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This article is about structures for water impoundment. For other uses, see Dam (disambiguation).

A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, 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 floodgates or levees (also known as dikes) are used to manage or prevent water flow into specific land regions.

Ancient dams were built in Mesopotamia, the Middle East, and China for water control. Possibly the earliest known dam is the Jawa Dam in Jordan, dating to 3,000 BC. Dams of a similar age have also been attributed to the Liangzhu culture, of the Yangtze Delta. 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.

Etymology

The English word "dam" is found in Middle English, and traces back to the word dam in Germanic languages Middle Low German, Middle Dutch, and Old Norse. Roots of the word include Gothic faur-dammjan ('to stop up'), and the Indo-European base *dhē- ('to set, put in place').[1]

History

Main article: History of dams
Image
The Proserpina Dam in Spain was built by the Romans, and has been in use for almost two millennia.[2]
Image
The Band-e Kaisar dam in Iran was built in the 3rd century CE.[3]
Image
The Kurit Dam was built around 1350 CE.[4]
A dam in arid hills, with water spilling over the crest.
The Elche Dam in Spain was the first true arch dam built in Europe since Roman times.[5]
A stone dam with a reservoir behind it, surrounded by trees
In Australia the Parramatta Dam tested the limits of how thin a dam could be.[6]

Antiquity

The earliest known dam is the Jawa Dam near Amman, Jordan, built around 3000 BCE. This embankment dam was part of an elaborate irrigation system, and was 28 m (92 ft) thick[a] and 5.5 m (18 ft) high.[8][b] Around 2600 BCE, Egyptians built the Sadd el-Kafara embankment dam near Cairo, although it failed around the time its construction was completed. Some of the stone blocks weighed 300 kg (660 lb).[10] The Sabaean peoples built a series of dams across the Wadi Danah, located in modern Yemen, starting around 1500 BCE, culminating in the Great Dam of Marib (built around 500 BCE) which was 700 m (2,300 ft) long and 20 m (66 ft) high.[11]

The Hittite Empire built several dams between the 17th and 13th centuries BCE, including the Eflatun Pınar dam and spring temple near modern Konya, Turkey.[12] An early dam in China – built by engineer Sunshu Ao around 580 BCE – impounded the Afengtang Reservoir which is still in existence today.[13] In Sri Lanka, several dams – including Tissa Wewa – were built around 370 BCE to create reservoirs; some of the dams were several kilometers long.[14][c]

Roman era

The Roman Empire constructed major waterworks – including aqueducts and tunnels – starting in the 5th century BCE, but they did not begin building significant dams until the first century CE.[16] Roman dams were typically masonry gravity dams with vertical faces on both upstream and downstream sides, although some were reinforced on the downstream side with buttresses or rock embankments.[17] The Romans were the first to use cement as a construction material, which could be mixed with small rocks to form concrete, or mixed with sand to form mortar to join bricks or stones. Some Roman cements, particularly those containing volcanic ash, were waterproof.[18][d]

One of the earliest dams built by the Romans was also the tallest they built: the Subiaco Dam, built around 60 CE, stood 40 m (130 ft) tall and 13.5 m (44 ft) thick.[19][e] The Romans built about 80 dams in Hispania (modern Spain),[20] including the Proserpina Dam, which impounded 6 million m3 of water. The dam was still operational in 2026.[2] Roman dam technology was applied by neighboring countries: after Persian king Shapur I defeated Roman emperor Valerian, he put defeated Romans to work building the Band-e Kaisar dam, which also functioned as a 40-arch bridge spanning the Karun River.[3]

Post-classical Asia and Middle Ages

One of the earliest dams built in Japan was the Sayama embankment, built near Osaka in 380 CE, which was 8 m (26 ft) high and 300 m (980 ft) long.[21] The Kurit Dam – the world's first large, thin arch dam – was built in Persia (modern-day Iran) around 1350 CE. Its height was initially 26 m (85 ft) and was later raised to 64 m (210 ft); it remained the world’s tallest dam until the start of the 20th century.[22] Dams in India were typically earthen dams with steep faces faced with stone. A notable example is the Veeranam Dam, built around 1020 CE in Tamil Nadu, which is 16 km (9.9 mi) long.[15]

In Europe, dams were used to power water wheels for milling and mining.[23][f] An early example was the Bazacle weir built around 1170 CE in France.[25] Dams to create fish ponds were common in Europe, and hundreds were built in Bohemia during the 15th and 16th centuries, creating ponds covering a total of 1,800 km2.[26] Dams for irrigation included the Almansa Dam – a gravity/arch dam built in 1384 in Spain; and the Elche Dam (built in 1640 and still standing) – the first true arch dam built in Europe since Roman times.[5] Several dams were built to supply Istanbul with water, including one designed by Mimar Sinan in 1560 to bring water from Belgrad Forest.[27] Another purpose of canals was transportation: the Saint-Ferréol Dam was built in France in 1675 to provide water for the Midi Canal. It remained the highest earthen dam in the world for over a century.[28] Several books on the subject of dam design and construction were published in the 1600s and 1700s, by authors including Jacob Leupold, Albert Brahms, Johann Silberschlag, and Oliver Evans.[29]

Industrial Revolution

In the late 18th century, the process of designing dams began to transform from an informal practice based on experience, to an engineering discipline rooted in science. Important figures that contributed to this evolution included French scientist Charles-Augustin de Coulomb who, in 1776, created a formula that described how soil reacts under stress,[30] a theory that was later given practical application to dams by Alexandre Collin.[31] Claude-Louis Navier developed the theory of elasticity in 1826.[32] In 1847, François Zola became the first engineer to design an arch dam based on an analytical consideration of stresses.[33] French engineer J. Augustine DeSazilly established that the best cross-section for a gravity dam was a triangle, with a vertical face on the upstream side.[34] Scottish physicist William John Macquorn Rankine developed a theory governing retaining walls in the 1850s which was applicable to dams.[35]

These scientific foundations led to safer, larger dams of all types. The Glencorse Dam in Britain (1824) was a 21 m (69 ft) high embankment dam that contained a clay core and had gently sloping faces.[36] In France, the Gouffre d'Enfer masonry gravity dam (1866) was 60 m (200 ft) tall.[37] The world's first large buttress dam was Mir Alam Dam (1804) in India.[6] In Australia, an arch dam – the Parramatta Dam (1856) – tested the limits of how thin a dam could be.[6]

Modern era

Image
The Three Gorges Dam in China

In the first half of the 20th century, many large dams were built, particularly in western Europe and the US.[38] After WW II, the availability of power construction machinery such as bulldozers, dump trucks, and scrapers contributed to an explosion in the number of large dams.[39] The 1933 invention of grout curtain technologies enabled dams to be safely built on top of porous soils.[40][g] This enabled the Aswan High Dam to be built across the Nile River, which has a deep, sandy riverbed: grout was pumped 208 m (682 ft) deep into the riverbed (spanning 57,000 m2), preventing water from flowing underneath the dam.[40]

Notable dams built in the modern era include:

  • Itaipu Dam (Brazil/Paraguay, 1984) An example of international cooperation.[47]

The modern era also saw the emergence of arguments against dam construction, starting as early as the 1870s with objections to the Thirlmere Dam in Britain.[50] In 1906, a seven-year battle was fought over the construction of the Hetch Hetchy Dam in California, which was eventually built and flooded a valley in Yosemite National Park that opponents claimed was as scenic as the famed Yosemite Valley.[50] After climate change became a global concern, debates emerged arguing whether the electricity produced by dams was as clean as solar power or wind generation. Although hydroelectricity itself is clean, dam opponents argue that adverse environmental impacts[h] cancel any benefits.[51]

Number of dams in the world

Dam types[52][i]
  1. Earthen embankment 37,592 (60.3%)
  2. Gravity 6,777 (10.9%)
  3. Rockfill embankment 4,565 (7.32%)
  4. Arch 2,302 (3.69%)
  5. Buttress 384 (0.62%)
  6. Barrage 343 (0.55%)
  7. Multiple arch 102 (0.16%)
  8. Other 10,298 (16.5%)

The number of large[j] dams in the world in 2025 was 62,362, according to the International Commission on Large Dams (ICOLD).[52] The total number of reservoirs (large and small) in 2011 was estimated to be 16.7 million.[53][k][l] These reservoirs store an estimated 8,070 km3 of water, which is about 10% of the volume of the Earth's natural freshwater lakes.[53][l] The reservoirs cover about 305,000 km2 of the planet's surface, which is about 7.3% of the area covered by natural lakes.[53][l] About 7.6% of the world's rivers are significantly impacted by reservoirs and 46.7% of large rivers are affected.[53][l] In 2015, the number of hydropower dams planned or under construction was 3,700, with most in China (highest total generation capacity), Brazil (highest number of planned dams), and India.[54]

Types

Dams can be classified by structure. The major structural types are embankment, gravity, buttress, and arch. Other forms include hybrid dams and rockslide dams.

Embankment dam

Main article: Embankment dam
Image
A typical embankment dam has a clay core and gradually sloping faces both upstream and downstream.[55]
Image
The Misogawa Dam is an embankment dam in Japan.

The most common dam structure is an embankment dam.[56] These dams consist of a pile of earth (rocks, clay, sand, gravel, soil, etc) placed in such a way to impound a reservoir or block the flow of water.[57] Embankment dams are the only type of modern dam not made of concrete.[58] There are several advantages of embankment dams: they can be built from locally available dirt and rocks, as opposed to importing rocks and cement required for concrete dams; they tend to be less expensive to build; and they can be built on softer soils because their broad base spreads their weight over a greater area (as opposed to heavy gravity dams that require bedrock foundations).[59]

The primary drawback to embankment dams is that they are inherently porous, so water can seep through the dam or underneath the dam.[60] Mitigation techniques to reduce seepage include placing a drainage system underneath the dam; injecting grout into the soil below the dam; and including a vertical layer of impervious material within the dam.[61] If an impervious layer is included, it may be made of clay, cement, or bitumen.[62][m] Failure to properly mitigate seepage can lead to dam failure caused by "piping": when water starts to flow through (or under) the dam in a small channel, which gradually enlarges until a large hole is pierced through the dam.[64]

Early embankment dams were often built of a single type of earth, but starting in the mid-16th century, dam engineers began to use several types of material, carefully layered in zones.[65] A typical zone pattern for embankment dams is a clay center (a vertical wall, extending from the riverbed to the crest of the dam), with gradually sloped banks of soil on both upstream and downstream sides, and both faces covered with large rocks.[66] Large rocks on the upstream face protect the structure from wave action.[67] The resistance to water seepage varies widely between the various materials: clay resists water seepage 10x more than silt, 10,000x more than sand, and 100,000,000x more than gravel.[68]

Gravity dam

Main article: Gravity dam
Image
A typical gravity dam has a nearly vertical upstream face, and a broad base.[37]
Image
The Grande Dixence Dam in Switzerland is one of the world's tallest gravity dams.[citation needed] The people atop the dam's crest show its size.

Gravity dams rely on their weight to remain immobile and resist the forces exerted by the upstream waters. In the past, gravity dams were built of masonry (stone, brick, or rubble) with mortar filling the joints; in the modern era, nearly all are made of concrete.[69][n] Concrete gravity dams can be solid or hollow.[71]

The crest (top) of gravity dams is a straight line stretching between the walls of the valley it crosses. When the crest of a gravity dam is curved (the convex side of the curve always faces upstream) it is called an arch-gravity dam (discussed below).[72][o] The cross-section of gravity dams is roughly triangular, with a flat bottom resting on the valley floor, and two inclined faces (upstream and downstream) that meet at the crest.[p] The upstream face is typically more vertical than the downstream face, to ensure stability.[74] The thickness of the base of a gravity dam is typically 70 to 85% of the height.[75][a]

Because gravity dams are so heavy, they must rest on bedrock; a gravity dam built over soil would compress the soil, cause the dam to settle, and perhaps crack and fail.[q] If the bedrock has cracks or defects, it must be prepared by injecting grout or placing concrete plugs.[77] A concern that designers must address is "uplift": if water seeps under the dam structure, the water pressure can apply extreme upward force on the dam structure, which may result in leaks or even dam failure. This risk can be mitigated with the use of grout curtains under the dam (which prevent water from seeping under the dam) and drainage systems under the dam, which lead water away when pressure increases.[78]

When concrete cures, it generates heat. For large dams this excess heat in the interior of the dam can cause the concrete to crack. To mitigate this issue, expansion joints can be included within the dam to permit the concrete to shrink without cracking. After the heat dissipates, the expansion joints are filled with grout.[79]

Buttress dam

Main article: Buttress dam
Image
A typical buttress dam has an inclined upstream face and multiple buttresses.[80]
Image
About 20 buttresses are visible in this photo of the Latyan buttress dam in Iran.[81][r]

A buttress dam consists of a flat upstream face supported on the downstream side by numerous triangular buttresses.[s] Most buttress dams are made of concrete.[84] Unlike a gravity dam (where the upstream face is nearly vertical) the upstream face of a buttress dam is sloped, typically with an inclination between 0.7 and 1.67.[t] The slant is required so the force of the upstream reservoir pushes downward onto the dam, forcing it into the ground, and increasing its stability (contrasted with gravity dams, where the dam's weight alone is sufficient to remain immobile).[80]

Buttress dams use much less concrete than a comparable gravity dam, but those cost savings are offset by a more complex construction process. Buttress dams are not as strong as gravity dams, and are suited only for lower heights. Because buttress dams have a much smaller footprint (the area of ground the dam covers) than gravity dams, the risks associated with uplift forces (from water under the dam) are lower in buttress dams.[85]

The individual buttresses may experience slight movements relative to each other. If the upstream face of the dam were a solid piece of concrete, the movements of the buttresses could introduce large stresses, resulting in cracking of the upstream dam face. To mitigate this, the upstream face is divided into multiple pieces, one per buttress, called the "buttress heads". Adjacent buttress heads are typically separated by a gap, and the gaps are filled with flexible seals.[82]

Arch dam

Main article: Arch dam
Image
A typical arch dam, viewed from above
Image
The Gordon Dam in Tasmania is an arch dam. The shape transfers the weight of the water into the valley walls.[86]

An arch dam is a curved dam that transfers the force of the impounded waters to the valley walls (in contrast to gravity or buttress dams, which transfer the force to the foundation below the dam).[86] Arch dams can only be built at a location in a valley where the valley is relatively narrow and has strong, steep rock walls.[87] Arch dams are relatively thin: the thickness of their base is less than half their height.[a] They are always made of concrete or masonry.[88] The central angle subtended by an arch dam can be relatively shallow or nearly semicircular: arch dams exist with central angles from 46 degrees to 140 degrees.[89][u]

All arch dams are curved, but there are a variety of shapes they may assume. Most older arch dams used a "constant radius" shape, which resembles a section of a vertical cylinder.[91][v] A more complex shape is the "constant angle" shape, which gradually reduces radius from the crest to the base.[w] Research into optimizing dam shapes for maximum strength led dam engineers to adopt the constant angle shape for many arch dams, with the first example built in 1914.[92] Another shape is the "double curved" or "cupola" which resembles a section of a dome, and is defined by incorporating curvature in the vertical direction, as well as the horizontal direction.[93]

Regardless of the shape of an arch dam's curvature, the dam must transfer the weight of the reservoir water into the valley walls. Tremendous forces are passed from the dam into the valley walls where they meet, so the valley walls must consist of strong rock. In some dams, concrete abutments must be constructed between the dam's arch and the valley walls to safely transfer the load.[87]

Hybrid structures

Image
The Sayano-Shushenskaya Dam in Russia is an example of an arch-gravity dam.
A painting of some dead fish, from the Tomb of Menna
Canada's Daniel-Johnson Dam is a multiple-arch dam.

Many dams combine features from two or more of the basic dam structures. An arch-gravity dam combines features from arch dams and gravity dams: the overall shape is an arch, but it is not a true arch dam because the thickness of the dam's base is more than half of its height – giving it a weight and footprint that is characteristic of gravity dams.[94][x][a]

A multiple-arch dam[y] combines features of arch dams with buttress dams. It is similar to a buttress dam, but the upstream face is not flat – rather, the face consists of a number of small arch dams: one arch connecting each pair of adjacent buttresses.[97][z]

Rockslide dams

Image
The Usoi Dam is a rockslide dam formed in 1911 by an earthquake. At 567 m (1,860 ft) high, it is the tallest dam in the world.[99]

A rockslide dam is a natural dam formed by a rockslide that slides into a valley and blocks the flow of a river, forming a lake on the upstream side.[100] There are thousands of rockslide dams around the world, including one created in 2010 in Pakistan that formed Attabad Lake.[100] Rockslide dams have the potential to cause catastrophic loss of life, if they fail and create an outburst flood. In 1786 in China, an earthquake created a rockslide dam on the Dadu River, which failed ten days later, killing 100,000 people.[101] Risks of outburst floods can be mitigated by building spillways on rockslide dams to lower the water level.[101] Engineers have used rockslide dams as foundations upon which to build new dams.[102] Rarely, engineers have used blasting on mountainsides to trigger a rockslide and create a crude embankment dam, called a "blast-fill" dam.[102] Not all natural dams are created by rockslides: volcanic dams are the result of volcanic activity, which can create dams from lava flows, lahar deposits, pyroclastic flows, or other debris.[103]

Uses

Primary purposes

Dam usage[52][aa]
  1. Irrigation 14,668 (47.5%)
  2. Hydropower 5,980 (19.4%)
  3. Water supply 3,394 (11.0%)
  4. Flood control 2,510 (8.14%)
  5. Tailings 1,510 (4.89%)
  6. Recreation 1,653 (5.36%)
  7. Navigation 63 (0.20%)
  8. Fish farming 77 (0.25%)
  9. Others 996 (3.23%)

The main purposes that dams serve include irrigation, hydropower, water supply, flood management, recreation, inland navigation, and fish farming.[104] Many dams – called "multi-purpose dams" – support two or more of these primary purposes.[105] For example, the Grand Coulee Dam in the US supports both hydropower and irrigation.[citation needed]

Irrigation is a critical application of dams: in 2006, between 12% and 15% of the world's population relied on food that was irrigated by water that originated in reservoirs impounded by dams.[106] In addition to directly moving water from the reservoir to irrigation canals, dams can also support irrigation by "dry-season releases": the dam impounds water during the wet season, and releases it downstream into the river during the dry season, thus ensuring water in the river year-round.[107]

Hydropower has been an important use of dams, not only to generate hydroelectricity, but also in the form of waterwheels that power mills.[24] As of 2004, global hydropower capacity totaled 740 GW, with a total annual production of 2,800 TW-hours per year, accounting for about 20% of the world's electricity supply.[24] More than 80% of the world's reservoir water storage capacity is used to generate hydropower.[106] Hydropower dams can act as an annual buffering system: the reservoir can be filled during the rainy season, then during the dry season (when it is typically hotter and electricity is needed to run air conditioning systems) the water can be released to generate electricity.[108] Some hydropower dams provide a pumped-storage capability: such dams can consume excess electricity (for example, from solar power on a sunny day) to drive pumps that lift water into their reservoir. When the electrical grid needs more power (for example, on a cloudy day) the water can be released to power the dam's generators to create hydroelectricity.[109][citation needed] A pumped-storage capability can also be used in a 24-hour cycle: during the night, when community use of electricity is low, conventional power sources (nuclear, oil) can power pumps to lift water into reservoirs; then – during the peak consumption hours in daytime – the water can be released through the dam's generators to generate electricity.[108][citation needed]

Many dams supply water for domestic or industrial use. In 2025, there were 3,394 large dams used for water supply.[52] Industrial usage is about twice domestic usage, but some of the water withdrawn from reservoirs (such as water used solely for cooling purposes) is returned to the river system.[110]

Flood management is an important role that many dams fill. In 2025, there were 2,510 large dams in the world devoted to flood management. These dams do not try to prevent all floodwaters from reaching downstream, instead they try to reduce the peak flood level (height) to a safe limit. Since floods are so unpredictable, these goals are typically expressed as statistical margins based on lengthy return periods.[ab] For example, a dam may be designed with the goal of reducing the flood peak by 50% for 1-in-100 year floods.[111]

Many dams are built on rivers for the purpose of keeping the water level sufficiently high to support transportation, including barges that carry freight. These dams are typically low, and are found in countries that have industries that require cargo to be transported on waterways.[112]

Some dams are designed with the primary goal of supporting recreation or fish farming.[112]

Other purposes

Not all dams are created to support the primary purposes listed above: some dams support other purposes (often in addition to primary purposes).

Image
A cluster of tailings dams in England
A large concrete structure in the middle of a river, kept dry by a steel wall surrounding it
The rusty steel panels are a temporary cofferdam which is keeping the worksite dry while a concrete bridge pier is being built in a river.[113]
Image
A weir in Spain

A tailings dam is a dam that impounds tailings – the waste from mining operations.[114] Most tailings dams are embankment structures.[115] Unlike a normal water-impound dam – which is almost always built in a valley – tailings dams may be built on flat ground, with the embankment constructed in a rectangular shape that encloses the tailings on all sides.[116] Tailings dams are unique because they are often enlarged over time: as mine operations continue, the embankments are repeatedly raised.[117] Tailings often include toxic by-products of mining, such as arsenic or lead. Therefore, tailings dams usually incorporate special protective measures to ensure that materials from the tailings do not contaminate the water supply outside the dam.[118]

A cofferdam is a temporary dam built at a construction site to keep water out of the site until the job is completed.[119] Cofferdams are commonly used when building bridge supports in lakes, rivers, and oceans.[120] When building a dam in a river valley, cofferdams are often required upstream, where they divert the river into temporary tunnels or channels that carry the river around the construction site, and then release the water downstream.[119]

A weir is a straight, flat, low structure built across a riverbed. Weirs are not designed to fully block a river, but rather to regulate the flow in a controlled way.[121] Some weirs are used to create a segment of the river that has a fixed level;[122] others are designed to minimize erosion of the river banks;[123] some are for landscaping or recreation purposes;[124] and other weirs are used as measuring gauges (the total water flow can be readily computed by measuring the depth of the water passing over the weir).[125]

A saddle dam raises the height of a saddle (low point) in the ridge surrounding a reservoir. Saddle dams supplement a primary dam, and are built at the same time. They are only needed if the ridge surrounding the primary dam's reservoir contains a low point which is below the primary dam's water level. The saddle dam will prevent overflow when the reservoir is filled.[126][ac]

A diversion dam that diverts a portion of a river's flow into a canal, which transports the water to another location where it is used for irrigation, hydropower, or other purposes.[128] A detention dam does not create a permanent reservoir, but instead regulates the flow of water in a valley to minimize the risk of flooding downstream.[129]

Underground dams are used to block the flow of groundwater and store it below the surface. Underground dams are small-scale structures constructed in arid regions where water is scarce. Some underground dams are built by digging a trench in the path of naturally flowing groundwater and placing a vertical, impervious barrier, then refilling the trench. Another design, used in sandy regions, is to build a low dam across a small valley so that occasional rainstorms will cause sand and water to accumulate behind the dam (the sand will inhibit evaporation of the groundwater). Under either design, a well or pipe is placed upstream of the barrier to withdraw the water.[130]

Design

Design process

The process of designing a dam can be undertaken in three stages: reconnaissance, feasibility, and project planning.[131] In the reconnaissance stage, designers visit the site, study it carefully, and gather all available geological, seismic, and topographical data. In the feasibility stage, detailed technical investigations are performed to assess the geology, hydrology, and hydraulics of the site. Inquiries are made into land acquisition, public utility availability, and location of construction materials (such as rocks and soil for landfill). Analysis of environmental and flood impacts are started.[131] In the planning stage, detailed design plans are created, a construction schedule is established, and cost estimates are prepared.[131]

Technical surveys and investigations

During the planning process for a dam, a large number of surveys and technical investigations are typically conducted. These investigations may be categorized as topographic, geological, and hydrological.[132] Topographic surveys are one of the first steps in planning a dam. Surveyors map the construction site and prepare detailed topographic maps of the region. The maps must be very precise, since virtually every aspect of the dam's construction will rely on the data.[133]

The geological investigations study the rocks and soil of the dam site. The dam  – and the water it impounds – will exert very large forces on the ground beneath the dam structure, on the valley walls where the dam abuts them, and on the ground beneath the reservoir. An accurate understanding of the strength of the ground, and identifying any faults, is essential to minimizing seepage and reducing the risk of dam failure.[134] The hydrological investigations examine all aspects of water flow in the vicinity of the dam. Data is produced which identifies the size of the upstream watershed and how much precipitation falls each year. Studies are performed to determine how much water flows through the dam site in an average year, how much it varies within a year, and how much it varies from year to year.[135]

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).[citation needed]

Environmental impact

Main article: Environmental impacts of reservoirs

Prior to building a new large dam, most countries require the developers to prepare an Environmental impact assessment (EIA) which documents the consequences the dam (and its reservoir) will have on communities and the environment.[136] The EIA identifies potential adverse impacts of the dam, and enables the developer and government to mitigate the impacts and compensate people adversely affected.[136]

There are many ways the environment can be impacted by construction of a dam and its reservoir. The land occupied beneath the reservoir is permanently lost to other uses, including agriculture, forest, or human habitation. Animals that lived in the area may permanently lose their habitat.[137] People who lived in the reservoir location must relocate to new homes, which can cause large-scale social disruption. For example, the Three Gorges Dam in China required the relocation of 1.4 million people.[138] Land adjacent to the reservoir may become saturated with water, impacting agriculture and increasing soil salinity.[139] The level of groundwater (underground water) surrounding the reservoir may rise, and the quality of groundwater that people pump from wells may degrade.[139]

Downstream of the dam, the flow of the river may be reduced, especially in the dry season. The quality of the downstream river water may also suffer.[139] Many rivers normally carry sediment, which can sometimes replenish soil downstream of the dam site – but sediment flow is reduced after a dam is constructed, because sediment accumulates in the reservoir.[140] Construction of some dams requires excavating a large amount of soil or rock, which must be piled somewhere – usually near to the construction site. The piles of excess soil can damage the surroundings and degrade air quality.[141] There is some evidence that the weight of the water in the reservoir can induce seismic activity, including earthquakes.[142]

Social and economic 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 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.[143]

The Three Gorges Dam on the Yangtze River in China is more than five times the size of the Hoover Dam (U.S.). It creates a reservoir 600 km (370 mi) 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.[144] During the 2010 China floods, the dam held back what would have been a disastrous flood and the huge reservoir rose by 4 m (13 ft) overnight.[48]

In 2008, it was estimated that 40–80 million people worldwide have been displaced from their homes as a result of dam construction.[145]

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 a clean energy source it is cheaper than both nuclear and wind power.[146] It is more easily regulated to store water as needed and generate high power levels on demand compared to wind power.[citation needed]

Aesthetics

A dam's appearance can be a factor when evaluating potential designs.[147] Dams with some curvature, such as arch dams, tend to be perceived as more attractive than those designed with entirely straight lines.[148] The advent of concrete after WWII as a material for building dams gave designers more flexibility to create pleasing dam designs.[149] Some dams  – such as the Hoover Dam and the Bratsk Dam – serve as objects that inspire admiration and pride, and can act as a symbol or icon of a community.[150] The Swiss dam engineer Niklaus Schnitter maintains that it is impossible to objectively determine if a dam and its reservoir will improve or detract from the pre-dam landscape, maintaining that it is a matter of taste.[151]

Selection of location, structure, and material

An important step in the design process is selecting the location, structure (arch, gravity, etc), and material (concrete, earth, etc). Factors that influence these decisions include topography (the shape of the valley), geology (especially as it relates to the strength of the ground below and to the side of the dam), the flow of water in the valley (hydrology), the availability of construction materials, and potential pathways for spillways.[152] The dam location should be chosen so the reservoir will be sufficiently large to meet project requirements. The location should also ensure that the ground is strong enough to support the forces that the dam structure and reservoir water will impose.[153]

If the dam is placed in a narrow valley, a gravity dam or arch dam may be most appropriate, especially if a tall dam is required. However, an arch dam can only be utilized if the walls of the valley are strong enough to support the large forces that the sides of the arch will impose.[154] A gravity dam structure is only feasible if the ground under the dam is strong bedrock.[155] Most gravity dams and arch dams are made of concrete, which is generally more expensive than earth or rock, and may influence the design choice.[156]

If the dam must span a wide valley, an embankment dam structure is often the optimal choice. Rock fill embankment dams are appropriate if rock is plentiful near the site, and an earth fill embankment dam may be used when rocks are not available.[157] For any embankment dam, an ideal site will be near impervious materials – such as clay – which can be used as a core layer within the dam.[157]

Auxiliary structures

Hydroelectricity

Main article: Hydroelectricity
Image
In this typical hydroelectricity installation of a dam, shown in cross section, the water is flowing from left to right.[158]

Many dams include power plants which run water through a generator to produce electricity. [159] The generator is always located near the bottom of the dam, enclosed in a powerhouse building.[158] Some powerhouses are located inside the dam structure; this is typically encountered in hollow gravity dams, particularly when no area is available downstream to put powerhouses.[160][citation needed]

Water is guided to the generator from upstream (often from a reservoir) via a passage called the penstock, which is always covered with a grate at the top to prevent debris from reaching the generator.[158][citation needed] The penstock feeds water into the generator, which uses the force of the water to rotate a turbine and generate electricity.[158] After the water leaves the generator, it goes through a passage (called the tailrace) and empties into the river downstream of the dam.[161][citation needed]

The amount of electricity that can be produced depends on the type of turbine, the amount of water fed into the generator, and the height of the upstream water level above the generator (this height is called the "head").[158]

Spillways and gates

Main articles: Spillway and Floodgate
Image
The Oroville embankment dam in US is the large rock structure on the right side of the image. Overflow water from the reservoir is cascading down the spillway on the left side.

Many dam projects include spillways, which are structures that provide a controlled release of excess water from the reservoir into the river downstream, preventing the dam from overflowing and possibly failing.[162]

Spillways can be integrated into the dam project in a variety of ways. Concrete gravity dams may position the spillway directly on the dam structure (in the middle or at the side). Other dams locate the spillway at a low point (saddle) of the ridge surrounding the reservoir; these saddle spillways convey the water downstream of the dam via a chute (channel) or through a tunnel. One particularly interesting spillway design is the bell mouth[ad] which is a vertical shaft in the interior of the reservoir, which leads to a tunnel that discharges downstream.[163]

Unusually large rainfall upstream may cause the reservoir to overflow. If the spillway is not large enough to safely transfer the overflow downstream, the water will spill over the dam structure, which could lead to significant damage or even total failure. Dam designers must perform detailed analysis of the variability of the region's rainfall and flooding; they use that data to design the spillway's capacity to handle a specific maximum flood.[164] For small dams, spillways are typically designed to safely handle the largest flood expected to occur once in 100 or 500 years. Large dams are typically required to handle the largest flood expected to occur once in 10,000 years.[165]

To operate effectively, the shape of the spillway must be carefully designed, usually adopting a parabolic shape at the top.[166] The bottom of the spillway must use special technologies that dissipate the energy of the rapidly flowing water as it discharges into the river, to minimize damage from erosion.[167] Some spillways use an ogee shape: the spillway starts horizontally at the dam top, becomes steeply inclined in the middle, then curves horizontally at the bottom (ensuring that the water shoots away from the dam structure, to minimize damage).[168]

Many dams include gates – usually positioned at the top of the spillway – to regulate the water level in the reservoir and control the rate at which overflow water is released downstream. Types of gates include vertical lift gates, drum gates, and radial gates.[169][ae]

Outlets

Dam outlets are structures – usually placed in the lower part of the dam structure – which permit the reservoir to be partially drained. Lowering the water level in a reservoir may be required for maintenance purposes, to purge sediment from the floor of the reservoir, to generate hydropower, or to increase the water flow downstream in the dry season.[170] Some dams create tunnels early in the dam construction process to divert river water around the construction site while the dam is being constructed. Those tunnels are sometimes converted into an outlet mechanism after the dam is completed.[171]

Locks and fish ladders

Main articles: Lock and Fish ladder

Construction

Cofferdams and diversion of river

Preparation, grouting, and foundations

Building the dam

Filling

Hydropower facilities

Operation and maintenance

Management procedures

Maintenance, leaks, and repairs

Inspection and monitoring

Sedimentation of reservoir

Dam removal

Main article: 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.[172] Some effects of dam removal include erosion of sediment in the reservoir, increased sediment supply downstream, increased river width and braiding, re-establishment of natural water temperatures and recolonization of habitats that were previously unavailable due to dams.[172]

The world's largest dam removal occurred on the Elwha River in the U.S. state of Washington (see Restoration of the Elwha River). Two dams, the Elwha and Glynes Canyon dams, were removed between 2011 and 2014 that together stored approximately 30 Mt of sediment.[172][173] As a result, the delivery of sediment and wood to the downstream river and delta was re-established. Approximately 65% of the sediment stored in the reservoirs eroded, of which ~10% was deposited in the riverbed. The remaining ~90% was transported to the coast. In total, renewed sediment delivery caused approximately 60 ha of delta growth, and also resulted in increased river braiding.[173]

Safety and Failures

Main article: Dam failure

Risk analysis and mitigation methods

Failures

Image
The earthen embankment Teton Dam failed in 1976 in the US.[citation needed]
Image
The concrete arch Malpasset Dam failed in 1959 in France.[citation needed]

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 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 grouting – pressure pumping Portland cement slurry into weak fractured rock.[citation needed]

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, Malpasset, Testalinden Creek dams), poor maintenance, especially of outlet pipes (Lawn Lake Dam, Val di Stava Dam collapse), extreme rainfall (Shakidor Dam), earthquakes, and human, computer or design errors (Buffalo Creek Flood, Dale Dike Reservoir, Taum Sauk pumped storage plant).[citation needed]

Since 2007, the Dutch IJkdijk foundation is developing, with an open innovation model, an early warning system for levee/dike failures. As 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.[citation needed]

Society and culture

International geopolitics

Profession and regulation

Most countries with large dams have statutes or regulations regulating dam construction and inspection practices. The regulations vary widely across countries. Some nations have a government agency responsible for inspecting dams, but many do not.[174] Some countries regulate dams at a federal level, but others regulate at a province/state level.[175] For example, Germany has no federal regulations; instead, each state has its own statutes. Dam owners are required to inspect their dams periodically with supervision by the government.[176] The regulations of most nations do not specify particular dam design parameters, but instead require compliance with “recognized rules of technology” or “state of the art in science and technology”.[177]

Art and culture

Wartime

During an armed conflict, a dam is to be considered 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 identification, a protective sign consisting of three bright orange circles placed on the same axis is defined by the rules of IHL.[citation needed]

A notable case of deliberate destruction of a dam was the Royal Air Force '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 and Eder rivers. This raid later became the basis for several films.[citation needed]

References

Footnotes

  1. ^ a b c d In the context of dams, "thickness" and "width" are synonymous and both mean the breadth of the dam cross section measured in the upstream/downstream direction. "Height" is the vertical distance from the foundation to the crest (top) of the dam. "Length" is the distance measured along the centerline of the crest (top) of the dam from one end to the other.[7]
  2. ^ Traces still remain today.[9]
  3. ^ Another early dam in Sri Lanka was the Kalabalala Tank, which is still in use today.[15]
  4. ^ The volcanic ash, called pozzolana, was used to create a variety of concrete called Roman concrete.
  5. ^ Around 100 CE the Romans repurposed the Subiaco Dam to supply water to the Aqua Anio Novus aqueduct. The dam stood until 1305 CE.[19]
  6. ^ More than 100,000 water-powered mills were built from the Middle Ages to 1900, mostly in Europe.[24]
  7. ^ The first grout curtain was created under a dam in Bou Hanifia, Algeria.[40]
  8. ^ Including deforestation the large amount of carbon introduced into the atmosphere by the manufacture of concrete.
  9. ^ Statistics include only large dams.[52]
  10. ^ The International Commission on Large Dams defines a large dam as "a dam with a height of 15 metres or greater from lowest foundation to crest or a dam between 5 metres and 15 metres impounding more than 3 million cubic metres".[52]
  11. ^ The count of reservoirs includes those created by all types of barriers, not all of which are dams.[53]
  12. ^ a b c d Data as of 2011.
  13. ^ Some embankment dams contain a solid, vertical steel wall or curtain in the center, extending from the bottom to the crest.[63]
  14. ^ The Nagarjuna Sagar Dam (1974) is an exception: it is a gravity dam made mostly of rubble.[70]
  15. ^ Some authorities use the term "straight gravity dam" to distinguish such dams from arch-gravity dams.[72]
  16. ^ In practice, most gravity dams have a flat crest (top) so a road can go across; thus the shape is a trapezoid.[73]
  17. ^ To prevent the dam from sliding horizontally, some gravity dams are locked into the bedrock by digging a large groove into the bedrock (parallel to the crest of the dam) so the dam structure is "keyed" into the bedrock. This lowers the risk of the dam shifting due to water pressure or earthquakes.[76]
  18. ^ Each buttress is separated from its neighbors by gaps at the top, which reduce stress within the dam structure.[82]
  19. ^ Some older buttress dams had a face that was vertical. In that configuration, the face was essentially a retaining wall supported by buttresses on the downstream side.[83]
  20. ^ The inclination of a dam face is the ratio of horizontal span to height. An inclination of 1.0 is a 45 degree angle; an inclination of 0.0 is fully vertical.
  21. ^ An arc angle of 133 degrees is optimal for minimizing the amount of concrete needed to build a constant angle arch dam.[90]
  22. ^ The word "radius" in the context of an arch dam refers to the radius of the central angle of the dam structure.
  23. ^ Also called "variable radius" curve.[92]
  24. ^ An early arch-gravity dam was the Monte Novo Dam in Portugal, built by the Romans.[95]
  25. ^ An equivalent term is "multiple-arch buttress dam".[96]
  26. ^ The earliest known multiple-arch buttress dam is the Esparragalejo Dam built by the Romans in the 1st century CE.[98]
  27. ^ Statistics include only single-purpose large dams.[52]
  28. ^ Dam designs sometimes use flood return periods of 50 year or 100 years.[111]
  29. ^ The Grand Ethiopian Renaissance Dam project includes a 5 km (3.1 mi) long saddle dam that holds back over 80% of the reservoir's live storage (i.e. the water that is above the lowest outlet of the reservoir).[127]
  30. ^ Also called a "shaft" or "morning glory" spillway
  31. ^ On some low dams, the dam structure itself acts as a spill way, and the gates are places on the top (crest) of the dam.

Citations

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  3. ^ a b Schnitter 1994, p. 76.
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  5. ^ a b Schnitter 1994, pp. 124–127.
  6. ^ a b c Schnitter 1994, pp. 151, 183.
  7. ^ "Glossary", Reclamation Library.
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  14. ^ Schnitter 1994, pp. 33–36.
  15. ^ a b Brown 2026, Section: "Early dams of East Asia".
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  17. ^ Schnitter 1994, p. 57.
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  21. ^ Schnitter 1994, p. 106.
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  32. ^
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  35. ^ Brown 2026, "Section: "The 19th century".
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  38. ^ Brown 2026, Section: "Development of modern structural theory".
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  49. ^ Ranjan 2024.
  50. ^ a b Brown 2026, Section: "Rise of environmental and economic concerns".
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  99. ^ Evans 2011, pp. 93, 506.
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  103. ^ Evans 2011, p. 519.
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  105. ^ Schnitter 1994, p. x, Forward.
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  113. ^ "I-205 Abernethy Bridge". Oregon Department of Transportation.
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  143. ^ Cullather, 110.
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  161. ^ "Hydroelectric Design Center", USACE, pp. A.3–A.7.
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  169. ^
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  172. ^ a b c Bellmore, J. R.; Duda, J. J.; Craig, L. S.; Greene, S. L.; Torgersen, C. E.; Collins, M. J.; Vittum, K. (2017). "Status and trends of dam removal research in the United States". WIREs Water. 4 (2) e1164. Bibcode:2017WIRWa...4E1164R. doi:10.1002/wat2.1164. ISSN 2049-1948. S2CID 114768364.
  173. ^ a b Ritchie, A. C.; Warrick, J. A.; East, A. E.; Magirl, C. S.; Stevens, A. W.; Bountry, J. A.; Randle, T. J.; Curran, C. A.; Hilldale, R. C.; Duda, J. J.; Gelfenbaum, G. R. (2018). "Morphodynamic evolution following sediment release from the world's largest dam removal". Scientific Reports. 8 (1): 13279. Bibcode:2018NatSR...813279R. doi:10.1038/s41598-018-30817-8. ISSN 2045-2322. PMC 6125403. PMID 30185796.
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Sources

Journals and websites

  • Brown, John Guthrie; et al. (2026). "Dam". Britannica. Retrieved 22 April 2026.
  • Schnitter, Niklaus (1978). "Römische Talsperren". Antike Welt. 8 (2): 25–32.

Books

  • Hodge, A. Trevor (2000). "Reservoirs and Dams". In Wikander, Örjan (ed.). Handbook of Ancient Water Technology. Technology and Change in History. Vol. 2. Leiden: Brill. pp. 331–339. ISBN 978-90-04-11123-3.
  • Nguyen, Josef (2012). "Tidal Power". In Pierce, Morris (ed.). Encyclopedia of Energy. Salem Press. pp. 1247–1249. ISBN 9781587658532. Retrieved 5 May 2026.
  • Vogel, Alexius (1987). "Die historische Entwicklung der Gewichtsmauer". In Garbrecht, Günther (ed.). Historische Talsperren. Vol. 1. Stuttgart: Verlag Konrad Wittwer. pp. 47–56 (50). ISBN 978-3-87919-145-1.

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