Short answers, pulled from the story.
The four basic types of dams are gravity, embankment, buttress, and arch. Gravity dams rely on their weight, embankment dams are large earthworks, buttress dams use a sloped concrete face supported by triangular buttresses, and arch dams use a curved wall to redirect water force into the valley walls.
The Jawa Dam near Amman, Jordan, is the oldest known dam, built around 3000 BCE. This embankment dam was part of an elaborate irrigation system, measuring 28 meters thick and 5.5 meters high.
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.
Dams provide irrigation, hydropower, water supply, flood management, recreation, inland navigation, and fish farming. About 20 percent of the world's arable land is irrigated from reservoirs impounded by dams, and global hydropower accounted for about 20 percent of the world's electricity supply as of 2022.
Dams fail from earthquakes, weak rock at the abutments, water leaking within or under the structure, or the dam sliding over its foundation. Earthen embankment dams face a unique risk called piping, where a small leak gradually erodes a channel through the dam. Between 1900 and 1994, only 100 large dams over 15 meters failed.
Dams built without the consent of downstream nations have led to international disputes involving Turkey, India, Ethiopia, and China. Examples include Turkey's Karakaya Dam on the Euphrates and Tigris, India's Farakka Barrage on the Ganges, and Ethiopia's Grand Ethiopian Renaissance Dam on the Blue Nile, built in 2020 over opposition from Sudan and Egypt.
Over 1,200 dams had been removed in the US as of 2016, restoring fisheries, sediment flow, and natural water temperatures. In 2025, over 600 dams were removed across Europe, restoring 3,740 kilometers of rivers and streams, supported by the European Union's Nature Restoration Law passed in 2024.