Dam

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Dam is open for . The scholarship allows level programm(s) in the field of taught at . The deadline of the scholarship is .

I -INTRODUCTION
Dam, structure that blocks the flow of a river, stream, or other waterway. Some dams divert the flow of river water into a pipeline, canal, or channel. Others raise the level of inland waterways to make them navigable by ships and barges. Many dams harness the energy of falling water to generate electric power. Dams also hold water for drinking and crop irrigation, and provide flood control.
The oldest known human-made dams were built more than 5,000 years ago in arid parts of the Middle East to divert river water to irrigate crops. Today there are more than 500,000 dams worldwide. The vast majority of these are small structures less than 3 m (10 ft) tall. Engineers regard dams that measure more than 15 m (50 ft) high as large dams. About 40,000 large dams exist in the world today.

II -WHY PEOPLE BUILD DAMS
People build dams to divert water out of rivers for use in other locations or to capture water and store it for later use. The volume of water flowing in any given river varies seasonally. In the spring and early summer, rivers typically swell with water from rainstorms and mountain snowmelt. In the drier months of late summer and autumn, many rivers slow to a trickle. Storage dams impound seasonal floodwater so it can be used during periods of little or no rainfall. The water that backs up against a storage dam forms an artificial lake, called a reservoir. Release of water from the reservoir can be controlled through systems of pipes or gates called outlet works.

A -Irrigation and Drinking Water
From ancient times to the present, people have built dams to capture water to irrigate crops in areas where rainfall does not provide enough ground moisture for plant growth. Simple irrigation systems often depend on small diversion dams that raise the height of a stream. Flowing water backs up against the dam until it overflows into a canal, ditch, or pipe that carries the water to fields.

Large storage dams support sophisticated modern irrigation systems that have dramatically altered the landscape of arid regions throughout the world. For example, large storage dams in the American West have transformed millions of acres of arid desert into productive cropland. Hoover Dam, which stretches across the Colorado River near Las Vegas, Nevada, stores about twice the annual flow of the river in its reservoir, Lake Mead. This reservoir holds enough water to cover the state of Pennsylvania to a depth of one foot. Lake Mead helps provide a dependable water supply for more than 400,000 hectares (1 million acres) of farmland in southern California and southwestern Arizona, and 162,000 hectares (400,000 acres) in Mexico.
Dams also replenish the water supply of cities and towns. Colorado River water impounded by Hoover Dam in Lake Mead helps provide water for drinking and other uses to more than 16 million people in greater Los Angeles, California, and portions of Arizona and southern Nevada.

B -Hydroelectric Power
Hydroelectric dams generate electricity (see Waterpower). Hydroelectric dams harness the energy of water released from the reservoir to turn hydraulic turbines. The turbines convert the energy of the falling water into mechanical energy, which is used to power electric generators.
The Grand Coulee Dam, on the Columbia River in Washington State, continuously generates more than 6,500 megawatts of electricity. This power is distributed to industrial and residential consumers throughout the western United States. The Itaipú Dam, on the Panará River between the countries of Brazil and Paraguay, continuously generates more than 12,600 megawatts of electricity. It supplies nearly 80 percent of the electric power used in Paraguay and 25 percent of the electricity used in Brazil.

Dams designed to generate electricity deliver water to a building, called a powerhouse, which contains highly specialized power-generating equipment. Large pipes called penstocks carry water from the reservoir down into the powerhouse. Water exits a penstock through small openings, which concentrate the flow and direct it onto the blades of a large hydraulic turbine. The force exerted by the falling water rotates the blades, and this action drives the shaft of an electric generator. The shaft spins giant magnets in the generator, creating an electric current (see Induction). Power lines transmit the current to consumers within a regional power network (see Electric Power Systems).

C -Flood Control
Dams also protect low-lying areas from floods (see Flood Control). Floods occur when more rain falls than the soil and vegetation can absorb. The excess water runs off the land in greater quantities than rivers, streams, ponds, and wetlands can contain. Such heavy rains, and also snowmelt, periodically cause rivers to overflow their banks, spilling onto the surrounding floodplain. Ensuing floods can damage property and endanger the lives of people and animals.

To control floodwaters in floodplains, engineers sometimes construct a group of dams and reservoirs along streams that feed into main rivers. Water from snowmelt and heavy rains is stored in the reservoirs, then released gradually into the main rivers during the dry season. This strategy is exemplified by the dams of the Tennessee Valley Authority (TVA), a federally sponsored corporation created in the 1930s by the President and Congress of the United States.

Most dams include an important safety feature called a spillway for use during extreme flood conditions. A spillway provides a way for excess floodwater flowing into a reservoir to be diverted around a dam. Without a spillway, the floodwater could overtop the dam’s crest and erode the backside of the dam, which might cause it to collapse. When this happens, millions of cubic meters of water can rush downstream, causing mass destruction. In 1889 more than 2,000 people were killed in Johnstown, Pennsylvania, when, during a heavy rainstorm, the South Fork Dam collapsed after a clogged spillway caused the reservoir to overtop the dam.
In some cases, the spillway is completely separate from the main body of the dam. This type of spillway usually comprises a gently sloped concrete channel that carries excess water around the dam and deposits it in the river below. In other cases, the spillway is part of the actual dam. Such spillways release water directly over the top of the dam through an overflow area that is slightly lower than the crest of the dam. In still other cases, excess water drains through a vertical shaft spillway, then into a gently sloped conduit that carries it through a tunnel and into the river downstream from the dam.

D -Navigation
Dams help make inland waterways accessible to ships and barges. By inundating shallow, rocky streambeds and controlling the release of water from reservoirs, dams make rivers deep enough for ships and barges to pass through without running aground. For example, to make the Ohio River in the east-central United States navigable throughout its length, engineers constructed a series of 13 dams. These dams enable commercial vessels to travel from Pittsburgh, Pennsylvania, to the Mississippi, one of the most important shipping rivers in the world.

When a dam obstructs a navigable river, engineers build a canal adjacent to the dam to permit ships and barges to bypass the dam. Canals may incorporate one or more locks, which contain mechanisms to control the water level. Ships and barges are raised or lowered with changes in the water level in the lock. One gate in the lock then opens, enabling a vessel to exit to a higher or lower section of the waterway. Locks prevent water from rushing uncontrolled through the canal.

E -Multiple Purposes
Many modern dams serve two or more purposes. For example, the TVA designed and built dams along the Tennessee River and its tributaries to provide flood control, generate electric power, and control river levels to permit year-round navigation.

While a dam can serve many different functions, it can prove impossible to operate at maximum efficiency for each purpose. For example, irrigation, power generation, flood control, and recreation may place conflicting demands on dams. A farmer who depends on a dam for irrigation wants water released from the reservoir only when crops need water during the summer growing season. On the other hand, an electric power company wants water released throughout the year to provide its customers with a steady source of power. Dams provide the most effective flood control when reservoir levels are relatively low, enabling them to easily absorb runoff from unexpected storms. But people who use reservoirs for recreational activities prefer the water levels to be high because it makes for better swimming and boating.

III -TYPES OF DAMS
Dams are classified by the type of material used in their construction and by their shape. Dams can be constructed from concrete, stone masonry, loose rock, earth, wood, metal, or a combination of these materials. Engineers build dams of different types, depending on the conditions of the riverbed, the geology of the surrounding terrain, the availability of construction materials, and the availability of workers. When more than one type of dam will suffice, engineers often opt to construct a type that they have built previously.

A -Gravity Dams
Gravity dams use only the force of gravity to resist water pressure—that is, they hold back the water by the sheer force of their weight pushing downward. To do this, gravity dams must consist of a mass so heavy that the water in a reservoir cannot push the dam downstream or tip it over. They are much thicker at the base than the top—a shape that reflects the distribution of the forces of the water against the dam. As water becomes deeper, it exerts more horizontal pressure on the dam. Gravity dams are relatively thin near the surface of the reservoir, where the water pressure is light. A thick base enables the dam to withstand the more intense water pressure at the bottom of the reservoir.

Most gravity dams are made from concrete, a mixture of portland cement, water, and aggregates (varying mixtures of sand and gravel). Concrete is well suited for dam construction because it is waterproof, extremely strong, and can be easily poured into forms. Concrete gravity dams make use of a triangular cross-section and steep upstream face that could not be constructed using loose material.

Concrete can also be expensive, making concrete gravity dams costly structures to build because they require so much concrete to resist the forces of the water pressure. The Grand Coulee Dam contains nearly 8 million cubic meters (almost 10 million cubic yards) of concrete, enough to build a sidewalk 1.2 m (4 ft) wide and 10 cm (4 in) thick twice around the equator. It is one of the most massive structures ever built, standing 168 m (550 ft) high and 1,592 m (5,223 ft) long.

B -Embankment Dams
An embankment dam is a gravity dam formed out of loose rock, earth, or a combination of these materials. The upstream and downstream slopes of embankment dams are flatter than those of concrete gravity dams. In essence, they more closely match the natural slope of a pile of rocks or earth.
Of the many different kinds of embankment dams that exist, rock-fill embankment dams and zoned-embankment dams are among the most common. Rock-fill embankment dams consist of a mound of loose rock covered with a waterproof layer on the upstream side to prevent excessive seepage and erosion. The waterproof layer may be made of concrete, flat stone panels, or other impervious materials. Zoned-embankment dams include an impervious core surrounded by a mound of material that water can penetrate. The supporting mound is usually made of loose rock or earth.

The core might be built from concrete, steel, clay, or any impervious materials. Like concrete gravity dams, embankment dams hold back water by the force of gravity acting upon their mass. Embankment dams require more material because loose rock and earth are less dense than concrete. Despite the huge volumes of material required to build an embankment dam, engineers often choose to build them if the materials are readily available. The Tarbela Dam, which crosses the Indus River in Pakistan, contains more than 126 million cubic meters (more than 165 million cubic yards) of earth and rock. This amounts to more than 15 times the volume of concrete used in the Grand Coulee Dam.

C -Arch Dams
Arch dams are concrete or masonry structures that curve upstream into a reservoir, stretching from one wall of a river canyon to the other. This design, based on the same principles as the architectural arch and vault, transfers some water pressure onto the walls of the canyon. Arch dams require a relatively narrow river canyon with solid rock walls capable of withstanding a significant amount of horizontal thrust. These dams do not need to be as massive as gravity dams because the canyon walls carry part of the pressure exerted by the reservoir. For example, the Glen Canyon Dam, which spans the Colorado River in Arizona, is the highest arch dam in the United States. It is 216 m (710 ft) high and 475 m (1,560 ft) long but contains less than 4 million cubic meters (under 5 million cubic yards) of concrete. Because they require less material than gravity dams, arch dams can be less expensive to build.

Not all concrete and masonry dams that curve into a reservoir qualify as arch dams. In some cases, engineers choose to use an arched shape even if it is not a structural necessity. For example, Hoover Dam features a prominent curve but the structure is actually thick enough to stand as a gravity dam. In many ways, the massive dam’s curvature comprises more of an aesthetic effect than it does a structural necessity.

D -Buttress Dams
A buttress dam consists of a wall, or face, supported by several buttresses on the downstream side. The vast majority of buttress dams are made of concrete that is reinforced with steel. Buttresses are typically spaced across the dam site every 6 to 30 m (20 to 100 ft), depending upon the size and design of the dam. Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a river valley.
Buttress dams fall into two basic categories: flat slab and multiple arch. Flat slab buttress dams have a flat upstream face. These dams are sometimes called Ambursen dams in recognition of Nils Ambursen, the Norwegian-born American engineer who popularized them in the early 20th century. An example of a flat slab buttress dam is the Stony Gorge Dam, which crosses Stony Creek near Orland, California. It stands 42 m (139 ft) tall, stretches 264 m (868 ft) long, and contains 33,000 cubic meters (43,100 cubic yards) of concrete.

Multiple arch buttress dams feature an upstream face formed by a series of arches. The arches rest on top of buttresses that extend down to the foundation. Bartlett Dam, on the Verde River near Phoenix, Arizona, is a multiple arch dam. It stands 94 m (309 ft) high, stretches 244 m (800 ft) long, and contains nearly 140,000 cubic meters (182,000 cubic yards) of concrete.

Like arch dams, buttress dams require less concrete than comparable gravity dams, but they are not necessarily less expensive to build than concrete gravity dams. Costs associated with the complex work of forming the buttresses or multiple arches may offset the savings in construction materials. Buttress dams may be desirable, however, in locations with foundations that would not easily support the massive size and weight of gravity dams.

IV -HOW DAMS ARE BUILT
The complexity of dam construction varies with the size of the dam. A small dam across a stream might be built in just a few weeks. But a large structure—such as Hoover Dam, which towers 221 m (726 ft) above the riverbed—can take several years to plan and several more years to actually build. Dams of this scale require extensive site testing to verify that the underlying rock can withstand the pressure exerted by the dam and the reservoir. The river must be temporarily diverted and the foundation cleared of earth and loose rock. In some cases roads and other distribution systems must be constructed to transport millions of tons of material and equipment as well as thousands of personnel to the site. Project planners may also have to construct temporary housing for workers to live in while they work at the site.

A -Site Testing
Before construction begins, engineers survey the geology of a proposed site to ensure that it will provide a foundation strong enough to support the weight of the dam. They evaluate the structural condition of the bedrock by drilling core samples that can be further studied in geological laboratories. The engineers must also determine if excessive amounts of water from the reservoir will seep into the rock, which could undermine the foundation and cause the structure to collapse or wash away.

Engineers also consider the possible stresses that the foundation will be subjected to, as it must withstand the weight of both the dam and the water in the reservoir. Larger dams exert greater stresses, which means that a foundation adequate for a dam 30 meters high may be inadequate for a structure 120 meters high. Much scientific study and analysis goes into the process of designing and building a dam and every dam site presents a unique set of conditions that engineers must evaluate individually.

B -Design
Site selection also helps determine the type of dam to be constructed. In locations with steep canyon walls of solid bedrock, engineers might opt to construct concrete arch dams. Gravity dams and buttress dams are well suited to wide, flat river valleys.

Once a site and the basic dam type have been selected, engineers use some basic mathematical formulas to determine the dam’s dimensions. The pressure exerted on a dam by water stored in a reservoir is directly proportional to the depth of water pushing against the dam. Water pressure is not affected by the total size of the reservoir; it depends only on the reservoir’s depth. This means that a reservoir 20 kilometers long and 25 meters deep exerts no more pressure against a dam than a reservoir 1 kilometer long and 25 meters deep.
Along with basic shape and dimensions, engineers analyze the weight and strength of available materials and calculate how much of each type would be required to safely hold back the reservoir. They use this information in formulas designed to ensure that a dam will be thick enough—and therefore strong enough—to hold back a reservoir without collapsing or being tipped over.

C -Site Preparation
A dam must rest on a strong foundation. Dam builders must sometimes dig 30 m (100 ft) or more through earth and loose rock to reach solid bedrock. When excavating a foundation, engineers have to divert the flow of river water through or around a site. When working with streams and rivers that do not flood heavily, engineers may choose to concentrate river flow into a specially built canal or flume and erect the dam around it. In the final stages of construction, they fill in the part of the dam that contains the canal or flume.

Large, volatile rivers require more elaborate diversion systems. In some cases, engineers excavate tunnels into the walls of the canyon for the river to flow through. They then build temporary diversion dams, called cofferdams, to direct the river water into the tunnels. The water passes through the diversion tunnels and eventually flows back into the river at a location downstream from the dam site. This technique enables engineers to completely remove water from the foundation and excavate down to a clean, solid base of bedrock. For example, to construct Hoover Dam across the flood-prone Colorado River, engineers drilled four tunnels 15 m (50 ft) in diameter and more than 0.8 km (0.5 mi) long. Excavation of the diversion tunnels alone took more than a year.

V -ECOLOGICAL IMPACT
Building a dam changes the ecology of the surrounding area. Among the most affected animals are fish that depend on free-flowing water to live. Some kinds of salmon, trout, and other fish species migrate downstream to spend part of their lives in the open ocean. As adults, they return upstream to lay their eggs in the gravel bottoms of the rivers where they were born. Large dams block the passage of such migratory fish.

Some dams incorporate a fish pass to allow fish a chance to swim around the dam and reach upstream spawning grounds. Fish passes called fish ladders comprise a series of small pools arranged like stair steps. Each pool is slightly higher than the previous one. Fish ladders are based on the idea that a fish swimming upstream cannot leap over a dam that is more than about 5 meters high, but it can leap up a series of pools, each slightly higher than the one below it. Despite fish passes and other efforts to help fish bypass dams, the cumulative effect of multiple dams built along the length of a river can exact a heavy toll on fish populations. In rivers blocked by many dams, salmon populations have dropped by as much as 95 percent, a decline many experts attribute, at least in part, to dams.

Dams also alter the water temperatures and microhabitats downstream. Water released from behind dams usually comes from close to the bottom of the reservoirs, where little sunlight penetrates. This frigid water significantly lowers the temperatures of sun-warmed shallows downstream, rendering them unfit for certain kinds of fish and other wildlife. Natural rivers surge and meander, creating small pools and sandbars that provide a place for young fish, insects, and other river-dwelling organisms to flourish. But dams alter the river flow, eliminating these microhabitats and, in some cases, their inhabitants.

Dams prevent nutrient-laden silt from flowing downstream and into river valleys. Water in a fast-moving river carries tiny particles of soil and organic material. When the water reaches a pool or a flat section of a river course, it slows down. As it slows, the organic matter it carries drops to the river bottom or accumulates along the banks. Following heavy rains or snowmelt, rivers spill over their banks and deposit organic matter on their floodplains, creating rich, fertile soil. Some of the organic matter makes it all the way to river mouths, where it settles into the rich mud of estuaries, ecosystems that nourish up to one-half of the living matter in the world’s oceans. Large dams artificially slow water to a near standstill, causing the organic matter to settle to the bottom of the reservoir. In such cases, downstream regions are deprived of nutrient-laden silt.

Dams can also wreak havoc on human populations. Reservoirs created by dams can inundate entire riverside communities that may be centuries old and filled with rich archaeological treasures. Community inhabitants are forced to seek out new places to live and work. Even those who do not have to leave suffer from forced change. People who depend on rivers for their livelihood may need to change their way of life when dams destroy natural river flows. For example, the culture of the Wishram and Wasco peoples once centered on Celilo Falls on the Columbia River. Standing on wooden platforms, members of these tribes dipped long-handled nets into the falls to catch Chinook salmon en route to upstream spawning grounds. Celilo Falls represented a unique confluence of conditions that made it one of the most productive fishing spots in North America. The banks of the falls also served as a marketplace, where traders gathered to exchange goods. Construction of the Dalles Dam in 1957 completely submerged Celilo Falls, forever eliminating them and the culture built around them.

VI -HISTORY
Dams rank among the oldest types of human-made structures. The earliest known dams were relatively small and built to provide water for irrigation in Mesopotamia, one of the first centers of urban civilization. Remnants of ancient dams persist today as ruins or parts of modern dams. Ruins of the Jawa Dam, believed to have been constructed around 3000 BC, still stand in Jordan. The Ma‘rib Dam, located in what is now Yemen, has been rebuilt several times since it was first constructed more than 2,700 years ago. The present structure apparently includes sections that date back more than 2,000 years.

A -Roman Empire
The engineers of ancient Rome were masters of collecting and distributing water. Beginning around the 1st century AD, they constructed a system of large dams to impound river water in regions surrounding the Mediterranean Sea. The Romans’ largest reservoir, the Lake of Homs, was created in AD 284 in what is now Syria. A dam impounded approximately 90 million cubic meters (117 million cubic yards) of water. Significant portions of the Roman-built Cornalvo and Proserpina dams in Spain remain in service after more than 1,700 years. The Romans often used buttresses to support dam walls. Historical data indicates that the Romans also understood the principle of arch dams and built one at Dara, on the present-day border between Turkey and Syria. However, no trace of the arched portion of the dam remains.
Dam construction waned in western Europe after the fall of the Roman Empire. In the 14th century the Il-khanid dynasty of the Mongol Empire built several major dams in present-day Iran. Their Kurit Dam, a masonry arch structure 58 m (190 ft) high, stood as the tallest dam in the world until the late 19th century.

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.
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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