doi-access=free}}</ref> Couto and Olden (2018)<ref name="Couto Olden 2018" /> conducted a global study and found 82,891 small hydropower plants (SHPs) operating or under construction. Technical definitions of SHPs, such as their maximum generation capacity, dam height, reservoir area, etc., vary by country.

==== Non-jurisdictional dams ====
A dam is non-jurisdictional when its size (usually "small") excludes it from being subject to certain legal regulations. The technical criteria for categorising a dam as "jurisdictional" or "non-jurisdictional" varies by location. In the United States, each state defines what constitutes a non-jurisdictional dam. In the state of [[Colorado]] a non-jurisdictional dam is defined as a dam creating a [[reservoir]] with a capacity of 100 acre-feet or less and a surface area of 20 acres or less and with a height measured as defined in Rules 4.2.5.1. and 4.2.19 of 10 feet or less.<ref>{{Cite web|title=DWR Dam Safety Non-Jurisdictional Dam |publisher=Colorado Information Marketplace |url=https://data.colorado.gov/Water/DWR-Dam-Safety-Non-Jurisdictional-Dam/6smc-zj6j/data |language=en|access-date=2020-05-11}}</ref> In contrast, the state of [[New Mexico]] defines a jurisdictional dam as 25 feet or greater in height and storing more than 15 acre-feet or a dam that stores 50 acre-feet or greater and is six feet or more in height (section 72-5-32 NMSA), suggesting that dams that do not meet these requirements are non-jurisdictional.<ref>{{Cite journal|date=7 December 2009|title=Evaluation of Non-Jurisdictional Dams|url=https://www.ose.state.nm.us/dams/submittal/EvaluationOfNonJurisdictionalDams.pdf|journal=Office of the State Engineer, Dam Safety Bureau}}</ref> Most US dams, 2.41&nbsp;million of a total of 2.5&nbsp;million dams, are not under the jurisdiction of any public agency (i.e., they are non-jurisdictional), nor are they listed on the [[National Inventory of Dams]] (NID).<ref name="Brewitt Colwyn 2020">{{Cite journal|last1=Brewitt|first1=Peter K.|last2=Colwyn|first2=Chelsea L. M.|date=2020|title=Little dams, big problems: The legal and policy issues of nonjurisdictional dams|journal=WIREs Water|language=en|volume=7|issue=1|pages=e1393|doi=10.1002/wat2.1393|issn=2049-1948|doi-access=free}}</ref>

Small dams incur risks similar to large dams. However, the absence of regulation (unlike more regulated large dams) and of an inventory of small dams (i.e., those that are non-jurisdictional) can lead to significant risks for both humans and ecosystems.<ref name="Brewitt Colwyn 2020" /> For example, according to the [[National Park Service|US National Park Service]] (NPS), "Non-jurisdictional—means a structure which does not meet the minimum criteria, as listed in the Federal Guidelines for Dam Safety, to be included in dam safety programs. The non-jurisdictional structure does not receive a hazard classification and is not considered for any further requirements or activities under the NPS dam safety program."<ref>{{Cite journal|date=25 May 2010|title=Director's Order #40: Dam Safety & Security Program|url=https://www.nps.gov/subjects/policy/upload/DO_40_5-25-2010.pdf|journal=United States Department of the Interior, National Park Service}}</ref> Small dams can be dangerous individually (i.e., they can fail), but also collectively,<ref>{{Cite journal|last1=Fencl|first1=Jane S.|last2=Mather|first2=Martha E.|last3=Costigan|first3=Katie H.|last4=Daniels|first4=Melinda D.|date=2015-11-05|editor-last=Deng|editor-first=Z. Daniel|title=How Big of an Effect Do Small Dams Have? Using Geomorphological Footprints to Quantify Spatial Impact of Low-Head Dams and Identify Patterns of Across-Dam Variation|journal=PLOS ONE|language=en|volume=10|issue=11|pages=e0141210|doi=10.1371/journal.pone.0141210|issn=1932-6203|pmc=4634923|pmid=26540105|bibcode=2015PLoSO..1041210F|doi-access=free}}</ref> as an aggregation of small dams along a river or within a geographic area can multiply risks. Graham's 1999 study<ref>{{Cite journal|last=Graham|first=W J|date=September 1999|title=A Procedure for Estimating Loss of Life Caused by Dam Failure|url=https://www.usbr.gov/ssle/damsafety/TechDev/DSOTechDev/DSO-99-06.pdf|journal=U.S. Department of the Interior, Bureau of Reclamation}}</ref> of US dam failures resulting in fatalities from 1960 to 1998 concluded that the failure of dams between 6.1 and 15 m high (typical height range of smaller dams<ref name="Pisaniello 2009">{{Cite journal|last=Pisaniello|first=John D.|date=2009|title=How to manage the cumulative flood safety of catchment dams|url=http://www.scielo.org.za/scielo.php?script=sci_abstract&pid=S1816-79502009000400001&lng=en&nrm=iso&tlng=en|journal=Water SA|volume=35|issue=4|pages=361–370|issn=1816-7950}}</ref>) caused 86% of the deaths, and the failure of dams less than 6.1 m high caused 2% of the deaths. Non-jurisdictional dams may pose hazards because their design, construction, maintenance, and surveillance is unregulated.<ref name="Pisaniello 2009" /> Scholars have noted that more research is needed to better understand the environmental impact of small dams<ref name="Couto Olden 2018" /> (e.g., their potential to alter the flow, temperature, sediment<ref>{{Cite journal|last1=Ashley|first1=Jeffrey T. F.|last2=Bushaw-Newton |first2=Karen|last3=Wilhelm|first3=Matt|last4=Boettner|first4=Adam|last5=Drames|first5=Gregg|last6=Velinsky|first6=David J.|date=March 2006|title=The Effects of Small Dam Removal on the Distribution of Sedimentary Contaminants |journal=Environmental Monitoring and Assessment|language=en|volume=114|issue=1–3|pages=287–312|doi=10.1007/s10661-006-4781-3|pmid=16565804|s2cid=46471207|issn=0167-6369}}</ref><ref name="Poff Hart 2002" /> and plant and animal diversity of a river).

=== By use ===
==== Saddle dam ====
A saddle dam is an auxiliary dam constructed to confine the reservoir created by a primary dam either to permit a higher water elevation and storage or to limit the extent of a reservoir for increased efficiency. An auxiliary dam is constructed in a low spot or "saddle" through which the reservoir would otherwise escape. On occasion, a reservoir is contained by a similar structure called a [[levee|dike]] to prevent inundation of nearby land. Dikes are commonly used for reclamation of arable land from a shallow lake, similar to a [[levee]], which is a wall or embankment built along a river or stream to protect adjacent land from flooding.

==== Weir ====
{{main|Weir}}
A weir (sometimes called an "overflow dam") is a small dam that is often used in a river channel to create an impoundment lake for water abstraction purposes and which can also be used for flow measurement or retardation.

==== Check dam ====
{{main|Check dam}}
A check dam is a small dam designed to reduce flow velocity and control soil [[erosion]]. Conversely, a [[wing dam]] is a structure that only partly restricts a waterway, creating a faster channel that resists the accumulation of sediment.

==== Dry dam ====
{{main|Dry dam}}

A dry dam, also known as a flood retarding structure, is designed to control flooding. It normally holds back no water and allows the channel to flow freely, except during periods of intense flow that would otherwise cause flooding downstream.

==== Diversionary dam ====
{{main|Diversionary dam}}

A diversionary dam is designed to divert all or a portion of the flow of a river from its natural course. The water may be redirected into a canal or tunnel for irrigation and/or hydroelectric power production.

==== Underground dam ====
Underground dams are used to trap [[groundwater]] and store all or most of it below the surface for extended use in a localized area. In some cases, they are also built to prevent saltwater from intruding into a freshwater aquifer. Underground dams are typically constructed in areas where water resources are minimal and need to be efficiently stored, such as in deserts and on islands like the [[Fukuzato Dam]] in [[Okinawa]], Japan. They are most common in [[northeastern Africa]] and the arid areas of [[Brazil]] while also being used in the [[southwestern United States]], Mexico, India, Germany, Italy, Greece, France and Japan.<ref>{{cite web |last=Yilmaz |first=Metin |title=Control of Groundwater by Underground Dams |url=http://etd.lib.metu.edu.tr/upload/1259621/index.pdf |publisher=The Middle East Technical University |access-date=7 May 2012 |date=November 2003}}</ref>

There are two types of underground dams: "sub-surface" and a "sand-storage". A sub-surface dam is built across an [[aquifer]] or drainage route from an impervious layer (such as solid bedrock) up to just below the surface. They can be constructed of a variety of materials to include bricks, stones, concrete, steel or PVC. Once built, the water stored behind the dam raises the water table and is then extracted with wells. A sand-storage dam is a weir built in stages across a stream or [[wadi]]. It must be strong, as floods will wash over its crest. Over time, sand accumulates in layers behind the dam, which helps store water and, most importantly, prevent [[evaporation]]. The stored water can be extracted with a well, through the dam body, or by means of a drain pipe.<ref>{{cite journal |last=Onder |first=H |author2=M. Yilmaz |title=Underground Dams—A Tool of Sustainable Development and Management of Ground Resources |journal=European Water |date=November–December 2005 |pages=35–45 |url=http://www.ewra.net/ew/pdf/EW_2005_11-12_05.pdf |access-date=7 May 2012}}</ref>

==== Tailings dam ====
{{main|Tailings dam}}
[[File:Bituminous geomembrane installation on a mine tailings storage facility.jpg|thumb|[[Bituminous Geomembranes (BGMs)|Bituminous geomembrane]]  installation on a mine tailings storage facility.]]

A tailings dam is typically an earth-fill embankment dam used to store [[tailings]], which are produced during [[mining]] operations after separating the valuable fraction from the uneconomic fraction of an [[ore]]. Conventional water retention dams can serve this purpose, but due to cost, a tailings dam is more viable. Unlike water retention dams, a tailings dam is raised in succession throughout the life of the particular mine. Typically, a base or starter dam is constructed, and as it fills with a mixture of tailings and water, it is raised. Material used to raise the dam can include the tailings (depending on their size) along with soil.<ref>{{cite book |last=Blight |first=Geoffrey E. |chapter=Construction of Tailings Dams |title=Case studies on tailings management |year=1998 |publisher=International Council on Metals and the Environment |location=Paris |isbn=978-1-895720-29-7 |pages=9–10 |chapter-url=https://books.google.com/books?id=qG9Bux3RYWMC&q=tailings+dam&pg=PA9 |access-date=10 August 2011}}</ref>

There are three raised tailings dam designs, the "upstream", "downstream", and "centerline", named according to the movement of the crest during raising. The specific design used is dependent upon [[topography]], geology, climate, the type of tailings, and cost. An upstream tailings dam consists of [[trapezoid]]al embankments being constructed on top but toe to crest of another, moving the crest further upstream. This creates a relatively flat downstream side and a jagged upstream side which is supported by tailings [[slurry]] in the impoundment. The downstream design refers to the successive raising of the embankment that positions the fill and crest further downstream. A centerlined dam has sequential embankment dams constructed directly on top of another while fill is placed on the downstream side for support and slurry supports the upstream side.<ref>{{cite web|title=Properties of Tailings Dams |url=http://www.mining.ubc.ca/faculty/meech/MINE290/Tailings%20Dam%20Construction%20Methods.pdf |publisher=NBK Institute of Mining Engineering |access-date=10 August 2011 |url-status=dead |archive-url=https://web.archive.org/web/20111001213037/http://www.mining.ubc.ca/faculty/meech/MINE290/Tailings%20Dam%20Construction%20Methods.pdf |archive-date=1 October 2011 }}http://mining.ubc.ca/files/2013/03/Dirk-van-Zyl.pdf</ref><ref name="taildam">{{cite book |editor-first=Raj K. |editor-last=Singhal |title=Environmental issues and management of waste in energy and mineral production: Proceedings of the Sixth International Conference on Environmental Issues and Management of Waste in Energy and Mineral Production: SWEMP 2000; Calgary, Alberta, Canada, May 30&nbsp;– June 2, 2000 |year=2000 |publisher=Balkema |location=[[Rotterdam]] [u.a.] |isbn=978-90-5809-085-0 |pages=257–260 |url=https://books.google.com/books?id=PqiYy538JFUC&q=tailings+dam&pg=PA257 |access-date=2015-11-09}}</ref>

Because tailings dams often store toxic chemicals from the mining process, modern designs incorporate an impervious [[geomembrane]] liner to prevent seepage.<ref>{{cite report |last1=McLeod|first1=Harvey  |last2=Bjelkevik|first2=Annika |date=2021 |title=TAILINGS DAM DESIGN Technology Update |publisher= ICOLD Committee on Tailings Dams|url= https://www.icoldchile.cl/boletines/181.pdf|doi= |page= 91}}</ref> Water/slurry levels in the tailings pond must be managed for stability and environmental purposes as well.<ref name="taildam" />

=== By material ===
==== Steel dams ====
{{main|Steel dam}}

[[File:RedridgeSteelDam02.jpg|thumb|[[Redridge Steel Dam]], built 1905, [[Michigan]]]]

A [[steel dam]] is a type of dam briefly experimented with around the start of the 20th century which uses steel plating (at an angle) and load-bearing beams as the structure. Intended as permanent structures, steel dams were an (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.

==== Timber dams ====
[[File:Dam Timber Crib.jpg|thumb|left|A timber crib dam in Michigan, 1978]]

[[Timber]] dams were widely used in the early part of the industrial revolution and in frontier areas due to ease and speed of construction. Rarely built in modern times because of their relatively short lifespan and the limited height to which they can be built, timber dams must be kept constantly wet in order to maintain their water retention properties and limit deterioration by rot, similar to a barrel. The locations where timber dams are most economical to build are those where timber is plentiful, [[cement]] is costly or difficult to transport, and either a low head diversion dam is required or longevity is not an issue. Timber dams were once numerous, especially in the [[North America]]n West, but most have failed, been hidden under earth embankments, or been replaced with entirely new structures. Two common variations of timber dams were the "crib" and the "plank".

Timber crib dams were erected of heavy timbers or dressed logs in the manner of a [[log house]] and the interior filled with earth or rubble. The heavy crib structure supported the dam's face and the weight of the water. [[Splash dam]]s were timber crib dams used to help float [[logging|logs]] downstream in the late 19th and early 20th centuries.

"Timber plank dams" were more elegant structures that employed a variety of construction methods using heavy timbers to support a water retaining arrangement of planks.

=== Other types ===
==== Cofferdams ====
{{main|Cofferdam}}
[[File:Dam Coffer.jpg|thumb|A cofferdam during the construction of [[canal lock|locks]] at the Montgomery Point Lock and Dam]]

A [[cofferdam]] is a barrier, usually temporary, constructed to exclude water from an area that is normally submerged. Made commonly of wood, [[concrete]], or [[steel]] sheet [[Deep foundation|piling]], cofferdams are used to allow construction on the [[Foundation (engineering)|foundation]] of permanent dams, bridges, and similar structures. When the project is completed, the cofferdam will usually be demolished or removed unless the area requires continuous maintenance. (See also [[causeway]] and [[retaining wall]].)

Common uses for cofferdams include the construction and repair of offshore oil platforms. In such cases, the cofferdam is fabricated from sheet steel and welded into place under water. Air is pumped into the space, displacing the water and allowing a dry work environment below the surface.

====Natural dams====
Dams can also be created by natural geological forces. [[Lava dam]]s are formed when lava flows, often [[basalt]]ic, intercept the path of a stream or lake outlet, resulting in the creation of a natural impoundment. An example would be the eruptions of the [[Uinkaret volcanic field]] about 1.8&nbsp;million–10,000 years ago, which created lava dams on the [[Colorado River]] in northern [[Arizona]] in the [[United States]]. The largest such lake grew to about {{convert|800|km|mi|abbr=on}} in length before the failure of its dam. [[Glacier|Glacial activity]] can also form natural dams, such as the damming of the [[Clark Fork River|Clark Fork]] in [[Montana]] by the [[Cordilleran Ice Sheet]], which formed the {{convert|7780|km2|mi2|abbr=on}} [[Glacial Lake Missoula]] near the end of the last Ice Age. [[Moraine]] deposits left behind by glaciers can also dam rivers to form lakes, such as at [[Flathead Lake]], also in Montana (see [[Moraine-dammed lake]]).

Natural disasters such as earthquakes and landslides frequently create [[landslide dam]]s in mountainous regions with unstable local geology. Historical examples include the [[Usoi Dam]] in [[Tajikistan]], which blocks the [[Murghab River]] to create [[Sarez Lake]]. At {{convert|560|m|ft|abbr=on}} high, it is the tallest dam in the world, including both natural and man-made dams. A more recent example would be the creation of [[Attabad Lake]] by a landslide on [[Pakistan]]'s [[Hunza River]].

Natural dams often pose significant hazards to human settlements and infrastructure. The resulting lakes often flood inhabited areas, while a catastrophic failure of the dam could cause even greater damage, such as the failure of western [[Wyoming]]'s [[Gros Ventre landslide]] in 1927, which wiped out the town of [[Kelly, Wyoming|Kelly]] resulting in the deaths of six people.

===== Beaver dams =====
{{main|Beaver dam}}
[[Beaver]]s create dams primarily out of mud and sticks to flood a particular habitable area. By flooding a parcel of land, beavers can navigate below or near the surface and remain relatively well hidden or protected from predators. The flooded region also allows beavers access to food, especially during the winter.

== Construction elements ==
=== Power generation plant ===
{{main|Hydroelectricity}}

[[File:Hydroelectric dam.svg|thumb|left|Hydroelectric dam in cross section]]
[[File:Water turbine - edit1.svg|thumb|upright|[[Water turbine|Hydraulic turbine]] and [[electric generator]]]]

{{As of|2005}}, hydroelectric power, mostly from dams, supplies some 19% of the world's electricity, and over 63% of [[renewable energy]].<ref name="REN21-2006">[http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_Update.pdf Renewables Global Status Report 2006 Update]  {{webarchive|url=https://web.archive.org/web/20110718181410/http://www.ren21.net/globalstatusreport/download/RE_GSR_2006_Update.pdf |date=18 July 2011 }}  {{cite web|url=http://www.ren21.net/Portals/0/documents/activities/gsr/RE_GSR_2006_Update.pdf |title=RENEWABLES GLOBAL STATUS REPORT 2006 Update |access-date=2015-11-09 |url-status=dead |archive-url=https://web.archive.org/web/20160514070611/http://www.ren21.net/Portals/0/documents/activities/gsr/RE_GSR_2006_Update.pdf |archive-date=14 May 2016 }}, ''[[REN21]]'', published 2006, accessed 16 May 2007</ref> Much of this is generated by large dams, although [[China]] uses small-scale hydro generation on a wide scale and is responsible for about 50% of world use of this type of power.<ref name="REN21-2006" />

Most hydroelectric power comes from the [[potential energy]] of dammed water driving a [[water turbine]] and [[electric generator|generator]]; to boost the power generation capabilities of a dam, the water may be run through a large pipe called a [[penstock]] before the [[turbine]]. A variant on this simple model uses [[pumped-storage hydroelectricity]] to produce electricity to match periods of high and low demand, by moving water between [[reservoir]]s 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. (For example, see [[Dinorwig Power Station]].)

=== Spillways ===
{{main|Spillway}}
[[File:Llyn Brianne spillway.jpg|upright|thumb|Spillway on [[Llyn Brianne]] dam, [[Wales]], soon after first fill]]

A spillway is a section of a dam designed to pass water from the upstream side of a dam to the downstream side. Many spillways have [[floodgate]]s designed to control the flow through the spillway. There are several types of spillway. A "service spillway" or "primary spillway" passes normal flow. An "auxiliary spillway" releases flow in excess of the capacity of the service spillway. An "emergency spillway" is designed for extreme conditions, such as a serious malfunction of the service spillway. A "[[fuse plug]] spillway" is a low embankment designed to be overtopped and washed away in the event of a large flood. The elements of a fuse plug are independent free-standing blocks, set side by side which work without any remote control. They allow increasing the normal pool of the dam without compromising the security of the dam because they are designed to be gradually evacuated for exceptional events. They work as fixed weirs at times by allowing overflow in common floods.

A spillway can be gradually [[erosion|eroded]] by water flow, including [[cavitation]] or [[turbulence]] of the water flowing over the spillway, leading to its failure. It was the inadequate design of the spillway and installation of fish screens that led to the 1889 over-topping of the [[South Fork Dam]] in [[Johnstown, Pennsylvania]], resulting in the [[Johnstown Flood]] (the "great flood of 1889").<ref>{{cite web|title=The Club and the Dam|url=http://www.jaha.org/attractions/johnstown-flood-museum/flood-history/the-club-and-the-dam/|website=Johnstown Flood Museum|publisher=Johnstown Area Heritage Association|access-date=15 January 2018}}</ref>

Erosion rates are often monitored, and the risk is ordinarily minimized, by shaping the downstream face of the spillway into a curve that minimizes turbulent flow, such as an [[ogee]] curve.

== Creation ==
=== Common purposes ===
{| class="wikitable plainrowheaders" style="margin-right: 0;"
|-
! scope="col" | Function
! scope="col" | Example
|-
! scope="row" | 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 Dam]] on the [[Paraná River]] in [[South America]] generates 14 [[Watt|GW]] and supplied 93% of the energy consumed by [[Paraguay]] and 20% of that consumed by [[Brazil]] as of 2005.
|-
! scope="row" | Water supply
| Many urban areas of the world are supplied with water taken from rivers pent up behind low dams or weirs. Examples include [[London]], with water from the [[River Thames]], and [[Chester]], with water taken from the [[River Dee, Wales|River Dee]]. Other major sources include deep upland reservoirs contained by high dams across deep valleys, such as the [[Claerwen]] series of dams and reservoirs.
|-
! scope="row" | Stabilize water flow / irrigation
| Dams are often used to control and stabilize water flow, often for [[agriculture|agricultural]] purposes and [[irrigation]].<ref>{{cite news |title=The Impact of Agricultural Development on Aquatic Systems and its Effect on the Epidemiology of Schistosomes in Rhodesia |quote=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. |work=The careless technology: Ecology and international development |editor1=M. Taghi Farvar |editor2=John P. Milton |author=C. J. Shiff |oclc=315029 |publisher=Natural History Press |date=1972 |pages=102–108}}</ref> 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]].<ref>{{cite news |title=Kazakhstan |publisher=Land and Water Development Division |year=1998 |quote=Construction of a dam (Berg Strait) to stabilize and increase the level of the northern part of the Aral Sea.}}</ref>
|-
! scope="row" | Flood prevention
| The [[Keenleyside Dam]] on the [[Columbia River]], Canada can store {{convert|8.76|km3||abbr=on|lk=on}} of floodwater, and the [[Delta Works]] protects the Netherlands from [[coastal flooding]].<ref>{{cite web|url=http://www.nae.usace.army.mil/recreati/bwd/bwdfc.htm |title=Blackwater Dam |publisher=US Army Corps of Engineers |quote=The principal objective of the dam and reservoir is to protect downstream communities |url-status=dead |archive-url=https://web.archive.org/web/20130228142956/http://www.nae.usace.army.mil/recreati/bwd/bwdfc.htm |archive-date=28 February 2013 }}http://www.nae.usace.army.mil/Missions/Recreation/BlackwaterDam.aspx</ref>
|-
! scope="row" | Land reclamation
| Dams (often called [[levee|dykes]] or [[levee]]s in this context) are used to prevent ingress of water to an area that would otherwise be submerged, allowing its [[Land reclamation|reclamation]] for human use.
|-
! scope="row" | Water diversion
| A typically small dam used to divert water for irrigation, power generation, or other uses, with usually no other function. Occasionally, they are used to divert water to another drainage or reservoir to increase flow there and improve water use in that particular area. See: [[diversion dam]].
|-
! scope="row" | Navigation
| Dams create deep reservoirs and can also vary the flow of water downstream. This can in return affect upstream and downstream [[Navigability|navigation]] by altering the river's depth. Deeper water increases or creates freedom of movement for water vessels. Large dams can serve this purpose, but most often weirs and [[lock (water transport)|locks]] are used.
|}

Some of these purposes are conflicting, and the dam operator needs to make dynamic tradeoffs. For example, power generation and water supply would keep the reservoir high, whereas flood prevention would keep it low. Many dams in areas where precipitation fluctuates in an annual cycle will also see the reservoir fluctuate annually in an attempt to balance these different purposes. Dam management becomes a complex exercise amongst competing stakeholders.<ref>{{cite web |title=Lake Diefenbaker Reservoir Operations Context and Objectives |url=https://www.wsask.ca/Global/Lakes%20and%20Rivers/Dams%20and%20Reservoirs/Operating%20Plans/Developing%20an%20Operating%20Plan%20for%20Lake%20Diefenbaker/DiefenbakerReservoirOperationsContextandObjectives.pdf |publisher=Saskatchewan Watershed Authority |access-date=27 June 2013 |archive-date=6 October 2014 |archive-url=https://web.archive.org/web/20141006081822/https://www.wsask.ca/Global/Lakes%20and%20Rivers/Dams%20and%20Reservoirs/Operating%20Plans/Developing%20an%20Operating%20Plan%20for%20Lake%20Diefenbaker/DiefenbakerReservoirOperationsContextandObjectives.pdf |url-status=dead }}</ref>

=== Location ===
[[File:Takato Dam discharge.jpg|thumb|The discharge of [[Takato Dam]]]]

One of the best places for building a dam is a narrow part of a deep river valley; the valley sides can then act as natural walls. The primary function of the dam's structure is to fill the gap in the natural reservoir line left by the stream channel. The sites are usually those where the gap becomes a minimum for the required storage capacity. The most economical arrangement is often a composite structure such as a [[masonry]] dam flanked by earth embankments. The current use of the land to be flooded should be dispensable.

Significant other [[engineering]] and [[engineering geology]] considerations when building a dam include:
* [[Permeability (fluid)|Permeability]] of the surrounding rock or soil
* [[Earthquake]] faults
* [[Landslide]]s and [[slope stability]]
* Water table
* Peak flood flows
* Reservoir silting
* [[Environmental impact of reservoirs|Environmental impacts]] on river fisheries, forests and wildlife (see also [[fish ladder]])
* 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 ===
Impact is assessed in several ways: the benefits to human society arising from the dam (agriculture, water, damage prevention and power), harm or benefit to nature and wildlife, impact on the geology of an area (whether the change to water flow and levels will increase or decrease stability), and the disruption to human lives (relocation, loss of [[archeological]] or cultural matters underwater).

==== Environmental impact ====
{{main|Environmental impacts of reservoirs}}
[[File:Dam-pollution.JPG|thumb|Wood and garbage accumulation due to a dam]]

Reservoirs held behind dams affect many ecological aspects of a river. Rivers topography and dynamics depend on a wide range of flows, whilst rivers below dams often experience long periods of very stable flow conditions or sawtooth flow patterns caused by releases followed by no releases. Water releases from a reservoir including that exiting a turbine usually contain very little suspended sediment, and this, in turn, can lead to scouring of river beds and loss of riverbanks; for example, the daily cyclic flow variation caused by the [[Glen Canyon Dam]] was a contributor to [[sand bar]] [[erosion]].

Older dams often lack a [[fish ladder]], which keeps many fish from moving upstream to their natural breeding grounds, causing failure of breeding cycles or blocking of migration paths.<ref>Silva, S., Vieira-Lanero, R., Barca, S., & Cobo, F. (2017). Densities and biomass of larval sea lamprey populations (Petromyzon marinus Linnaeus, 1758) in north-western Spain and data comparisons with other European regions. ''Marine and Freshwater Research'', 68(1), 116–122.</ref> Even fish ladders do not prevent a reduction in fish reaching the [[spawn (biology)|spawning]] grounds upstream.<ref>Tummers, J. S., Winter, E., Silva, S., O'Brien, P., Jang, M. H., & Lucas, M. C. (2016). Evaluating the effectiveness of a Larinier super active baffle fish pass for European river lamprey Lampetra fluviatilis before and after modification with wall-mounted studded tiles. ''Ecological Engineering'', 91, 183–194.</ref> In some areas, young fish ("smolt") are transported downstream by [[barge]] during parts of the year. Turbine and power-plant designs that have a lower impact upon aquatic life are an active area of research.

At the same time, however, some particular dams may contribute to the establishment of better conditions for some kinds of fish and other aquatic organisms. Studies have demonstrated the key role played by tributaries in the downstream direction from the main river impoundment, which influenced local environmental conditions and beta diversity patterns of each biological group.<ref name="Lansac-Tôha, Fernando Miranda 2019">Lansac-Tôha, Fernando Miranda (2019).</ref> Both replacement and richness differences contributed to high values of total beta diversity for fish (average = 0.77) and phytoplankton (average = 0.79), but their relative importance was more associated with the replacement component for both biological groups (average = 0.45 and 0.52, respectively).<ref name="Lansac-Tôha, Fernando Miranda 2019"/> A study conducted by de Almeida, R. A., Steiner, M.T.A and others found that, while some species declined in population by more than 30% after the building of the dam, others increased their population by 28%.<ref name="Almeida, Ricardo 2018">Almeida, Ricardo (2018).</ref> Such changes may be explained by the fact that the fish obtained "different feeding habits, with almost all species being found in more than one group.<ref name="Almeida, Ricardo 2018"/>

A large dam can cause the loss of entire [[ecology|ecospheres]], including [[endangered species|endangered]] and undiscovered species in the area, and the replacement of the original environment by a new inland lake.  As a result, the construction of dams have been opposed in various countries with some, such as Tasmania's Franklin Dam project, being cancelled following environmentalist campaigns.<ref>{{Cite web |last=Stobbe Reimer |first=Asha Miriam |date=2021-09-30 |title=Tasmanian Wilderness Society blocks dam construction (Franklin River Campaign) 1981-83 |url=https://commonslibrary.org/tasmanian-wilderness-society-blocks-dam-construction-franklin-river-campaign-1981-83/ |access-date=2023-07-07 |website=The Commons Social Change Library |language=en-AU}}</ref>

Large reservoirs formed behind dams have been indicated in the contribution of [[earthquake|seismic activity]], due to changes in water load and/or the height of the water table. However, this is a mistaken assumption, because the relatively marginal stress attributed to the water load is orders of magnitude lesser than the force of an earthquake. The increased stress from the water load is insufficient to fracture the Earth's crust, and thus does not increase the severity of an earthquake.<ref>{{cite book |last1=Jain |first1=Sharad K. |last2=Singh |first2=V. P. |title=Water Resources Systems Planning and Management |date=12 September 2003 |publisher=Elsevier |isbn=978-0-08-054369-7 |page=408 |url=https://books.google.com/books?id=dR1sFWlUpPUC&pg=PA408 |language=en}} "However, a reservoir, at worst, can only advance an earthquake which would have occurred otherwise too. The magnitude of forces associated with an earthquake is several orders bigger compared to the additional load of water in the reservoir. The change in stresses due to water load is too small to cause fracture in the Earth's crust (Srivastava, 1993). Therefore, the presence of a reservoir does not increase the severity of an earthquake."</ref>

Dams are also found to influence [[global warming]].<ref>{{cite news|ssrn=1108425|title=The Potential of Water Power in the Fight Against Global Warming|first=Lea-Rachel|last=Kosnik|date=1 March 2008}}</ref> The changing water levels in reservoirs are a [[Greenhouse gas#Sources|source for greenhouse gases]] like [[methane]].<ref>{{cite web |url=http://frenchtribune.com/teneur/1212763-water-reservoirs-behind-rising-greenhouse-gases |title=Water Reservoirs behind Rising Greenhouse Gases |work=[[French Tribune]] |date=9 August 2012 |access-date=9 August 2012}}</ref> While dams and the water behind them cover only a small portion of earth's surface, they harbour biological activity that can produce large quantities of greenhouse gases.<ref>{{cite web|url=http://timesofindia.indiatimes.com/home/environment/global-warming/Dams-the-latest-culprit-in-global-warming/articleshow/15403985.cms |title=Dams the latest culprit in global warming |work=[[The Times of India]] |date=8 August 2012 |access-date=9 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120809003651/http://timesofindia.indiatimes.com/home/environment/global-warming/Dams-the-latest-culprit-in-global-warming/articleshow/15403985.cms |archive-date=9 August 2012 }}</ref>

==== Human social impact ====
Dams' impact on human society is significant. [[Nick Cullather]] argues in ''Hungry World: America's Cold War Battle Against Poverty in Asia'' that dam construction requires [[Government|the state]] to displace people in the name of the [[common good]], and that it often leads to abuses of the masses by planners. He cites [[Morarji Desai]], Interior Minister of India, in 1960 speaking to villagers upset about the [[Pong Dam]], who threatened to "release the waters" and drown the villagers if they did not cooperate.<ref>Cullather, 110.</ref>

The [[Three Gorges Dam]] on the [[Yangtze River]] in [[China]] is more than five times the size of the [[Hoover Dam]] ([[United States|U.S.]]). It creates a reservoir {{convert|600|km|abbr=on}} long to be used for [[flood control]] and hydropower generation. Its construction required the loss of over a million people's homes and their mass relocation, the loss of many valuable archaeological and cultural sites, and significant ecological change.<ref name="Bbc060520">{{cite news |publisher=China Embassy |url=http://www.china-embassy.org/eng/zt/sxgc/t36502.htm |title=Three Gorges dam wall completed |date=20 May 2006 |access-date=21 May 2006}}</ref> During the [[2010 China floods]], the dam held back a what would have been a [[Yangtze#Periodic floods|disastrous flood]] and the huge reservoir rose by 4&nbsp;m (13&nbsp;ft) overnight.<ref>{{Cite news|url=https://www.bbc.co.uk/news/mobile/world-asia-pacific-10695272|publisher=BBC News|title= China's Three Gorges dam faces flood test|date=20 July 2010}}</ref>

In 2008, it was estimated that 40–80&nbsp;million people worldwide have been displaced from their homes as a result of dam construction.<ref>{{cite web |url=http://internationalrivers.org/en/way-forward/world-commission-dams/world-commission-dams-framework-brief-introduction |title=World Commission on Dams Report |website=International Rivers |date=29 February 2008 |access-date=16 August 2012 |archive-date=13 September 2008 |archive-url=https://web.archive.org/web/20080913152808/http://internationalrivers.org/en/way-forward/world-commission-dams/world-commission-dams-framework-brief-introduction |url-status=dead }}</ref>

==== Economics ====
Construction of a [[hydroelectric plant]] requires a long lead time for site studies, [[hydrological]] studies, and [[environmental impact assessment]]s, and are large-scale projects in comparison to carbon-based power generation. The number of sites that can be economically developed for hydroelectric production is limited; new sites tend to be far from population centers and usually require extensive [[power transmission]] lines. Hydroelectric generation can be vulnerable to major changes in the [[climate]], including variations in [[rainfall]], ground and surface [[water level]]s, and glacial melt, causing additional expenditure for the extra capacity to ensure sufficient power is available in low-water years.

Once completed, if it is well designed and maintained, a hydroelectric power source is usually comparatively cheap and reliable. It has no fuel and low escape risk, and as an [[Sustainable energy|clean energy]] source it is cheaper than both nuclear and wind power.<ref>{{cite web|url=http://en.openei.org/apps/TCDB/|title=Transparent Cost Database – Transparent Cost Database|website=en.openei.org}}</ref> It is more easily regulated to store water as needed and generate high power levels on demand compared to [[wind power]].

==== Reservoir and dam improvements ====
Despite some positive effects, the construction of dams severely affects river ecosystems leading to degraded riverine ecosystems as part of the hydrological alteration.<ref name="Ren, Kang 2019">Ren, Kang (2019).</ref> One of the main ways to reduce the negative impacts of reservoirs and dams is to implement the newest nature-based reservoir optimization model for resolving the conflict in human water demand and riverine ecosystem protection.<ref name="Ren, Kang 2019"/>

==== Dam removal ====
{{Main|Dam removal}}
[[Water]] and [[sediment]] flows can be re-established by removing dams from a river. Dam removal is considered appropriate when the dam is old and [[maintenance costs]] exceed the expense of its removal.<ref name="Bellmore etal 2017">{{Cite journal|last1=Bellmore|first1=J. R.|last2=Duda|first2=J. J.|last3=Craig|first3=L. S.|last4=Greene|first4=S. L.|last5=Torgersen|first5=C. E.|last6=Collins|first6=M. J.|last7=Vittum|first7=K.|date=2017|title=Status and trends of dam removal research in the United States|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/wat2.1164|journal=WIREs Water|language=en|volume=4|issue=2|pages=e1164|doi=10.1002/wat2.1164|s2cid=114768364 |issn=2049-1948}}</ref> Some effects of dam removal include [[erosion]] of sediment in the [[reservoir]], increased [[Sediment transport|sediment supply]] downstream, increased river width and [[Braided river|braiding]], re-establishment of natural water temperatures and [[Recolonization|recolonisation]] of [[habitat]]s that were previously unavailable due to dams.<ref name="Bellmore etal 2017" />

The world's largest [[dam removal]] occurred on the [[Elwha River|Elwha river]] in the [[Washington (state)|U.S. state of Washington]] (see [[Restoration of the Elwha River]]). Two dams, the [[Elwha Dam|Elwha]] and [[Glines Canyon Dam|Glynes Canyon dams]], were removed between 2011 and 2014 that together stored approximately 30 [[Megatonnes|Mt]] of sediment.<ref name="Bellmore etal 2017" /><ref name="Ritchie etal 2018">{{Cite journal|last1=Ritchie|first1=A. C.|last2=Warrick|first2=J. A.|last3=East|first3=A. E.|last4=Magirl|first4=C. S.|last5=Stevens|first5=A. W.|last6=Bountry|first6=J. A.|last7=Randle|first7=T. J.|last8=Curran|first8=C. A.|last9=Hilldale|first9=R. C.|last10=Duda|first10=J. J.|last11=Gelfenbaum|first11=G. R.|date=2018|title=Morphodynamic evolution following sediment release from the world's largest dam removal|url= |journal=Scientific Reports|language=en|volume=8|issue=1|pages=13279|doi=10.1038/s41598-018-30817-8|issn=2045-2322|pmc=6125403|pmid=30185796|bibcode=2018NatSR...813279R}}</ref> As a result, the delivery of [[sediment]] and [[wood]] to the downstream [[river]] and [[River delta|delta]] were [[Restoration of the Elwha River|re-established]]. Approximately 65% of the sediment stored in the [[reservoir]]s eroded, of which ~10% was deposited in the [[Stream bed|riverbed]]. The remaining ~90% was transported to the [[coast]]. In total, renewed sediment delivery caused approximately 60 ha of [[River delta|delta]] growth, and also resulted in increased [[Braided river|river braiding]].<ref name="Ritchie etal 2018" />

== Failure ==
{{main|Dam failure}}
[[File:(IDAHO-L-0010) Teton Dam Flood - Newdale.jpg|thumb|[[Teton Dam]] failure]]

Dam failures are generally catastrophic if the structure is breached or significantly damaged. Routine [[deformation monitoring]] and monitoring of seepage from drains in and around larger dams is useful to anticipate any problems and permit remedial action to be taken before structural failure occurs. Most dams incorporate mechanisms to permit the reservoir to be lowered or even drained in the event of such problems. Another solution can be rock [[grout]]ing&nbsp;– [[Pressure grouting|pressure pumping]] [[Portland cement]] [[slurry]] into weak fractured rock.

[[File:International special sign for works and installations containing dangerous forces.svg|thumb|left|International special sign for works and installations containing dangerous forces]]
During an armed conflict, a dam is to be considered as an "installation containing dangerous forces" due to the massive impact of possible destruction on the civilian population and the 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 may cause severe losses among the civilian population. To facilitate the identification, a [[protective sign]] consisting of three bright orange circles placed on the same axis is defined by the rules of IHL.

[[File:The Great Conemaugh Valley Disaster.jpg|thumb|[[South Fork Dam]] failure and resulting [[flood]] that destroyed [[Johnstown, Pennsylvania|Johnstown]] in Pennsylvania in 1889]] 
The main causes of dam failure include inadequate spillway capacity, piping through the embankment, foundation or abutments, spillway design error ([[South Fork Dam]]), geological instability caused by changes to water levels during filling or poor surveying ([[Vajont Dam|Vajont]], [[Malpasset]], [[Testalinden Creek]] dams), poor maintenance, especially of outlet pipes ([[Lawn Lake Dam]], [[Val di Stava Dam collapse]]), extreme rainfall ([[Shakidor Dam]]), [[earthquake]]s, and human, computer or design error ([[Buffalo Creek Flood]], [[Dale Dike Reservoir]], [[Taum Sauk pumped storage plant]]).

A notable case of deliberate dam failure (prior to the above ruling) was the [[Royal Air Force]] [[No. 617 Squadron RAF|'Dambusters']] raid on [[Germany]] in [[World War II]] (codenamed "[[Operation Chastise]]"), in which three German dams were selected to be breached in order to damage German infrastructure and manufacturing and power capabilities deriving from the [[Ruhr (river)|Ruhr]] and [[Eder (Fulda)|Eder]] rivers. This raid later became the basis for several films.

Since 2007, the Dutch [[IJkdijk]] foundation is developing, with an [[open innovation]] model and early warning system for levee/dike failures. As a part of the development effort, full-scale dikes are destroyed in the IJkdijk fieldlab. The destruction process is monitored by sensor networks from an international group of companies and scientific institutions.

== See also ==
{{commons|Dam}}
* {{annotated link|Bunding}}
* {{annotated link|Grout curtain}}
* {{annotated link|Ice jam|Ice dam}}
* {{annotated link|Inflatable rubber dam}}
* {{annotated link|International Commission on Large Dams}}
* [[List of dams and reservoirs]]
* [[List of largest dams]]
* [[List of tallest dams]]
* {{annotated link|List of tidal barrages}}
* {{annotated link|Lock (water navigation)}}
* {{annotated link|Reservoir safety }}

== Notes==
{{Reflist}}

== Sources ==
* {{cite journal
  | last1 = Arenillas
  | first1 = Miguel
  | last2 = Castillo
  | first2 = Juan C.
  | title = Dams from the Roman Era in Spain. Analysis of Design Forms (with Appendix)
  | journal = 1st International Congress on Construction History [20th–24th January]
  | year = 2003
  | url = http://www.traianvs.net/textos/presas_in.htm#_ednref4
  }}
* {{cite journal
  | last1 = Almeida
  | first1 = Ricardo
  | title = A case study on environmental sustainability: A study of the trophic changes in fish species as a result of the damming of rivers through clustering analysis
  | journal = Computers & Industrial Engineering
 | year = 2018
  | volume = 135
 | pages = 1239–1252
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==Further reading==
* Khagram, Sanjeev. ''Dams and Development: Transnational Struggles for Water and Power''. Ithaca: Cornell University Press 2004.
* McCully, Patrick. ''Silenced Rivers: The Ecology and Politics of Large Dams''. London: Zed. 2001.

== External links ==
{{Wiktionary|dam}}
* [http://www.des.nh.gov/organization/commissioner/pip/factsheets/db/documents/db-1.pdf Basic Terms of Dam Characteristics] {{Webarchive|url=https://web.archive.org/web/20201130015246/https://www.des.nh.gov/organization/commissioner/pip/factsheets/db/documents/db-1.pdf |date=30 November 2020 }}
* [https://web.archive.org/web/20130324052009/http://www.mathalino.com/reviewer/fluid-mechanics-and-hydraulics/analysis-gravity-dam Gravity Dam Analysis]
* [http://en.structurae.de/structures/stype/index.cfm?doi=3 Structurae: Dams and Retaining Structures]

{{Portal bar|Engineering}}

{{Hydropower}}
{{Authority control}}

[[Category:Barrages (dam)]]
[[Category:Dams| ]]
[[Category:Hydraulic structures]]