DAMS

With the exception of the Great Wall of China, dams are the largest structures ever built. Throughout history, big dams have prevented flooding, irrigated farmland, and generated tremendous amounts of electricity. Without dams, modern life as we know it would simply not be the same.
Since the first large-scale dam was built in Egypt more than 5,000 years ago, engineers have devised various types of dams to withstand the forces of a raging river.
ARCH DAMS
are good for narrow, rocky locations. They are curved, and the natural shape of the arch holds back the water in the reservoir. Arch dams, like the El Atazar Dam in Spain, are thin and require less material than any other type of dam.
BUTTRESS DAMS
may be flat or curved, but one thing is certain: a series of supports, or buttresses, brace the dam on the downstream side. Most buttress dams, like the Bartlett Dam in Arizona, are made of reinforced concrete.
EMBANKMENT DAMS
are the most commonly built dams in the United States. They are massive dams made of earth and rock. Like gravity dams, embankment dams rely on their heavy weight to resist the force of the water. But embankment dams are also armed with a dense, waterproof core that prevents water from seeping through the structure. Tailings dams -- large structures that hold back mining waste -- are a type of embankment dam.
GRAVITY DAMS
are massive dams that resist the thrust of water entirely by their own weight. Most gravity dams, like the Grand Coulee Dam in Washington, are expensive to build because they require so much concrete. Still, many people prefer its solid appearance to the thinner arch and buttress dams.