Wikipedia

Dam

A dam is a structure that impounds water or restricts water flow. Dams are categorized by their structure: gravity dams are large, heavy masses made of concrete or masonry, and rely on their weight to remain immobile and resist the forces exerted by the upstream water. Embankment dams are large earthworks consisting of rocks, clay, sand, soil, or gravel. A buttress dam consists of a sloped, concrete face supported on the downstream side by numerous triangular buttresses. An arch dam is a curved concrete wall that transfers the force of the impounded waters to the valley walls.

The earliest known dam is the Jawa Dam in Jordan, built around 3000 BCE. The Hittite Empire built several dams between the 17th and 13th centuries BCE in Turkey. The Roman Empire began building significant dams until the first century CE – typically masonry gravity dams with vertical faces on both upstream and downstream sides. In medieval Europe, dams were used to power water wheels for milling and mining. In the late 18th century, the process of designing dams began to transform from an informal practice to an engineering discipline rooted in science. In the 20th century, the widespread availability of concrete and power construction machinery led to an explosion in the number of large dams built around the world. 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.

Dams provide for irrigation, hydropower, water supply, flood management, recreation, inland navigation, and fish farming. 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. Dams can generate hydropower, which is a clean and renewable source of electricity. Many dams supply water for domestic or industrial use. Dams that support flood management don't entirely eliminate all possibility of flood, but generally reduce the peak flood level (height) to a safe limit.

Dams are often one component of a larger project. Many dams include power plants that run water through a generator to produce electricity. 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. Dam outlets are structures which permit the reservoir to be partially drained for the purpose of purging sediment from the floor of the reservoir, generating hydropower, or increasing the water flow downstream. When a dam is placed in a river where it would prevent the movement of boats, locks may be incorporated into the dam project.

Dams occasionally fail, sometimes causing large floods and loss of life. Many principles governing the design of safe dams have been developed based on lessons learned from dam failures. Dams can fail for many reasons: rock weakness at the abutments, erosion of the foundation under the dam, shearing where the dam meets rock, or sliding over its foundation. Dams built in Turkey, India, Ethiopia, and China without the consent of downstream nations have led to notable international disputes, amid increasing water scarcity driven by population growth and the impacts of climate change.

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

The Proserpina Dam in Spain was built by the Romans and has been in use for almost two millennia.[2]
The Band-e Kaisar dam in Iran was built in the 3rd century CE.[3]
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 and trial and error – to an engineering discipline rooted in science.[30] Important figures that contributed to this evolution included the French scientist Charles-Augustin de Coulomb who, in 1776, created a formula that described how soil reacts under stress, a theory that was later given practical application to dams by Alexandre Collin.[30] Claude-Louis Navier developed the theory of elasticity in 1826.[31] In 1847, François Zola became the first engineer to design an arch dam based on an analytical consideration of stresses.[32] The 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.[33] The Scottish physicist William John Macquorn Rankine developed a theory governing retaining walls in the 1850s which was applicable to dams.[34]

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.[35] In France, the Gouffre d'Enfer masonry gravity dam (1866) was 60 m (200 ft) tall.[36] 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

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.[37] 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.[38] The 1933 invention of grout curtain technologies enabled dams to be safely built on top of porous soils.[39][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.[39]

Notable dams built in the modern era include:

  • Itaipu Dam (Brazil/Paraguay, 1984) An example of international cooperation.[46]

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.[49] 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.[49] 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.[50]

Number of dams in the world

Dam types[51][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).[51] The total number of reservoirs (large and small) in 2011 was estimated to be 16.7 million.[52][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.[52][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.[52][l] About 7.6% of the world's rivers are significantly impacted by reservoirs and 46.7% of large rivers are affected.[52][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.[53]

Types

Dams can be classified by their structural type: embankment, gravity, buttress, and arch. Other forms include hybrid dams and rockslide dams.[54]

Advantages and disadvantages of dam types

Advantages and characteristics of dam types[55]
Dam type Advantages Characteristics and drawbacks
Embankment dam  • Ground beneath dam can be weak, since it is under little stress
 • Little excavation required
 • Low sensitivity to earthquakes
 • Settlement of ground beneath dam is acceptable
 • Materials for dam may be found locally		  • Large amount of material required
Gravity dam  • Minimal stress within concrete
 • Integrated spillways possible
 • Handles temperature fluctuations well		  • Lots of expensive concrete required
 • Sensitive to earthquakes and ground settlement
 • Refrigeration may be required while curing
 • Large amount of excavation
Buttress dam  • Less concrete and fewer cooling issues vs gravity dams
 • Minimal ground settlement issues
 • Spillway integration		  • High sensitivity to earthquakes
 • Face of dam sensitive to temperature changes
 • Large amount of excavation
 • More formwork and labor vs gravity dams
Arch dam  • Little concrete required
 • Small excavation
 • Earthquake tolerant
 • Few concerns with ground settlement		  • High stress within concrete
 • Complex and risky abutment where dam meets valley walls
 • Spillway placement may be difficult

Embankment dam

A typical embankment dam has a clay core and gradually sloping faces both upstream and downstream.[56]
The Misogawa Dam is an embankment dam in Japan.

The most common dam structure is an embankment dam.[57] These dams consist of a pile of earth (rocks, clay, sand, gravel, soil, etc) formed into the shape of a large levee with a broad trapezoidal cross-section.[m] Embankment dams are categorized as rockfill or earthfill – depending on if their primary material is rocks or soil.[59]

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).[60]

The primary drawback to embankment dams is that they are inherently porous, so water can seep through the dam or underneath the dam.[61] 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.[62] If an impervious layer is included, it may be made of clay, cement, or asphalt.[63][n] Failure to properly mitigate seepage can lead to dam failure caused by "piping" – 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.[65]

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.[66][o] 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 sloping banks of soil on both upstream and downstream sides, and both faces covered with large rocks.[68] Large rocks on the upstream face protect the structure from wave action.[69] 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.[70]

Gravity dam

A typical gravity dam has a nearly vertical upstream face, and a broad base.[36]
The Grande Dixence Dam in Switzerland is one of the world's tallest gravity dams. The people atop the dam's crest illustrate its size.[71][p]

Gravity dams rely on their weight to remain immobile and resist the forces exerted by the upstream water. 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.[72][q] The cost of a concrete dam is proportional to the volume of concrete required – so solid concrete gravity dams can be expensive to build. An approach to reduce cost is to incorporate large hollow chambers inside the dam.[74][r]

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).[76][s][t] 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.[u] The upstream face is typically more vertical than the downstream face, to provide stability; The inclination of the downstream face of a gravity dam is typically 0.75 to 0.8.[79][v] To ensure that the dam is stable and will not tip over, the profile must conform to the middle-third rule, which states that the forces acting on the dam (gravity, water pressure, etc) must produce a net force that is directed at the middle portion of the base (rather than directed near the downstream edge of the base).[81] In addition, the thickness of the base should be 70 to 85% of the height.[82][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.[w] If the bedrock has cracks or defects, it must be prepared by injecting grout or placing concrete plugs.[84] 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.[85]

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.[86]

Buttress dam

A typical buttress dam has an inclined upstream face and multiple buttresses.[87]
About 20 buttresses are visible in this photo of the Latyan buttress dam in Iran.[88][x]

A buttress dam consists of a flat upstream face supported on the downstream side by numerous triangular buttresses.[y] Most buttress dams are made of concrete.[91] 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 0.3 and 1.0.[v] 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).[87]

Buttress dams use much less concrete than a comparable gravity dam, but those cost savings are offset by a more complex construction process.[z] 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 structure rests upon) than gravity dams, the risks associated with uplift forces (from water under the dam) are lower in buttress dams.[93]

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.[89]

Arch dam

A constant-radius arch dam, viewed from above
The Gordon Dam in Tasmania is an arch dam. The shape transfers the weight of the water into the valley walls.[94]

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).[94] Arch dams can only be built at a location in a valley where the valley is relatively narrow and has strong, steep rock walls.[95] Arch dams are relatively thin: the thickness of their base is less than half their height.[t] They are always made of concrete or masonry.[97] 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.[98][aa]

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.[100][ab] A more complex shape is the "constant angle" shape, which gradually reduces radius from the crest to the base.[ac] 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.[101] 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.[102]

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.[95]

Hybrid structures

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.

Soms dams combine features from two 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.[103][ad][t]

A multiple-arch dam[ae] 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: each arch connects one pair of adjacent buttresses.[106][af]

Rockslide dams

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.[108]

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.[109] There are thousands of rockslide dams around the world, including one created in 2010 in Pakistan that formed Attabad Lake.[109] 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.[110] Risks of outburst floods can be mitigated by building spillways on rockslide dams to lower the water level.[110] Engineers have used rockslide dams as foundations upon which to build new dams.[111] Rarely, engineers have used blasting on mountainsides to trigger a rockslide and create a crude embankment dam, called a "blast-fill" dam.[111] 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.[112]

Uses

Primary purposes

Dam usage[51][ag]
  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.[113] Many dams – called "multi-purpose dams" – support two or more of these primary purposes.[114]

Irrigation is a critical application of dams: about 20% of the world's arable land is irrigated by water that originated in reservoirs impounded by dams (as of 2022).[115][ah] 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.[117]

Hydropower provides clean, renewable energy in the form of hydroelectricity. As of 2022, global hydropower capacity accounted for about 20% of the world's electricity supply.[118][ai] More than 80% of the world's reservoir water storage capacity is used to generate hydropower.[116] 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.[119]

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 from a low reservoir to a higher 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.[119] 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.[119]

Many dams supply water for domestic or industrial use. In 2025, there were 3,394 large dams used for water supply.[51] 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.[120]

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.[aj] For example, a dam may be designed with the goal of safely regulating 1-in-100 year floods.[121]

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.[122]

Some dams are designed with the primary goal of supporting recreation or fish farming.[122]

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).

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.[123]
A weir in Spain

A tailings dam is a dam that impounds tailings – the waste from mining operations.[124] Most tailings dams are embankment structures.[125] 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.[126] Tailings dams are unique because they are often enlarged over time: as mine operations continue, the embankments are repeatedly raised.[127] 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.[128]

A cofferdam is a temporary dam built at a construction site to keep water out of the site until the job is completed.[129] Cofferdams are commonly used when building bridge supports in lakes, rivers, and oceans.[130] 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.[129]

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.[131] Some weirs are used to create a segment of the river that has a fixed level;[132] others are designed to minimize erosion of the river banks;[133] some are for landscaping or recreation purposes;[134] 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).[135]

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.[136][ak]

A diversion dam 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.[138] 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.[139]

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). In both of these designs, a well or pipe is placed upstream of the barrier to withdraw the water.[140]

Design

Design process

The process of designing a dam can be undertaken in three stages: reconnaissance, feasibility, and project planning.[141] 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 the location of construction materials (such as rocks and soil for landfill). Analysis of environmental and flood impacts is started.[141] In the planning stage, detailed design plans are created, a construction schedule is established, and cost estimates are prepared.[141]

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.[142] 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.[143]

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.[144] A critical part of the geological assessment is understanding the likelihood and magnitude of earthquakes and other seismic events.[145]

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.[146]

Impact assessment

Dams can provide significant benefits to a community in the form of irrigation, water supply, and hydroelectricity. However, many adverse impacts can follow from dam construction, leading some to oppose new dam construction.[147] To evaluate these concerns, countries require developers of large dams to prepare an Environmental Impact Assessment (EIA) that documents the consequences the dam (and its reservoir) will have on communities and the environment.[148] The EIA enables the developer and government to assess the desirability of a dam, to mitigate its impacts, and to compensate people adversely affected.[149] An EIA may address impacts in the following areas: air quality, climate change, water flow, reservoir, downstream region, socioeconomic, and infrastructure.[150] The EIA should identify potential mitigation approaches to minimize adverse impacts. Mitigations include changing the dam's location, size, or design; compensating those impacted; or cancelling the project altogether.[151]

Air quality may be impacted by dams in several ways: During construction, there may be large amounts of particulate matter in the air. After the dam is operating: the reservoir's water and associated humidity may have impacts on the microclimate near the dam site.[152]

Although hydropower from dams is much cleaner than power from coal or oil plants, concrete dams are responsible for putting large amounts of greenhouse gases into the atmosphere, which contribute to climate change.[152] To produce one cubic meter of concrete, around 200 kg of carbon dioxide (CO2) is put into the atmosphere.[153][al] For example, the Three Gorges Dam – containing 28 million m3 concrete – put roughly 5.6 billion kg of CO2 into the atmosphere.[155][am] CO2 is also emitted from reservoir water as organic matter decomposes; the organic matter includes all plants and trees submerged by the reservoir, as well as plant life carried into the reservoir from upstream.[152]

Dams impact water flow, which can have several adverse impacts. 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.[156] 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. Fish migration may be seriously impacted, since the dam may prevent fish from swimming upstream to spawn.[157]

Reservoirs impounded by dams can impact the environment. Fish and plants that lived in or near the river will die, and perhaps become extinct in that locality. Terrestrial animals that lived in the valley will lose habitat. The reservoir may cause deforestation, if a large number of trees are submerged under the waters. There is evidence that the weight of the water in the reservoir can trigger landslides, seismic activity, and earthquakes.[158]

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.[156] 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.[159]

Communities that live near the dam and its reservoir may be severely impacted. People who live within the reservoir boundaries must relocate to new homes, which can cause large-scale social disruption. The Aswan High Dam in Egypt displaced 50,00 Nubians and devastated the Nubian community; it also forced the relocation of the Abu Simbel temples. The Three Gorges Dam in China required the relocation of 1.4 million people.[160] Land adjacent to the reservoir may become saturated with water, impacting agriculture and increasing soil salinity. The level of groundwater (underground water) surrounding the reservoir may rise, and the quality of groundwater that people pump from wells may degrade.[161]

Many dam projects require extensive modifications to the local infrastructure. New housing may be built for workers; electrical power transmission lines will be needed if the dam produced hydroelectricity; bridges and roads may need to be created or re-routed.[162]

Selection of location, structure, and material

Important steps in the design process are 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.[163] 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.[164] The site selection must also consider seismic factors: when fault lines are discovered under or near the dam, designers must determine if they pose a risk.[165][an]

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.[167] A gravity dam structure is only feasible if the ground under the dam is strong bedrock.[168] 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.[169]

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.[170] 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.[170]

Aesthetics

A dam's appearance can be a factor when evaluating potential designs.[171] Dams with some curvature, such as arch dams, tend to be perceived as more attractive than those designed with entirely straight lines.[172] The advent of concrete after WWII as a material for building dams gave designers more flexibility to create pleasing dam designs.[173] 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.[174] 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.[175]

Auxiliary structures

Hydroelectricity

In this typical hydroelectricity installation of a dam, shown in cross section, the water is flowing from left to right.[176]
The generators of this dam are in the low, horizontal building lower-left. Five penstocks feed water from the reservoir into the power plant.

Many dams include power plants that run water through a generator to produce electricity.[177][ao] The generator is typically located at a level near the bottom of the dam, enclosed in a powerhouse building.[176] 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.[179]

Designing a hydropower facility for a particular dam requires analysis of the amount of electricity desired, the amount of water available to feed into the turbine, and the height of the upstream water level above the generator (this height is called the "head") Those factors will determine which turbine style is optimal; turbine styles commonly used in dams include Francis turbine, Pelton turbine, and Kaplan turbine.[180]

Water is guided to the generator from upstream (often from a reservoir) via a passage – called the penstock – that feeds the water into the generator, which uses the force of the water to rotate a turbine and generate electricity.[181][ap]

Spillways and gates

The Oroville embankment dam in the 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.
The spillway of the Grand Coulee Dam is integrated into the main structure of the dam.

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.[182]

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 via a chute (channel) or through a tunnel to discharge downstream of the dam. One particularly interesting spillway design is the bell-mouth,[aq] which is a vertical shaft in the interior of the reservoir that leads to a tunnel that discharges downstream.[183]

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 a 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.[184] 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.[185]

To operate effectively, the shape of the spillway must be carefully designed, usually adopting a parabolic shape at the top.[186] 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.[187] 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).[188]

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.[189][ar]

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, to increase the water flow downstream in the dry season, or to reduce stress on the dam structure in an emergency situation.[190] 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.[191]

Locks and fish bypasses

Boats cannot pass over this dam (right side of image) so a lock is incorporated into the dam's structure (left side). In this photo, a tugboat is pushing barges into the lock.
This dam in Japan incorporates a fish ladder to enable fish to swim upstream. The ladder is the long diagonal structure with water cascading down.

When a dam is placed in a river where it would prevent the movement of boats, locks may be incorporated into the dam project. Locks enable boats to pass the dam in both directions. Locks consist of one or more rectangular chambers with large doors at both ends, and piping that permits each chamber to be filled and emptied.[192]

Some rivers are important migration paths for fish. If a dam is built on such a river, it could cause significant ecological damage. The impacts can be mitigated by including a fish bypass mechanism in the dam project. Designs for a fish bypass include fish ladders, fish lifts, or artificial creeks that mimic a natural river.[193]

Construction

Constructing a dam requires a significant amount of planning and scheduling.[194] A typical three-stage planning sequence is: reconnaissance and research, investigations and surveys, and preparation of design documents and drawings.[195]

Cofferdams and diversion of river

A temporary cofferdam can be built to divert river water around a dam construction site.[196]

Many dams require more than one year to build, and so they cannot be built within a single dry season. For those dams, the flow of river water must be diverted around the dam construction site.[197] Diversion can be accomplished by building a temporary tunnel or channel around the side of (or underneath) the dam, and building a temporary cofferdam to direct the river into the tunnel/channel.[196] Cofferdams are typically removed after construction; but for some embankment dams, they are incorporated into the dam itself.[198] Channels and tunnels are also typically plugged up or demolished; although some dam projects retain tunnels or channels as part of the permanent dam project, for example, as an outlet or a spillway.[198]

Another diversion technique is to build a cofferdam which forces the river to one side of the valley, and build half the dam on the other side (within the cofferdam). After the first half of the dam is complete, the cofferdam is moved to direct the water to the first half-dam (which has passages to let the water through) while the second half of the dam is built within the cofferdam.[199]

For some embankment dams, the cofferdam and diversion may be avoided altogether by channeling the river into a narrow course in the center of the valley and building the dam inward from both sides of the valley, leaving a gap in the middle for the river to flow through. Then, in the dry season – when the flow of the river is small – the central portion of the dam is rapidly completed, as the water rises behind the dam.[200]

Preparation, grouting, and drainage system

Workers are drilling holes in a dam site. Grout will be injected into the holes to strengthen the ground and reduce water seepage.

An early step in the construction process is preparing the foundation, which is the rock upon which the dam structure will rest. For heavy dams – gravity dams, arch dams, or buttress dams – the dam is very heavy and must rest on strong, solid, bedrock. Any soil, gravel, loose rock, or poor-quality rock must be removed to expose bedrock before building the dam. Tools used to expose and prepare the bedrock include high-power water jets and blasting. Embankment dams have wide bases, and do not subject the ground to as much pressure as heavy dams; so – in some situations – they may be built on loose rocks or soil.[201]

A major risk in any dam project is water seeping under the dam or around its sides. An important technique to mitigate that risk – performed before the dam is built – is to inject grout into the ground below the dam and into the valley walls on either side.[202] Two types of grouting processes are utilized for dams: consolidation grouting locates rocks below and to the sides of the dam that may have cracks or defects, and injects grout under high pressure at those locations. The purpose is to fill any cracks or voids, thus limiting seepage by reducing the size and number of underground passages water can take around the dam.[203] The other kind of grouting is curtain grouting, where grout is injected deep into the rock through boreholes drilled in a pattern calculated to create a solid wall of grout below and to the sides of the dam. The depth of the grout curtain is typically 30% to 70% of the planned depth of the water behind the dam.[204] Another technique employed to mitigate seepage is a drainage system. The purpose of a drainage system is to reduce the risk of "piping" (water inside the dam eroding the dam structure) or uplift (water under the dam pushing the dam upward). The drainage system collects water from inside or under the dam and carries it downstream. The drainage may consist of pipes, or a crushed rock "blanket" under the dam.[205]

Building the dam

Concrete dams require a concrete plant which uses conveyor belts to move rocks and concrete.[206]
The machine on the left is vibrating the roller compacted concrete of this dam in Turkey.

The process of building the dam structure depends on the dam material: concrete dams and earth/rock embankment dams use distinct processes.

Concrete dams require a concrete plant to be built near the dam site. The plant combines aggregate (rocks), cement, fly ash, and water to produce concrete.[207] The concrete is delivered from the concrete plant to the dam structure by means of conveyor belts, buckets, dump trucks or cranes. Formwork is built at the dam location to contain the concrete when it is placed. The concrete must be vibrated after it is placed into the dam structure.[208] Dam structures consist of concrete walls and structures that are very thick – much thicker than, for example, the wall of a building. Concrete that is used for such thick structures – called mass concrete – generates a large amount of heat as it cures.[209] The heat requires special treatment to ensure it does not crack when curing, such as refrigeration systems which circulate coolant through the concrete by means of piping.[210] Gaps or joints between large segments of the concrete help the curing proceed faster and reduce cracking. The gaps are later filled with grout, and their upstream edge is sealed with strips of metal, rubber, or plastic.[211]

A recent innovation is roller compacted concrete (RCC) which uses less cement, permits use of aggregate up to 100 mm in size, and does not generate as much heat as conventional concrete. RCC has several benefits over conventional concrete: it permits tracked bulldozers to immediately drive on top of it after it is placed; it reduces construction costs because less formwork and labor is required; and it reduces the need to manage the heat generated while curing.[212]

Embankment dams are built primarily of soil and rock, and use a different approach to construction than concrete dams. Typically, vast amounts of soil and rock are required to build an embankment dam, so the material is usually excavated from "borrow areas" near the dam site. The soil and rocks are laid down in successive layers called "lifts".[213] After laying down a layer, it is compacted with heavy machinery.[214] The layers must be carefully monitored to ensure that they contain the correct materials, and are not overly wet, and that they are sufficiently compacted. Instruments are embedded within the dam as it is built, and are continually monitored so any defects can be quickly corrected.[215]

Operation

Maintenance and repairs

After a dam is completed and becomes operational, management processes are employed to ensure that it continues to fulfill its purposes (irrigation, hydropower, etc), avoids safety incidents, and achieves its intended lifespan. These management processes include technical activities such as:[216]

  • Prioritization and planning
  • Operation
  • Maintenance
  • Testing
  • Inspection, monitoring and surveillance
  • Dam safety reviews
  • Investigation and rectification of deficiencies
  • Emergency planning and response
  • External Advice and management reviews
  • Communication and records
  • Continuous improvement

Inspection and monitoring

An essential task of dam operators is surveilling and inspecting the dam to identify potential safety issues.[217] Problems may arise due to inherent design flaws, improper construction procedures, or stresses imposed on the dam.[citation needed] Stresses that act on a dam include:[218]

  • Force of the reservoir water pressing on the dam
  • Waves in the reservoir striking the dam
  • Earthquakes and minor seismic events
  • Weight of the dam compressing the ground underneath the dam (and – for arch dams – on the sides of the dam) causing the dam to settle or move
  • Water seeping under the dam and applying an uplift pressure on the dam structure
  • Internal compression and tension stresses within the structure
  • Expansion and contraction of the dam structure due to temperature fluctuations

Dam operators continually inspect and monitor the dam. In the event that any anomalies are detected, dam operators will implement corrective actions.[219] Monitoring the condition of a modern dam relies on data provided by permanent sensors within and around the dam, which measure stresses and movements. Sensors typically used in dams are tiltmeters, joint meters, strain meters, deflectometers, thermometers, deformation meters, and piezometers.[218] Dam personnel monitor these sensors, and if irregular data is reported, they investigate the underlying cause, and – if needed – implement repairs or mitigations.[220]

Sedimentation of reservoir

Most reservoirs gradually accumulate sediment, decreasing the amount of water that the reservoir can hold. When the water capacity is reduced, the dam's ability to perform its intended purposes (irrigation, hydropower, water supply, flood control, etc) is correspondingly reduced. Sediment enters a reservoir in the form of soil suspended in river water; as the river empties into the reservoir, the water velocity slows down, and the sediment settles to the bottom of the reservoir.[221] Sediment can also enter a reservoir from wind-blown soil, landslides, construction work near the water, and erosion from irrigation or rainfall.[222] To mitigate sedimentation, dam operators implement strategies to reduce the amount of sediment entering the reservoir.[223] Some sedimentation can be reduced by planting plants and trees in the reservoir's drainage basin, or by building terraces.[224]

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.[225] 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.[225]

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 Glines Canyon dams, were removed between 2011 and 2014 that together stored approximately 30 Mt of sediment.[225][226] 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.[226]

Dam failure

The concrete arch Malpasset Dam failed in 1959 in France.[227]
The earthen embankment Teton Dam failed in 1976 in the US.[228]

Many principles governing the design of safe dams have been developed based on lessons learned from dam failures.[229] Dams can fail for many reasons. Arch dam failures can result from rock weakness at the abutments (where the sides of the dam press into the valley walls), erosion of the foundation under the dam, or shearing (slipping) where the dam meets rock. Gravity dams and buttress dams can fail due to the dam sliding over its foundation.[230]

The first modern arch dam to fail was the Malpasset Dam in France. The failure in 1959 was not due to the concrete dam's shape or strength, but rather the presence of an underground slip line which moved due to the combined weight of the dam and the reservoir water. The movement caused the dam to crack, and the resultant flood killed several hundred people downstream.[231]

In some cases, it is not possible to determine the cause of the failure. The Baldwin Hills embankment dam in the US failed in 1963 but – despite extensive investigations – a definitive cause was not found. Potential factors included: weak foundation soil which subsided; irregular settling of the dam structure; fault lines under the dam; a nearby oil field that was depleted of oil then later re-pressurized; and the act of emptying and filling the reservoir.[232]

An unusual dam failure was the Vajont Dam in Italy. This concrete arch dam was built in a valley that had steep sides which were prone to landslides. Designers failed to fully understand the geology of the valley, and built the dam in 1959 – then the tallest dam in the world at 267 m (876 ft). In 1963 a huge landslide slid down the hillside above the reservoir, displacing nearly all the water in the reservoir, and causing a 125 m (410 ft) tall wave to overtop the dam. The resulting flood killed over 2,000 people downstream. The dam structure itself suffered only minor damage.[233]

A risk unique to earthen embankment dams is "piping": a small leak through or under the permeable dam structure gradually erodes the soil, until a small channel is formed, which – if unaddressed – may lead to dam failure. Piping caused the Teton Dam in the US to fail in 1976, killing 11 people.[234]

Society and culture

International disputes

A photograph of large concrete dam and the reservoir it creates. The photo is taken from an aerial viewpoint, looking down from above. Large plumes of white water spray over a spillway.
Ethiopia built the Grand Ethiopian Renaissance Dam on the Blue Nile River in 2020, over the objections of Egypt and Sudan.[235]

World population increases and the impacts of climate change have led to water scarcity conditions which are responsible for international conflicts over sharing the water of transnational rivers.[236] Organizations such as the United Nations and the International Law Association have recommended negotiation and collaboration to facilitate sharing, yet most countries believe they are entitled to unilaterally build dams within their borders without consulting downstream nations.[237] Some authorities have predicted that water supply may be used as a weapon in future conflicts.[238] Dams built in Turkey, India, Ethiopia, and China without the consent of downstream nations have led to notable international disputes.[236]

Turkey's Southeastern Anatolia Project is a major water project which includes many dams, one of which is the large Karakaya Dam. Most of these dams are on the Euphrates and Tigris rivers, which flow downstream into neighboring nations Syria and Iraq. These downstream nations have protested to Turkey about potential water supply issues.[239]

India and Bangladesh have a long-standing dispute about sharing the waters of the Ganges river, which focuses on the Farakka Barrage built in 1972. A treaty was signed in 1996.[240] The Indus River is the primary river of Pakistan, with headwaters in several countries, including India. The Indus Waters Treaty was signed between India and Pakistan in 1960, but India subsequently built large dams on the Indus over the opposition of Pakistan, including the Baglihar Dam and Kishanganga Dam.[241]

The Nile River has been the source of tensions between the arid downstream nations (Egypt and Sudan) and the upstream nations where most of the Nile's water originates as rainfall. Throughout the 20th century and into the 21st, negotiations have led to various treaties and initiatives, such as the Nile Basin Initiative. In 2020, Ethiopia built the Grand Ethiopian Renaissance Dam (GERD) on the Blue Nile, in spite of opposition from Sudan and Egypt.[235]

The Mekong River traverses several countries. In 1995, Thailand, Cambodia, Laos and Vietnam signed a treaty creating the Mekong River Commission to regulate the river's water supply. China did not participate in discussions leading to the treaty, and later built many dams on the river, including the Xiaowan Dam and Nuozhadu Dam.[242]

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.[243] Some countries regulate dams at a federal level, but others regulate at a province/state level.[244] 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.[245] 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”.[246]

Art and culture

Dams are featured in novels, movies, documentaries, songs, banknotes, popular histories, and propaganda posters. In the mid-20th century, Soviet artists such as Isaak Brodsky and Gustav Klutsis highlighted large dams in artworks intended to glorify workers and industry. Their works portrayed the Mingachevir Dam, Kayrakkum Dam, Bratsk Dam, and Dnieper Dam.[247] Folk musician Woody Guthrie wrote the songs Grand Coulee Dam and Roll On, Columbia, Roll On (both 1941) as part of a series of songs he wrote about the Columbia Basin Project.[248]

The docudrama war film The Dam Busters (1955) portrayed the Operation Chastise military operation during World War II, when the Allies successfully bombed a dam in Germany.[249] Books about dams include The Johnstown Flood (1968) by David McCullough – a popular history of the 1889 dam failure, and subsequent flood, in Pennsylvania.[250] Edward Abbey wrote the novel The Monkey Wrench Gang (1975) about environmental activists who sought to destroy the Glen Canyon Dam.[251] The Patagonia company produced the documentary movie DamNation (2014) which advocates for dam removal to restore ecosystems and fish populations.[252]

References

Footnotes

  1. 1 2 3 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.[39]
  8. ↑ Including deforestation and the large amount of carbon introduced into the atmosphere by the manufacture of concrete.
  9. ↑ Statistics include only large dams.[51]
  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".[51]
  11. ↑ The count of reservoirs includes those created by all types of barriers, not all of which are dams.[52]
  12. 1 2 3 4 Data as of 2011.
  13. ↑ Embankment dams are the only type of modern dam not made of concrete.[58]
  14. ↑ Some embankment dams contain a solid, vertical steel wall or curtain in the center, extending from the bottom to the crest.[64]
  15. ↑ Many modern embankment dams are still made of a single type of earth.[67]
  16. ↑ Grande Dixence Dam is the world's tallest gravity dam as of 2025.
  17. ↑ The Nagarjuna Sagar Dam (1974) is an exception: it is a gravity dam made mostly of rubble.[73]
  18. ↑ A hollow gravity dam is not the same as a gravity dam that contains galleries. Many dams contain galleries – long, horizontal passages – that permit workers to perform inspections, and may contain equipment such as hydropower generators.[75]
  19. ↑ The term "curved gravity dam" is an alternative to "arch gravity dam".[77] In some contexts, the term "straight gravity dam" may be used to emphasize that there is no curve or arch.[76]
  20. 1 2 3 Arch dams are classified as "thin arch" (thickness less than 25% of height) and "thick arch" (thickness between 25% and 50% of height). Above 50%, it is an arch-gravity dam.[96][a]
  21. ↑ In practice, most gravity dams have a flat crest (top) so a road can go across; thus the shape is a trapezoid.[78]
  22. 1 2 The term "inclination" in the context of dams is the ratio of horizontal span to height. Inclination is the inverse of slope. An inclination of 1.0 is a 45 degree angle; an inclination of 0.0 is fully vertical. Inclination can also be expressed as a percentage: inclination 0.3 is the same as 30%.[80]
  23. ↑ 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.[83]
  24. ↑ Each buttress is separated from its neighbors by gaps at the top, which reduce stress within the dam structure.[89]
  25. ↑ The upstream face is essentially flat, but each buttress may have a bulge (convex side upstream). 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.[90]
  26. ↑ Buttress dams can use up to 89% less concrete than a comparable gravity dam.[92].
  27. ↑ An arc angle of 133 degrees is optimal for minimizing the amount of concrete needed to build a constant angle arch dam.[99]
  28. ↑ The word "radius" in the context of an arch dam refers to the radius of the central angle of the dam structure.
  29. ↑ Also called "variable radius" curve.[101]
  30. ↑ An early arch-gravity dam was the Monte Novo Dam in Portugal, built by the Romans.[104]
  31. ↑ An equivalent term is "multiple-arch buttress dam".[105]
  32. ↑ The earliest known multiple-arch buttress dam is the Esparragalejo Dam built by the Romans in the 1st century CE.[107]
  33. ↑ Statistics include only single-purpose large dams.[51]
  34. ↑ Between 12% and 15% of the world's population relied on food irrigated by dams (as of 2006).[116]
  35. ↑ In 2006, hydroelectricity capacity was 740 GW, with a total annual production of 2,800 TW-hours per year.[118]
  36. ↑ Dam designs sometimes use flood return periods of 50 year or 100 years.[121]
  37. ↑ 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).[137]
  38. ↑ To manufacture one ton of cement, about 0.8 tons of CO2 are emitted into the air.[154] Each cubic meter of concrete uses about 250 kg of cement and contributes around 200 kg of CO2 into the atmosphere..[153] These carbon figures for a dam only include CO2 from the manufacture of cement and do not include exhaust from vehicles and machinery used while building the dam.
  39. ↑ 200 kg/m3 x 28M m3 yields 5,600M kg.[153]
  40. ↑ Some fault lines are part of ancient rock systems and are no longer active.[166]
  41. ↑ Some hydropower plants – called "run of the river" plants – operate without a dam or reservoir: a long channel diverts some water from a river and carries it to a downstream location, where the hydropower plant is located.[178]
  42. ↑ The channel that conveys water to the generator may consist of two parts: the headrace (a horizontal channel that is not under pressure); and the penstock (a sloping or vertical channel that must be strong enough to withstand the weight of the water). The intake is always covered with a trash rack (grate) at the top to prevent debris from reaching the generator.[181]
  43. ↑ Also called a "shaft" or "morning glory" spillway
  44. ↑ On some low dams, the dam structure itself acts as a spillway, and the gates are placed on the top (crest) of the dam.

Citations

  1. ↑ "Dam", Collins English Dictionary.
  2. 1 2 Schnitter 1994, pp. 62–64.
  3. 1 2 Schnitter 1994, p. 76.
  4. ↑ Schnitter 1994, p. 90.
  5. 1 2 Schnitter 1994, pp. 124–127.
  6. 1 2 3 Schnitter 1994, pp. 151, 183.
  7. ↑ "Glossary", Reclamation Library.
  8. ↑
  9. ↑ Schnitter 1994, pp. 18–21.
  10. ↑
  11. ↑ Schnitter 1994, pp. 24–27.
  12. ↑ Schnitter 1994, pp. 13–14.
  13. ↑ Schnitter 1994, pp. 41–43.
  14. ↑ Schnitter 1994, pp. 33–36.
  15. 1 2 Brown 2026, Section: "Early dams of East Asia".
  16. ↑ Schnitter 1994, pp. 55–58.
  17. ↑ Schnitter 1994, p. 57.
  18. ↑ Schnitter 1994, pp. 56–57.
  19. 1 2 Schnitter 1994, p. 58.
  20. ↑ Schnitter 1994, p. 59.
  21. ↑ Schnitter 1994, p. 106.
  22. ↑
  23. ↑
  24. ↑ Lemperiere 2006, p. 1065.
  25. ↑ Schnitter 1994, p. 108.
  26. ↑ Schnitter 1994, pp. 120–122.
  27. ↑ Schnitter 1994, pp. 131–132.
  28. ↑
  29. ↑ Schnitter 1994, p. 152.
  30. 1 2
  31. ↑
  32. ↑
  33. ↑
  34. ↑ Brown 2026, "Section: "The 19th century".
  35. ↑ Schnitter 1994, p. 157.
  36. 1 2 Schnitter 1994, pp. 170–172.
  37. ↑
  38. ↑ Schnitter 1994, pp. 165–167.
  39. 1 2 3 Schnitter 1994, p. 213.
  40. ↑ "Zuiderzee", Britannica.
  41. ↑
  42. ↑
  43. ↑ Berlow 1998, pp. 10, 213.
  44. ↑ Jansen 2012, pp. 5–6.
  45. ↑ Berlow 1998, pp. 113, 195.
  46. ↑ Berlow 1998, pp. 84, 213.
  47. ↑ "Three Gorges Dam", BBC.
  48. ↑ Ranjan 2024, pp. 18, 27–33.
  49. 1 2 Brown 2026, Section: "Rise of environmental and economic concerns".
  50. ↑
  51. 1 2 3 4 5 6 7 "World Register of Dams", ICOLD.
  52. 1 2 3 4 5 Lehner 2011.
  53. ↑ "Research with GDW Data", Global Dam Watch.
  54. ↑
  55. ↑
  56. ↑
  57. ↑
  58. ↑ Denny 2010, p. 187.
  59. ↑
  60. ↑
  61. ↑
  62. ↑
  63. ↑ Siddiqui 2009, pp. 162–163.
  64. ↑ Schnitter 1994, pp. 160–162.
  65. ↑
  66. ↑
  67. ↑ Schleiss 2022, p. 61.
  68. ↑
  69. ↑ Siddiqui 2009, pp. 161, 163–164.
  70. ↑ Brown 2026, Section: "Embankment dams".
  71. ↑ "Grande Dixence Fact Sheet", Grande Dixence SA.
  72. ↑
  73. ↑ Schnitter 1994, p. 172.
  74. ↑
  75. ↑ Siddiqui 2009, pp. 139–141.
  76. 1 2
  77. ↑ Schleiss 2022, p. 52.
  78. ↑ Siddiqui 2009, pp. 137.
  79. ↑
  80. ↑
  81. ↑ Siddiqui 2009, pp. 148, 151–152.
  82. ↑
  83. ↑
  84. ↑
  85. ↑
  86. ↑
  87. 1 2
  88. ↑ Brown 2026, Section: "Concrete buttress and multiple-arch dams".
  89. 1 2
  90. ↑ Schnitter 1994, pp. 61, 117–118.
  91. ↑
  92. ↑ Schnitter 1994, p. 182.
  93. ↑
  94. 1 2
  95. 1 2
  96. ↑
  97. ↑
  98. ↑
  99. ↑
  100. ↑
  101. 1 2
  102. ↑ Schnitter 1994, pp. 186, 205–206.
  103. ↑
  104. ↑ Schnitter 1994, pp. 68–69.
  105. ↑ Schnitter 1994, p. 118-120.
  106. ↑
  107. ↑ Schnitter 1994, p. 65-67.
  108. ↑ Evans 2011, pp. 93, 506.
  109. 1 2 Evans 2011, p. ix.
  110. 1 2 Evans 2011, p. x.
  111. 1 2 Evans 2011, p. xi.
  112. ↑ Evans 2011, p. 519.
  113. ↑
  114. ↑ Schnitter 1994, p. x, Forward.
  115. ↑ Schleiss 2022, p. 69.
  116. 1 2 Lemperiere 2006, p. 1066.
  117. ↑
  118. 1 2 |Schleiss 2022, p. 69. |Lemperiere 2006, p. 1065. }}
  119. 1 2 3
  120. ↑ Lemperiere 2006, pp. 1066–1067.
  121. 1 2
  122. 1 2 Lemperiere 2006, p. 1067.
  123. ↑ "I-205 Abernethy Bridge". Oregon Department of Transportation.
  124. ↑
  125. ↑ Blight & Jewell 1998, pp. 6, 8, 29.
  126. ↑ Blight & Jewell 1998, p. 9.
  127. ↑ Blight & Jewell 1998, pp. 9–10.
  128. ↑
  129. 1 2 Siddiqui 2009, pp. 240–244.
  130. ↑ Gerwick 2014, pp. 148–155.
  131. ↑ Kitchen 2016, pp. 13–18.
  132. ↑ Kitchen 2016, pp. 13–14.
  133. ↑ Kitchen 2016, pp. 16–17.
  134. ↑ Kitchen 2016, pp. 17–18.
  135. ↑
  136. ↑
  137. ↑ El-Askary 2026.
  138. ↑ Siddiqui 2009, p. 130.
  139. ↑ Siddiqui 2009, p. 131.
  140. ↑ Onder & Yilmaz 2005.
  141. 1 2 3 Siddiqui 2009, pp. 1–3.
  142. ↑ Siddiqui 2009, pp. 4, 17, 49.
  143. ↑ Siddiqui 2009, pp. 4–6.
  144. ↑
  145. ↑ Fell 2014, pp. 686–689, 695–705, 766-769 975-985.
  146. ↑ Siddiqui 2009, pp. 49–90.
  147. ↑
  148. ↑
  149. ↑ Siddiqui 2009, pp. 91–94.
  150. ↑ Schleiss 2022, pp. 70–75.
  151. ↑ Schleiss 2022, pp. 76–77.
  152. 1 2 3 Schleiss 2022, pp. 70–71.
  153. 1 2 3 "Concrete CO2 Fact Sheet", NRMCA, p. 7.
  154. ↑ Bărbulescu 2025, p. 3, Section 2.1.
  155. ↑ "Three Gorges Dam", Britannica, Section: "Physical description".
  156. 1 2 Siddiqui 2009, p. 92.
  157. ↑
  158. ↑
  159. ↑
  160. ↑
  161. ↑
  162. ↑ Schleiss 2022, pp. 74–75.
  163. ↑ Siddiqui 2009, pp. 131–133.
  164. ↑ Siddiqui 2009, pp. 133–134.
  165. ↑
  166. ↑ Arnold 2012, p. 119.
  167. ↑ Siddiqui 2009, p. 132.
  168. ↑ Siddiqui 2009, pp. 131–132.
  169. ↑
  170. 1 2 Siddiqui 2009, p. 131-132.
  171. ↑ Turpin 2008, pp. 114–116, 120–122.
  172. ↑ Turpin 2008, pp. 106, 112–113.
  173. ↑ Turpin 2008, pp. 110–111.
  174. ↑ Turpin 2008, pp. 123–137.
  175. ↑ Turpin 2008, p. 138.
  176. 1 2 Kaygusuz 2016, pp. 361–362.
  177. ↑ Kaygusuz 2016.
  178. ↑ Kaygusuz 2016, p. 361.
  179. ↑
  180. ↑
  181. 1 2
  182. ↑
  183. ↑
  184. ↑ Schnitter 1994, pp. 214–215.
  185. ↑
  186. ↑
  187. ↑
  188. ↑ Siddiqui 2009, p. 201.
  189. ↑
  190. ↑
  191. ↑ Schnitter 1994, p. 225.
  192. ↑ "Locks", Britannica.
  193. ↑ Pelikan 1997, pp. 61–66.
  194. ↑
  195. ↑ Siddiqui 2009, pp. 1–4.
  196. 1 2 Siddiqui 2009, pp. 240–241, 244.
  197. ↑ Siddiqui 2009, p. 239.
  198. 1 2 Siddiqui 2009, p. 241,244.
  199. ↑ Siddiqui 2009, pp. 240–241.
  200. ↑ Siddiqui 2009, pp. 241–244.
  201. ↑
  202. ↑
  203. ↑ Siddiqui 2009, pp. 158–159.
  204. ↑
  205. ↑
  206. ↑ Schrader 2012, pp. 553–554, 556, 560–566.
  207. ↑ Schrader 2012, pp. 553–554, 556, 566.
  208. ↑
  209. ↑
  210. ↑ Schrader 2012, pp. 567–569.
  211. ↑
  212. ↑
  213. ↑ Fetzer 2012a, pp. 342–350.
  214. ↑ Fetzer 2012a, pp. 344–350.
  215. ↑ Fetzer 2012a, pp. 349–352.
  216. ↑
  217. ↑
  218. 1 2
  219. ↑
  220. ↑
  221. ↑ Siddiqui 2009, pp. 269–270.
  222. ↑ Siddiqui 2009, p. 270.
  223. ↑
  224. ↑ Siddiqui 2009, pp. 271–275.
  225. 1 2 3 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.
  226. 1 2 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.
  227. ↑ James 2012a, p. 17,25-27.
  228. ↑ James 2012a, p. 34-41.
  229. ↑
  230. ↑
  231. ↑ James 2012a, p. 17-27.
  232. ↑ James 2012a, p. 8-16.
  233. ↑ James 2012a, p. 41-53.
  234. ↑
  235. 1 2 Ranjan 2024, pp. 18–33.
  236. 1 2
  237. ↑ Ranjan 2024, p. 19,32.
  238. ↑ Chellaney 2015, pp. xiii, xviii, 2, 50, 56, 176–180.
  239. ↑ Chellaney 2015, pp. 48, 179–180, 197–202, 225.
  240. ↑
  241. ↑ Chellaney 2015, pp. 44–49, 185–187, 192–197, 262–263.
  242. ↑
  243. ↑ "Regulation of Dam Safety", ICOLD, pp. 10–11.
  244. ↑ "Regulation of Dam Safety", ICOLD, p. 12.
  245. ↑ "Regulation of Dam Safety", ICOLD, pp. 41–44.
  246. ↑ "Regulation of Dam Safety", ICOLD, p. 14.
  247. ↑ Fowkes 2025.
  248. ↑ Cray 2004, pp. 207–213.
  249. ↑ "Bombs Away", The Guardian.
  250. ↑ Sutor 2018.
  251. ↑ Lichtenstein 1976.
  252. ↑ Horn 2014.

Sources

Journals, news, and websites

  • Brown, John Guthrie; et al. (2026). "Dam". Britannica. Retrieved 22 April 2026.

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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