Dams are built for a number of purposes: to generate hydroelectric power; control flooding; safeguard water supplies for irrigation, domestic, or industrial use; provide for recreation; or ease navigation.
The earliest known dam was built by the Egyptians across the Garawi Valley in 2800 B.C.E. and measured 370 feet (113 m) along its crest. The masonry shell was filled with earth and rubble, but as it was not sealed against water, the center of the dam was soon washed away. This failure discouraged the Egyptians from further forays into dam construction.
The Romans, armed with their knowledge of concrete, were more successful. Their constructions initially relied on sheer weight of material to resist the water, but in the first century they built the first arch- type dam at Glanum in France. The apex of the arch pointed upstream, transferring the force along the dam and into the solid bedrock of the valley sides. This design was also favored by the Mongols in fourteenth- century Iran, but it was otherwise little used until the nineteenth century when French engineer Francois Zola designed his eponymous arch dam, using rational stress analysis for the first time.
In the latter half of the nineteenth century concrete was used as the primary construction material for the first time in a gravity dam in New York and an arch dam in Queensland, Australia. More complex structures were now within reach, and multiple arch, cupola, and buttress dams sprang up around the United States.
China is home to the world's largest dam project, the Three Gorges Dam, which is expected to be fully operational in 2009. It spans the Yangtze River and has been constructed to ease flooding on the Yangtze and provide hydroelectric power for millions.
