Dam

• 1. Introduction

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  A dam is a structure that impounds or restricts the flow of water, and the oldest one we know sits near Amman in modern Jordan. The Jawa Dam was built around 3000 BCE, an earthwork 28 meters thick and 5.5 meters high, part of an elaborate irrigation system. Five thousand years later, the International Commission on Large Dams counted 62,362 large dams worldwide in 2025. The total number of reservoirs, large and small, was estimated at 16.7 million in 2011, holding roughly 8,070 cubic kilometers of water. That is about 10 percent of the volume held by Earth's natural freshwater lakes. How did a pile of earth in the desert grow into something that reshapes rivers across continents? What separates a structure that holds back a sea of water from one that buckles and kills thousands? And why do these silent walls so often become the spark for war?

• 2. Four Ways To Hold A River

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  Gravity dams win by sheer mass, relying on their own weight to resist the force of impounded water. Historically built of masonry with mortar in the joints, nearly all modern examples are concrete. The cross-section is roughly triangular, and to keep the structure from tipping, the profile must obey the middle-third rule. That rule demands the net force land on the middle portion of the base rather than near the downstream edge. The thickness of the base should be 70 to 85 percent of the height. Because these dams are so heavy, they must rest on bedrock, since a gravity dam built over soil would compress it, settle, and perhaps crack.

  Embankment dams are the most common structure, a pile of earth shaped into a broad trapezoidal levee. They split into rockfill or earthfill depending on the primary material, and they can be built from whatever lies near the site. That makes them cheaper than concrete dams needing imported rock and cement. Their broad base spreads weight over a wide area, so they can sit on softer soils. Their weakness is porosity. Water can seep through or underneath, and a failure called piping begins when a small channel forms and gradually enlarges until a hole is pierced through the dam. Clay resists seepage 10 times more than silt, 10,000 times more than sand, and 100 million times more than gravel.

  Buttress dams present a flat upstream face supported on the downstream side by triangular buttresses. The upstream face is sloped, typically between 0.3 and 1.0, so the reservoir pushes the dam down into the ground and adds stability. They use far less concrete than a comparable gravity dam, but a more complex construction process offsets those savings. They are not as strong as gravity dams and suit only lower heights. The individual buttresses can shift slightly against each other, so the face is split into separate buttress heads, with flexible seals filling the gaps between them.

  Arch dams curve to transfer the force of impounded water into the valley walls rather than the foundation below. They can only be built where a valley is relatively narrow with strong, steep rock walls. They are thin, their base less than half their height, and always made of concrete or masonry. The central angle can run from 46 degrees to 140 degrees. Most older arch dams used a constant radius shape resembling a section of a vertical cylinder. The constant angle shape, which reduces radius from crest to base for maximum strength, first appeared in a dam built in 1914.

• 3. From Trial And Error To Science

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  The Hittite Empire built several dams between the 17th and 13th centuries BCE, including the Eflatun Pinar dam and spring temple near modern Konya, Turkey. In China, an engineer named Sunshu Ao impounded the Afengtang Reservoir around 580 BCE, and it still exists today. The Sabaean peoples raised dams across the Wadi Danah in modern Yemen starting around 1500 BCE, building toward the Great Dam of Marib around 500 BCE, which stood 700 meters long and 20 meters high. In Sri Lanka, dams including Tissa Wewa were built around 370 BCE, some of them several kilometers long.

  The Romans were the first to use cement as a construction material, mixing it with small rocks to form concrete or with sand for mortar. Some Roman cements, particularly those with volcanic ash, were waterproof. Around 60 CE they finished the Subiaco Dam, both one of their earliest and their tallest, standing 40 meters high and 13.5 meters thick. They built about 80 dams in Hispania, modern Spain, including the Proserpina Dam, which impounded 6 million cubic meters of water and remained operational in 2026. After the Persian king Shapur I defeated the Roman emperor Valerian, he set captured Romans to work on the Band-e Kaisar, which doubled as a 40-arch bridge over the Karun River.

  The Kurit Dam, the world's first large thin arch dam, was built in Persia around 1350 CE. Initially 26 meters high and later raised to 64 meters, it remained the world's tallest dam until the start of the 20th century. In France, the Bazacle weir around 1170 CE powered water wheels for milling and mining. Spain's Elche Dam, built in 1640 and still standing, was the first true arch dam in Europe since Roman times.

  In the late 18th century, dam design began shifting from trial and error to a discipline rooted in science. In 1776, the French scientist Charles-Augustin de Coulomb created a formula for how soil reacts under stress, later applied to dams by Alexandre Collin. Claude-Louis Navier developed the theory of elasticity in 1826. In 1847, Francois Zola became the first engineer to design an arch dam from an analytical consideration of stresses. The Scottish physicist William John Macquorn Rankine developed a theory of retaining walls in the 1850s that applied to dams.

• 4. What Dams Are For

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  Irrigation draws on about 20 percent of the world's arable land, watered from reservoirs impounded by dams as of 2022. Beyond moving water straight into canals, dams support farming through dry-season releases, holding water during the wet season and letting it out when the river would otherwise run low. This keeps water in the river year-round.

  Global hydropower capacity accounted for about 20 percent of the world's electricity supply as of 2022. More than 80 percent of the world's reservoir water storage capacity goes to generating hydropower. Some hydropower dams add a pumped-storage capability, consuming excess electricity to lift water from a low reservoir to a higher one. When the grid needs more power, that water is released back through the generators. The cycle can run daily, using cheap nighttime power from nuclear or oil plants to pump water up, then releasing it during peak daytime demand.

  Flood-management dams numbered 2,510 large structures worldwide in 2025. They do not try to stop all floodwater, but instead reduce the peak flood height to a safe limit. Because floods are unpredictable, the goals are expressed as statistical margins based on long return periods, such as safely regulating a 1-in-100 year flood. Water supply relied on 3,394 large dams in 2025, with industrial usage about twice domestic usage.

  A tailings dam impounds the waste from mining operations, and most are embankment structures. Unlike a normal water dam built in a valley, a tailings dam may sit on flat ground, its embankment enclosing the tailings on all sides. These dams are often raised over time as mining continues. Because tailings can include toxic by-products such as arsenic or lead, they usually carry special protective measures to keep contaminants out of the surrounding water supply.

• 5. When Walls Give Way

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  The Vajont Dam in Italy failed without the dam itself breaking. Built in 1959, this concrete arch dam was then the tallest in the world at 267 meters, set in a valley with steep sides prone to landslides. Designers failed to fully understand the geology. In 1963 a huge landslide slid down the hillside above the reservoir, displacing nearly all the water and sending a 125-meter wave over the top. The flood killed over 2,000 people downstream, while the dam suffered only minor damage.

  The earliest known dam failure struck around 2600 BCE, when Egypt's Sadd el-Kafara dam failed near the time of its completion, apparently from a flood that severely damaged the structure. The first modern arch dam to fail was the Malpasset Dam in France in 1959. The cause was not the concrete's shape or strength, but an underground slip line that moved under the combined weight of the dam and reservoir. The dam cracked, and the flood killed several hundred people downstream.

  The Baldwin Hills embankment dam in the US failed in 1963, and despite extensive investigation no definitive cause was found. Suspects ranged from weak foundation soil to fault lines, irregular settling, and a nearby oil field depleted then re-pressurized. Between 1900 and 1994, only 100 large dams over 15 meters failed, as dam failure rates dropped exponentially through the 20th century. Yet the potential for downstream damage rose, because dam heights and reservoir capacities kept growing.

  Natural dams carry their own danger. A rockslide dam forms when a slide blocks a river, creating a lake upstream, and there are thousands of them around the world. One created in 2010 in Pakistan formed Attabad Lake. In 1786 in China, an earthquake created a rockslide dam on the Dadu River, which failed ten days later and killed 100,000 people. Engineers have even used blasting on mountainsides to trigger a rockslide and form a crude embankment, called a blast-fill dam.

• 6. Drowned Homes And Lost Wonders

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  The Three Gorges Dam in China required the relocation of 1.4 million people. People living within reservoir boundaries must leave for new homes, which can cause large-scale social disruption. Land next to a reservoir may grow saturated with water, hurting agriculture and raising soil salinity. The level of groundwater around a reservoir may rise, and the quality people pump from wells may degrade.

  The Aswan High Dam in Egypt, built in 1960 across the Nile, displaced 50,000 Nubians and devastated the Nubian community. To build it across a deep, sandy riverbed, grout was pumped 208 meters deep across 57,000 square meters to stop water flowing underneath. The same dam forced the relocation of the Philae and Abu Simbel temples. The Itaipu Dam flooded the spectacular Guaira Falls.

  Concrete dams put large amounts of greenhouse gases into the atmosphere. Producing one cubic meter of concrete releases roughly 200 kilograms of carbon dioxide. A dam the size of Three Gorges, containing 28 million cubic meters of concrete, would release roughly 5.6 billion kilograms of carbon dioxide. More gas escapes from reservoir water as submerged plants and trees decompose.

  Downstream, the flow of a river may fall, especially in the dry season, and water quality may suffer. Rivers normally carry sediment that can replenish soil downstream, but that flow drops once a dam traps sediment in the reservoir. Roughly half the sediment of the world's rivers is trapped by dams, about 8 to 16 cubic kilometers per year. Fish migration may be seriously harmed, since a dam can block fish from swimming upstream to spawn.

• 7. Taking Them Down

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  Over 800,000 dams and barriers obstruct rivers and streams in the US. More than 1,200 had been removed as of 2016, with over 600 of those removed between 1996 and 2016. When a dam comes down, fisheries are restored, sediment flows are re-established, the river widens, braiding grows more pronounced, and natural water temperatures return.

  Between 2014 and 2018, the Elwha Dam and the Glines Canyon Dam were removed from the Elwha River in the US, together having stored about 30 million tonnes of sediment. Their removal restored the delivery of sediment and wood downstream and re-established the river delta. A group of four dams, including the Iron Gate Dam, came off the Klamath River between 2020 and 2024, the result of a sustained campaign by Native Americans and environmentalists. One goal was to restore one of the largest salmon migration routes on the Pacific coast of North America.

  Europe held over 1,000,000 dams and barriers in 2021, with at least 150,000 of them no longer required. River fragmentation is a major cause of a 55 percent decline in migratory fish populations and an 80 percent decline in fish biodiversity. In 2024 the European Union passed the Nature Restoration Law, which encourages removing unneeded dams. In 2025, over 600 dams were removed across Europe, restoring 3,740 kilometers of rivers and streams.

• 8. Water As A Weapon

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  Twenty documented attacks on dams occurred between 1917 and 1993. The Geneva Conventions were amended in 1949 to prohibit attacks on dams if they would cause severe losses among the civilian population. World population growth and climate change have pushed water scarcity into international conflict over transnational rivers. Most countries believe they may unilaterally build dams within their borders without consulting downstream nations.

  Turkey's Southeastern Anatolia Project includes many dams, among them the large Karakaya Dam, most on the Euphrates and Tigris rivers flowing down into Syria and Iraq. Those downstream nations have protested over potential water-supply issues. India and Bangladesh have a long-standing dispute over the Ganges, centered on the Farakka Barrage built in 1972; a treaty was signed in 1996, but tensions persist. The Indus Waters Treaty was signed between India and Pakistan in 1960, yet India later built large dams on the Indus over Pakistan's opposition, including the Baglihar Dam and Kishanganga Dam.

  In 2020, Ethiopia built the Grand Ethiopian Renaissance Dam on the Blue Nile, despite opposition from Sudan and Egypt. On the Mekong, Thailand, Cambodia, Laos, and Vietnam signed a treaty in 1995 creating the Mekong River Commission, but China stayed out and later built many dams, including the Xiaowan Dam and Nuozhadu Dam. Against these conflicts stands the Itaipu Dam of 1984, on the border of Brazil and Paraguay, an example of international cooperation. Edward Abbey caught that tension in fiction with The Monkey Wrench Gang in 1975, a novel about activists seeking to destroy the Glen Canyon Dam.