Dam

B

Forerunners of Modern Dams

By the 17th century Spanish builders had erected a number of prominent storage dams, including the Almansa Dam on the Vega de Belén River, which measures 18 m (60 ft) high and is still standing today. In the 17th century the Spanish also built the first true arch dam in Europe since Roman times, the Elche Dam, which stands 21 m (70 ft) high.

The need for large dams did not become widespread until the 19th century, when the populations of urban centers swelled. Urban growth increased the need for water and electricity, which fostered construction of large dams on an unprecedented scale. Engineers began to apply mathematical formulas and structural theory to make dams safer. In the 1850s French engineer Augustin de Sazilly used principles of mathematics to minimize the amount material necessary to build a masonry gravity dam. De Sazilly proposed that the most advantageous shape for a gravity dam was a triangle with a vertical upstream face. De Sazilly’s innovation took hold and lives on in the triangular profile and near-vertical upstream face of many modern concrete gravity dams.

In the late 19th century, dam engineers resurrected the use of concrete, which had not been used in dam construction since ancient Roman times. Among the first of the modern concrete dams are Boyd’s Corner Dam, built to provide water to residents of New York City in 1872, and the San Mateo Dam near San Francisco, California, completed in 1890.

C

Dams of the 20th Century

During the 20th century dam engineers expanded upon the mathematical formulas and structural designs pioneered in the 19th century. Twentieth-century designs incorporated sophisticated mathematics and materials science, giving rise to higher and stronger dams than ever before. These engineering marvels captured the attention of the general public, who regarded them as major symbols of civic achievement. Dams tamed raging rivers. In so doing they eliminated floods, provided people with water and electricity, and caused arid deserts to yield thriving agricultural crops.

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In 1902 President Theodore Roosevelt signed the National Reclamation Act. This act created the United States Reclamation Service (renamed the U.S. Bureau of Reclamation in 1923) to help make the arid desert lands of the American West suitable for economic development. The agency was intended to use the proceeds from the sale of public lands in the West to design and construct dams to impound water for irrigation. The Theodore Roosevelt Dam, which impounds the Salt River near Phoenix, Arizona, was one of the agency’s first major projects. This dam helped convert 240,000 acres of Arizona desert into fertile cropland.

Irrigation projects of this nature were being undertaken around the world. For example, the original Aswān Dam, completed in 1902, was built to control annual flooding of the Nile River in Egypt and to increase irrigation in the Nile River delta.

Hydroelectric power also gained importance during the early years of the 20th century. When the Keokuk Dam, on the Mississippi River at Keokuk, Iowa, began operation in 1913, it was the largest hydroelectric dam in the world. By 1920 hydroelectric power plants accounted for 40 percent of the electric power produced in the United States.

During the Great Depression of the 1930s, dam construction attracted significant attention because it provided a highly visible means of putting people to work. Dams also symbolized progress and success in the face of economic adversity. Hoover Dam was constructed between 1931 and 1936, during the height of the Great Depression. The huge project provided people with a sense of national pride and put thousands of people to work during this difficult time. Pouring the concrete for the dam, just one part of its construction, required the work of approximately 5,000 men for more than two years. Hoover Dam was only one of many dams constructed during the Great Depression. Others included the Norris Dam, built by the Tennessee Valley Authority; the Fort Peck Dam across the Missouri River near Glasgow, Montana, built by the U.S. Corps of Engineers; and Grand Coulee Dam, built by the U.S. Bureau of Reclamation on the Columbia River in central Washington. When the Grand Coulee Dam was completed in the early 1940s, it was proudly proclaimed the first human-made structure to exceed the size of the largest pyramids built in ancient Egypt.

The fervent pace of dam construction continued and even accelerated after World War II (1939-1945). The postwar period saw the construction of some of the most impressive structures in the world. The Grand Dixence Dam, a concrete gravity dam on the Dixence River in the Swiss Alps, was built between 1951 and 1961. At 285 m (935 ft) high, it is one of the tallest gravity dams in the world. The world’s two largest storage dams were also built during this period. Owen Falls Dam in Uganda, built in 1954, stores more than 204 million cubic meters (267 million cubic yards) of water in its reservoir, Lake Victoria. When the Kariba Arch Dam, on the Zambezi River between Zimbabwe and Zambia, was completed in 1959, it created Lake Kariba, the second largest reservoir in the world. This lake stores more than 180 million cubic meters (236 million cubic yards) of water.

In the late 20th century large dams continued to serve as a source of pride throughout the world. This is perhaps best exemplified by the efforts of the People's Republic of China to build the Three Gorges Dam across the Yangtze River. More than 200 m (656 ft) high and 1.6 km (1 mi) long, the dam will create a reservoir 650 km (400 mi) long for irrigation of the Yangtze Valley when it is completed in 2009. The dam’s hydroelectric power plant is expected to generate more than 18,000 megawatts of electricity, which will be distributed to users throughout central China.

D

Opposition to Large Dams

In the late 20th century widespread concern developed over the environmental effects of large dams. Such concerns were first raised in Great Britain more than 100 years ago with protests over the construction of Lake Thirlmere Dam in northwestern England’s scenic Lake District. In the early 20th century, American naturalist John Muir raised public awareness of the adverse environmental effects of Hetch Hetchy Dam (see Conservation). This dam was built in Yosemite National Park to provide water for the city of San Francisco. By the 1960s citizens across the United States expressed concern about the number of river valleys flooded by reservoirs and the amount of wilderness lost to economic progress. At the end of the 20th century, the issue embroiled environmentalists and dam proponents around the world.

In 1999 the U.S. government ordered the removal of Edwards Dam on the Kennebec River in Maine to restore populations of Atlantic salmon in the region. The decision to breech Edwards Dam was based on economic concerns because the dam produced a relatively small amount of hydroelectric power but blocked significant growth of a large commercial fishery. The issue of dam removal also captured the attention of the American public when environmentalists began campaigning for removal of four dams on the lower Snake River in the Pacific Northwest. Together, these dams generate more than 2 million kilowatts of electric power, but populations of spawning fish have plummeted since their completion in the 1960s and 1970s. Activists have also proposed the removal of the Glen Canyon Dam, on the Colorado River near Page, Arizona. Water backing up against the Glen Canyon Dam inundated hundreds of square kilometers of the Colorado River Basin, creating Lake Powell, one of the largest reservoirs in the United States. Lake Powell devastated the fragile ecosystems it flooded and completely changed the river environment downstream. Dam removal advocates argue that Glen Canyon Dam causes more harm than good.

Proponents of large dams argue that their benefits outweigh their negative consequences. Expanding urban populations depend more than ever on the electric power and fresh water provided by dams. Dams also protect the lives of inhabitants of floodplains, prevent property damage, and provide a way to transport food and other goods from one region to another.

Such controversy is not unique to North America. Conflict over the construction of the mammoth Three Gorges Dam on the Yangtze River in China rages on, even as the dam takes shape. At stake are not only wildlife but also 2 million people who must move from their ancestral homes and farms to make way for the dam and its reservoir. The debate between dam proponents and removal advocates promises to continue for many years to come.

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