A dam is a structure for impeding and controlling the flow of water in a water course, and which increases the water elevation to create the hydraulic head. The reservoir creates, in effect, stored energy.
Dams are one of the earliest known man-made structures, records existing from c. 2900 BC of a 15-meter-high dam on the Nile. Construction methods were largely empirical until 1866, when the first scientifically designed dam was built in France.
Dams are classified by profile and building materials, these being determined by availability and site. They must be strong enough to hold back water; withstand ice, silt, and uplift pressures, and stresses from temperature changes and earthquakes. The site must have stable earth or rock that will not unduly compress, squeeze out or let water seep under the dam. Borings, seismic tests, structural models, and computer simulations are all design aids.
Masonry or concrete dams are typically used for blocking streams in narrow gorges. The highest are around 300 meters high. A gravity dam holds back water by its own weight and may be solid, sloping downstream with a thick base, or buttressed, sloping upstream and strengthened by buttresses which transfer the dead weight sideways; these require less concrete. Arch dams, with one or more arches pointing upstream, are often built across a canyon and transfer some water pressure to its walls. Hoover Dam, built in 1936, is a combination of arch and gravity types. Embankment or earthfill dams are large barriers of rock, sand, silt, or clay for controlling broad streams. As in a gravity dam, their weight deflects the horizontal water thrust downward toward the broad base. The materials may be uniformly mixed or there may be zones of waterproof material such as concrete either on the upstream face or inside the dam. During construction, temporary cofferdams are built to keep water away from the site. Automatic spillways for excess water, intakes, gates, and bypasses for fish and ships all form part of a dam complex.