Dams

Introduction

Dams are structures built across streams or rivers, usually to block water flow and cause the formation of a pond or lake upstream of the dam. The water impounded behind a dam can be used for drinking, irrigation of crops, or generating electricity. Electricity generated by dams, termed hydroelectricity, is made by allowing water to run downhill through fanlike turbines, a modern application of the ancient waterwheel principle for extracting work from falling water. The turbines turn generators that produce electricity. About 19% of the world’s electricity was hydroelectricity as of 2007.

Dams have many effects on their local environments and have long been criticized for flooding landscapes, destroying or damaging downstream ecosystems, closing off fish migration routes, and in some cases forcing the evacuation of millions of people. However, for many years it was assumed that dams are a zero-pollution, climate-neutral way to produce electricity, since they burn no chemical fuel and emit no carbon dioxide (CO₂) in daily operation. However, in the 1990s, researchers realized that some dams, especially in the tropics, are significant sources of methane, a potent greenhouse gas. Research published in 2006 reported that dams may account for more than 4% of anthropogenic (human-caused) greenhouse warming.

Historical Background and Scientific Foundations

Overview

The first known large dam was built by the Egyptians around 2800 BC. It was a massive barrier 37 ft (11 m) tall and capable of impounding 20 million cubic ft (566,000 cubic m) of water. The dam failed after a few years when water overflowed its crest, eroding the downstream side of the structure and weakening it until the dam collapsed.

Since that time, many civilizations have built dams of various sizes. However, truly large dams were not built until the 1930s, when engineers began building dams large enough to block whole river basins. Hoover Dam was completed across the Colorado River in the United States in 1936, becoming the world’s largest dam at that time. When the lake behind the dam had filled, it backed 110 mi (177 km) upstream and covered 247 square mi (639 square km). Hoover is still the second-tallest dam in the United States.

Since Hoover, thousands of dams have been built worldwide. Over half (172 out of 292) of the world’s large rivers are now dammed at one point or more. All of the world’s largest 21 rivers are dammed. Globally, there are more than 45,000 large dams, that is, dams over 49 ft (15 m) high, which altogether store more than 1,557 cubic mi (6,500 cubic km) of water, an amount equal to 15% of the world’s annual freshwater runoff. There are over 300 very large dams, namely those over 492 ft (150 m) high or impounding over 6 cubic mi (25 cubic km of water). The latest of these giant dams, and the most controversial and environmentally destructive to date, is the Three Gorges dam across the Yangtze River in China. It will impound over nine cubic mi (39 cubic km) of water after filling and is forcing the evacuation of over a million people.

As of 2004, 2.2% of the world’s primary energy was obtained from hydroelectric dams, or about 19% of the world’s electricity. Irrigation water from dams is used to grow 12-16% of the world’s food.

Dams and Climate Change

There are several connections between dams and climate change. First, hydroelectricity was long assumed to have less global-warming impact than electricity generated

WORDS TO KNOW

ANAEROBIC BACTERIA: Bacteria that grow without oxygen, also called anaerobes.

ANTHROPOGENIC: Made by humans or resulting from human activities.

CLEAN DEVELOPMENT MECHANISM: One of the three mechanisms set up by the Kyoto Protocol of 2007 to, in theory, allow reductions in greenhouse-gas emissions to be implemented where they are most economical. Under the Clean Development Mechanism, polluters in wealthy countries can obtain carbon credits (greenhouse pollution rights) by funding reductions in greenhouse emissions in developing countries.

FOSSIL FUELS: Non-renewable fuels formed by biological processes and transformed into solid or fluid minerals over geologic time. Fossil fuels include coal, petroleum, and natural gas.

GREENHOUSE GAS: A gas whose accumulation in the atmosphere increases heat retention.

HYDROELECTRICITY: Electricity generated by causing water to flow downhill through turbines, which are fan-like devices that turn when fluid flows through them. The rotary mechanical motion of each turbine is used to turn an electrical generator.

METHANE: An odorless, colorless, flammable gas, with the formula CH₄, that is the simplest hydrocarbon compound and the major constituent of natural gas.

WORLD BANK: International bank formed in 1944 to aid in reconstruction of Europe after World War II, now officially devoted to the eradication of world poverty through funding of development projects.

using fossil fuels. Although CO₂ is emitted during the manufacture of the cement, concrete, and steel that go into making a modern dam, these emissions, for a typical dam, amount to less than 10% of the emissions from generating the same amount of electricity by burning fossil fuels. However, starting in 1993, researchers began noting that some large dams might be significant sources of the greenhouse gas methane (CH₄), which has about 23 times the global-warming effect of CO₂, ton for ton. As of 2007, this issue continued to be hotly debated, with some scientists challenging the objectivity of scientists working for (or partly funded by) the hydropower industry, who have done most of the research on dam methane to date.

Methane is produced after the dam is constructed. When a typical tropical dam is built, large areas of upstream forest are drowned. Wood and other types of organic (carbon-containing) matter that decay under water are mostly broken down by anaerobic bacteria—bacteria that thrive in the absence of oxygen—using digestive chemistry that produces CH₄ as a waste product; in contrast, microorganisms that digest organic matter where oxygen is present produce CO₂. The anaerobic rotting of wood and other organic matter under water produces methane.

An initial pulse of methane is released as the trees, soil carbon, and forest litter covered by the filling of a new dam lake rot. However, this is not the major source of dam methane, which continues to be produced as long as the dam stands. In most tropical areas, rainy seassons alternate with dry seasons. During the dry season, water continues to flow through the dam and the lake water level lowers. This creates large mud flats and shallow zones, because tropical river valleys are generally wide and gently sloping rather than narrow and steep. Water weeds and other soft, quick-growing vegetation flourish in these flats and shallows.

When the rainy season comes, the water level rises again, drowning these plants, which quickly decay underwater, releasing methane. In effect, the rising and falling lake is a vast, solar-powered machine for harvesting carbon from CO₂ in the air and re-releasing it as CH₄ with greatly increased greenhouse warming effect.

In 2005 Philip Fearnside, one of the most-cited researchers on methane production by dams, found that in 1990, 13 years after its lake filled, the Curua-Una Dam in Brazil emitted 3.6 times more greenhouse gas than oil-burning plants generating the same amount of electricity would have emitted. That is, the dam released enough methane to cause 3.5 times as much global warming as the CO₂ released by the oil plants would have.

Ivan Lima and colleagues, among other scientists, have suggested that at least some of this methane could be collected and burned to produce electricity. This would both extract useful energy and convert the methane (CH₄) to CO₂, greatly reducing its greenhouse potential. Most dams allow deep, high-pressure, methane-rich water to run through their turbines. This causes the release of much of the methane in the water and may account for up to 70% of methane emissions from dam lakes (the rest is emitted from the lake surface). Some or most of this methane might be collected for burning. As of 2007, however, such technologies had not yet been demonstrated on a practical scale.

There are other relationships between dams and climate change. Dams in high latitudes (far from the equator) do not produce large amounts of methane, and so can indeed produce electricity with less greenhouse impact than comparable fuel-burning plants would. This does not reduce their other environmental impacts, however. Counting all dams worldwide, the greenhouse impact of dams is less than half of the least-polluting fuel-burning alternatives.

Dams are also linked to climate change because they may serve as storage reservoirs in places where precipitation patterns and flood risk may be altered by climate change. Thus, some dams may help with mitigation of and adaptation to the effects of climate change. Finally, some dams may have to contend with decreased rainfall and water flow from changing climate, which might reduce water and energy benefits from those dams.

Impacts and Issues

Reports in the 1990s of significant methane emissions from dams led to intense debate on the climate-change mitigation merits of large dams. In 1997 a workshop was convened by the International Union for the Conservation of Nature and Natural Resources (an 83-state organization headquartered in Geneva, Switzerland) and the World Bank to discuss the question. The two groups established the World Commissionon Dams to investigate the effectiveness and costs of dams, including their relationship to climate change.

In November 2000 the commission released its report. Although stating that “dams have made an important and significant contribution to human devel-

opment, and benefits derived from them have been considerable,” the report also said that “in too many cases an unacceptable and often unnecessary price has been paid to secure those benefits, especially in social and environmental terms, by people displaced, by communities downstream, by taxpayers and by the natural environment.” The commission estimated that 40 to 80 million people had been displaced by dams and many species driven to extinction; it also stated that most of the benefits from 1% to 28% of anthropogenic (human-caused) greenhouse emissions might come from dams. A study by Ivan Lima and colleagues published in 2006 stated that 4% of global emissions are from dams.

The World Commission on Dams made a number of recommendations for how proposed dam projects should be judged, including whether people who would be displaced consent to the project. Its findings have been controversial because one of the commission’s founders, the World Bank, is the world’s largest funder of dam projects, having provided a total of $75 billion (1998 dollars) for 538 large dams in 92 countries since its inception in 1945. The World Bank has not accepted the commission’s recommended international standards for dam-building. Europe’s two largest public banks, the European Investment Bank and the European Bank for Reconstruction and Development, announced in 2005 that they would abide by the commission’s standards. A directive of the European Union mandates that carbon-trading hydropower projects under the CleanDevelopment Mechanism must abide by the commission’s standards.

Primary Source Connection

The following news article recognizes that in the United States, more than 87,000 dams need to be repaired. Many of these dams have already proved to be hazardous, such as a dam in Hawaii that collapsed and killed 7 people in 2006. Such obstacles as the high number of aging dams starting to deteriorate, the low number of dam inspectors, and a lack of funding for the dam safety program has interfered with the upkeep of the country’s dams.

PROBLEM DAMS ON THE RISE IN US

The Kaloko dam in Hawaii stood 116 years—until last year when it collapsed after heavy rains, killing seven.

Potential disaster was averted in April in Hollis, N.H., when a dozen families were evacuated and engineers made a controlled breach of an old pond dam to keep it from failing.

Such incidents are warning signs that many of the nation’s more than 87,000 dams are in need of repair. Last month’s high-profile collapse of the 1-35 bridge in Minneapolis focused America’s attention on bridge problems. The nation’s dams are worse off.

In 2005, the last time the American Society of Civil Engineers rated America’s infrastructure, bridges received a “C” grade; dams earned a “D.”

Even that rating may be generous, a Monitor analysis of dam-inspection data shows. Since 1999, the number of “high-hazard” dams rated “deficient” has more than doubled, according to data from the Association of State Dam Safety Officials (ASDSO) in Lexington, Ky. High-hazard dams are those whose failures could cause fatalities. In 1999, the US had 546 such dams rated deficient. By last year, it had 1,333.

A second category of “significant-hazard” dams (so-called because they threaten substantial property loss) saw a rise from 339 to 949 deficient dams over the same period. In all, 2.6 percent of the nation’s dams are deficient, according to the ASDSO.

“The growth of deficient high-hazard dams in this country is a major issue,” says Brad Larossi, legislative chairman for the ASDSO, which represents dam-safety inspectors in all states. “The trend is rising at such a steep slope, much faster than states can do [dam] rehabilitation. Without question the overall trends are clear.”

Several factors are behind the rise. Old dams continue to deteriorate or may fail suddenly because of inadequate spillways and trees growing on dams. Many states don’t have enough dam engineers to keep up proper maintenance, causing the repair backlog to grow. And as more homes and businesses are built closer to dams, the hazards increase, a phenomenon dam-safety experts call “hazard creep.”

Some experts claim that some of the rise is due to better reporting, an encouraging sign. “To be frank, there’s been in the past a reluctance in some quarters to identify too many dams as deficient,” says Mark Ogden, administrator for dam-safety engineering at the Ohio Department of Natural Resources in Columbus. “But there’s also been a strong effort by our association to increase awareness of this problem. We all are realizing we need an honest assessment.”

Some states are seeing a faster rise in deficient dams than others. Pennsylvania leads the pack with 215 deficient high-hazard dams, 172 more than in 1999. Not far behind is Ohio, with an increase of 158. Other states, such as Colorado, New Jersey, and California have seen declines. Some of that is due to better funding, experts say. All three have boosted dam budgets by a third or more since 1999.

Those increases are in contrast to federal dam spending. The nation’s dam-safety program, which helps fund safety inspector and engineer training, has not been fully funded in at least five years, Mr. Larossi says. Actual funding is about $5.9 million, well below the $9 million budgeted, he says.

As a result, the number of full-time inspectors has not increased since 1997 (excluding Florida, which claims to have hired 45 inspectors). That leaves each inspector responsible for about 195 dams on average; the ASDSO recommends no more than 50.

“We have seen increased awareness over the importance of adequate funding for state inspectors, but these offices are still understaffed,” says Stephanie Lindloff, of American Rivers, an environmental group.

Mark Clayton

CLAYTON, MARK. “PROBLEM DAMS ON THE RISE IN US” CHRISTIAN SCIENCE MONITOR (SEPTEMBER 13, 2007).

See Also Clean Development Mechanism

BIBLIOGRAPHY

Periodicals

Fearnside, Philip M. “Do Hydroelectric Dams Mitigate Global Warming? The Case of Brazil’s Curua-Una Dam.” Mitigation and Adaptation Strategies for Global Change 10 (2005): 675-691.

Lima, Ivan B. T., et al. “Methane Emissions from Large Dams as Renewable Energy Resources: A Developing Nation Perspective.”Mitigation and Adaptation Strategies for Global Change (February 2007).

Yardley, William. “Climate Change Adds Twist to Debate Over Dams.” New York Times (April 23, 2007).

Web Sites

International Rivers Network. “Citizen’s Guide to the World Commission on Dams.” http://www.irn.org/wcd/wcdguide.pdf (accessed March 29, 2008).

New Scientist. “Hydroelectric Power’s Dirty Secret Revealed.” http://www.newscientist.com/article/dn7046.html (accessed March 29, 2008).

United Nations Environment Programme. “Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams.” http://www.dams.org/docs/kbase/contrib/env253.pdf (accessed March 29, 2008).

World Commission on Dams. “Dams and Development: A New Framework for Decision-Making.” http://www.dams.org//docs/report/wcdreport.pdf (accessed March 29, 2008).

Larry Gilman

Dams

Classification of dams

Dam construction

Impact of dams

Three Gorges

Resources

Dams are structures designed to restrict the flow of a stream or river, thus forming a pond, lake, or reservoir behind the wall. Dams are used for flood control, for production of hydroelectric power, to store and distribute water for agriculture and human populations, and as recreation sites.

Classification of dams

Dams may be classified according to the general purpose for which they are designed. These include storage, diversion, and detention.

Storage dams are built to provide a reliable source of water for short or long periods of time. Small dams, for example, are often built to capture spring runoff for use by livestock in the dry summer months. Storage dams can be further classified by the specific purpose for which the water is being stored, such as municipal water supply, recreation, hydroelectric power generation, or irrigation.

Diversion dams are typically designed to raise the elevation of a water body to allow that water to be conveyed to another location for use. The most common applications of diversion dams are supplying irrigation canals and transferring water to a storage reservoir for municipal or industrial use.

Detention dams are constructed to minimize the impact of flooding and to restrict the flow rate of a particular channel. In some cases, the water trapped by a detention dam is held in place to recharge the subsurface groundwater system. Other detention dams, called debris dams, are designed to trap sediment carried by floods and debris flows.

Large dams frequently serve more than one of these purposes and many combine aspects of each of the three main categories. Operation of such multi-purpose dams is complicated by sometimes opposing needs. In order to be most effective, storage behind a flood control dam should be maintained at the lowest level possible. After a detention event occurs, water should be released as quickly as possible, within the capacity of the downstream channel. Conversely, the efficient and economic operation of storage and diversion dams requires that water levels be maintained at the highest possible levels. Releases from these reservoirs should be limited to the intended user only, such as the power generating turbines or the municipal water user. Operators of multipurpose dams must balance the conflicting needs of the various purposes to maintain the reliability, safety, and economic integrity of the dam. Operators must use a variety of information to predict the needs of the users, the expected supply, and the likelihood of any abnormal conditions that might impact the users or the dam itself. Failure to do so can threaten even the largest of dams.

During the El Ninö of 1983, climate and hydro-logic forecasts failed to predict abnormally heavy spring runoff in the Rocky Mountains. Dam operators along the Colorado River maintained high water storage levels, failing to prepare for the potential of the flooding. By the time operators began to react, water was bypassing the dams via their spillways and wreaking havoc throughout the system. Ultimately, the Glen Canyon Dam in Arizona was heavily affected with flood flows eroding large volumes of rock from within the canyon walls that support the dam. Fortunately, the flooding peaked and control was regained before the dam was breached.

Dam construction

There are four main types of dams: arch, buttress, gravity, and embankment dams. The type of construction for each dam is determined by the proposed use of the structure, qualities of the intended location, quantity of water to be retained by the structure, materials available for construction, and funding limitations.

Arch dams use an upstream-facing arch to help resist the force of the water. They are typically built in narrow canyons and are usually made of concrete. Good contact between the concrete and the bedrock are required to prevent leakage and ensure stability. A dome dam is a special variant with curves on the vertical and horizontal planes, while the arch dam is only curved on the horizontal plane. In addition, dome dams are much thinner than arch dams.

A buttress dam is characterized by a set of angled supports on the downstream side that help to oppose the force of the water. This design can be employed in wide valleys where a solid bedrock foundation is not available. Because of the steel framework and associated labor needed for construction, these dams are no longer economically viable.

The gravity dam withstands the force of the water behind it with its weight. Made of cement or masonry, this type of dam normally utilizes a solid rock foundation but can be situated over unconsolidated material if provisions are made to prevent the flow of water beneath the structure. The solid, stable nature of this dam is favored by many and often incorporated into the spillway designs of embankment dams.

An embankment dam uses the locally available material (rocks, gravel, sand, clay, etc.) in construction. As with gravity dams, the weight of embankment dams is used to resist the force of the water. The permeability of the materials that make up these dams allows water to flow into and through the dam. An impervious membrane or clay core must be built into them to counteract the flow and protect the integrity of the structure. Because the materials are locally available and the construction of these dams is relatively simple, the cost of construction for this type of dam is much lower than the other types. Embankment dams are the most common.

Impact of dams

Dams have long been acknowledged for providing electricity without the pollution of other methods, for flood protection, and for making water available for agriculture and human needs. Within recent decades, however, the environmental impact of dams has been debated. While dams do perform important functions, their effects can be damaging to the environment.

The damming of a river will have dramatic consequences on the nature of the environment both upstream and downstream of the dam. The magnitude of these effects are usually directly related to the size of the dam.

Prior to dam construction, most natural rivers have a flow rate that varies widely throughout the year in response to varying conditions. Once a dam is constructed, the flow rate of the river below a dam is restricted. The dam itself and the need to control water releases for the various purposes of the particular dam result in a flow rate that has a smaller range of values and peaks that occur at times related to need rather than the dictates of nature. In cases where the entire flow has been diverted for other uses, there may no longer be any flow in the original channel below the dam.

Because water is held behind the dam and often released from some depth, the temperature of the water below the dam is usually lower than it would be prior to dam emplacement. The temperature of the water flow is often constant, not reflecting the natural seasonal variations that would have been the case in the free-flowing river. Similarly, the chemistry of the water may be altered. Water exiting the lake may be higher in dissolved salts or have lower oxygen levels than would be the case for a free-flowing river.

Impoundments increase the potential for evaporation from the river. Because the surface area of a lake is large compared to that of the river that supplies it, the loss of water to evaporation must be considered. In some desert areas, potential annual evaporation can be greater than 7 ft (2.1 m), meaning that over the course of one year, if no water flowed into or out of the system, the reservoir would drop in elevation by 7 ft (2.1 m). At Lake Mead on the Colorado River in Arizona and Nevada, evaporation losses in one year can be as great as 350 billion gal (1.3 trillion l).

The impoundment of water behind a dam causes the velocity of the water to drop. Sediment carried by the river is dropped in the still water at the head of the lake. Below the dam, the river water flows from the clear water directly behind the dam. Because the river no longer carries any sediment, the erosive potential of the river is increased. Erosion of the channel and banks of the river below the dam will ensue. Even further downstream, sediment deprivation affects shoreline processes and biological productivity of coastal regions.

This problem has occurred within the Grand Canyon below Glen Canyon Dam. After the construction of the dam was completed in 1963, erosion of the sediment along the beaches began because of the lack of incoming sediment. By the early 1990s, many beaches were in danger of disappearing. In the spring of 1996, an experimental controlled flood of the river below Glen Canyon Dam was undertaken to attempt to redistribute existing sediments along the sides of the channel. While many of the beaches were temporarily rebuilt, this redistribution of sediments was short lived. Research on this issue is continuing, however, the fundamental problem of the lack of input sediment for the river downstream of the dam remains unresolved.

The environmental changes described above create a new environment in which native species may or may not be able to survive. New species frequently invade such localities, further disrupting the system. Early photographs of rivers in the southwest desert illustrate the dramatic modern invasion of non-native plants. Entire lengths of these rivers and streams have been transformed from native desert plants to a dense riparian environment. Native species that formerly lived in this zone have been replaced as a result of the changes in river flow patterns. The most commonly cited species affected by the presence of dams is the salmon. Salmon have been isolated from their spawning streams by impassable dams. The situation has

KEY TERMS

Arch dam— A thin concrete dam that is curved in the upstream direction.

Buttress dam— A dam constructed of concrete and steel that is characterized by angled supports on the downstream side.

El Niño— The phase of the Southern Oscillation characterized by increased sea water temperatures and rainfall in the eastern Pacific, with weakening trade winds and decreased rain along the western Pacific.

Embankment dam— A simple dam constructed of earth materials.

Gravity dam— A massive concrete or masonry dam that resists the force of the water by its own weight.

Impoundment— The body of water, a pond or lake, that forms behind a dam.

Permeability— The capacity of a geologic material to transmit a fluid, such as water.

Spillway— A passage for water to flow around a dam.

been addressed through the use of fish ladders and by the use of barges to transport the fish around the obstacles, but with only limited success.

Three Gorges

Dam The world’s largest dam is the Three Gorges Dam near Sandouping, China. The 610 ft high concrete structure extends approximately 1.3 miles across the Yangtze River valley and will create a reservoir that will extend about 350 miles upstream. Although the dam itself was completed in 2006, its hydroelectric facility was not scheduled for completion until 2009 at an estimated total cost of $30 billion. It is intended to produce hydroelectric power, provide flood control, and allow large ships to navigate up the river.

The section of the Yangtze River affected by the Three Gorges dam has experienced more than 200 catastrophic floods during the past 2, 000 years. Some 4, 000 people were killed and more than 1 million left homeless after a 1998 flood that caused an estimated $24 billion in economic losses. A 1954 flood is reported to have killed about 30, 000 people. Strong and continuous growth of China’s industrial and consumer economies also increased the demand for electrical power, much of which has historically been supplied by coal burning power plants with minimal pollution control. With a design capacity of more than 18, 000 megawatts of electricity, the dam is planned to produce the same amount of power as 15 nuclear power plants.

Despite the benefits of flood control, electricity, and improved navigation, the Three Gorges Dam has been criticized for its potential environmental impact. Filling of the reservoir behind the dam will flood 13 cities, 140 towns, and 1, 300 villages upstream from the dam and displace about 1.5 million residents. Numerous mines and factories will also be flooded without removal of potentially hazardous waste, which will likely enter the water supply as reservoir level increases. The reservoir may also concentrate a large proportion of the 700 million tons of sediment and 265 billion tons of raw sewage that enter the river each year. Finally, more than a thousand known archeological sites will be lost beneath the reservoir. Although international environmental groups protested the planning and construction of the dam, the Chinese government maintained that the benefits of dam construction will outweigh any environmental costs. Concerns about construction quality and the long term safety of the dam were also raised.

Resources

BOOKS

Billington, D.P. and D.C. Jackson. Big Dams of the New Deal Era: A Confluence of Engineering and Politics. Norman, Okla.: University of Oklahoma Press, 2006.

Brooks, K.B. and Cronon, W. Public Power, Private Dams: The Hells Canyon High Dam Controversy. Seattle: University of Washington Press, 2006.

Chetham, D. Before the Deluge: The Vanishing World of the Yangtze’s Three Gorges. (Reprint ed.) New York: Palgrave Macmillan, 2004.

David B. Goings

Dams

Introduction

Dams are structures built across streams or rivers, usually to block water flow and cause the formation of a lake upstream of the dam. The water accumulated or impounded behind a dam can be used for drinking, irrigation of crops, or generating electricity. Electricity generated by dams, termed hydroelectricity, is made by allowing water to run downhill through fan-like turbines, a modern application of the ancient waterwheel principle for extracting work from falling water. The turbines turn generators that produce electricity. About 19% of the world's electricity is hydroelectricity.

Dams have long been criticized for flooding landscapes, destroying ecosystems, and in some cases, forcing the evacuation of millions of people. However, for many years it was assumed that dams are a climate-neutral way to produce electricity since they burn no chemical fuel and produce no carbon dioxide (CO₂) emissions in daily operation. In the 1990s, researchers realized that some dams, especially in the tropics, are significant sources of methane, a potent greenhouse gas. Research published in 2006 reported that dams may account for more than 4% of anthropogenic warming.

Historical Background and Scientific Foundations

Overview

The first large dam was built by the Egyptians around 2800 BC. It was a massive barrier 37 ft (11 m) tall and capable of impounding 20 million cubic ft (566,000 cubic m) of water. The dam failed after a few years when water overflowed its crest, eroding the downstream side of the structure and weakening it until the dam collapsed.

Since that time, most civilizations have built dams of various sizes. However, the building of truly large dams did not occur until the twentieth century. During the 1930s, engineers began building dams large enough to block whole river basins. The Hoover Dam was completed across the Colorado River in the United States in 1936, becoming the world's largest at that time. When the lake behind the dam had filled, it backed 110 mi (177 km) upstream and covered 247 square mi (639 square km). Hoover is still the second-tallest dam in the United States.

Since Hoover, thousands of dams have been built worldwide. Over half (172 out of 292) of the world's large rivers are now dammed at one point or more. All of the world's largest 21 rivers are dammed. Globally, there are more than 45,000 large dams, that is, dams over 49 ft (15 m) high, which altogether store more than 1,557 cubic mi (6,500 cubic km) of water, an amount equal to 15% of the world's annual freshwater runoff. There are over 300 very large dams, namely those over 492 ft (150m) high or impounding over 6 cubic mi (25 cubic km) of water. The latest of these giant dams, and the most controversial and environmentally destructive to date, is the Three Gorges dam across the Yangtze River in China. It will impound over 9 cubic mi (39 cubic km of water) after filling and is forcing the evacuation of over a million people.

As of 2004, 2.2% of the world's primary energy was obtained from hydroelectric dams, or about 19% of the world's electricity. Irrigation water from dams is used to grow 12–16% of the world's food.

Dams and Climate Change

There are several connections between dams and climate change. First, hydroelectricity was long assumed to have less global-warming impact than electricity generated using fossil fuels. Although CO₂ is emitted during the manufacture of the cement, concrete, and steel that go into making a modern dam, these emissions, for a typical dam, amount to less than 10% of the emissions from generating the same amount of electricity by burning fossil fuels. However, starting in 1993, researchers began

noting that some large dams might be significant sources of the greenhouse gas methane (CH₄), which has about 23 times the global-warming effect of CO₂, ton for ton. As of 2008, this issue continued to be hotly debated, with some scientists challenging the objectivity of scientists working for (or partly funded by) the hydropower industry, who have done most of the research on dam methane to date.

Methane is produced after the dam is constructed. When a typical tropical dam is built, large areas of upstream forest are drowned. Wood and other types of organic (carbon-containing) matter that decay under water are mostly broken down by anaerobic bacteria—bacteria that thrive in the absence of oxygen—using digestive chemistry that produces CH₄ as a waste product; in contrast, microorganisms that digest organic matter where oxygen is present produce CO₂. The anaerobic rotting of wood and other organic matter under water produces methane.

An initial pulse of methane is released as the trees, soil carbon, and forest litter covered by the filling of a new dam lake rot. However, this is not the major source of dam methane, which continues to be produced as long as the dam stands. In most tropical areas, rainy seasons alternate with dry seasons. During the dry sea- son, water continues to flow through the dam and the lake water level lowers. This creates large mud flats and shallow zones, because tropical river valleys are generally wide and gently sloping rather than narrow and steep. Water weeds and other soft, quick-growing vegetation flourish in these flats and shallows.

When the rainy season comes, the water level rises again, drowning these plants, which quickly decay underwater, releasing methane. In effect, the rising and falling lake is a vast, solar-powered machine for harvesting carbon from CO₂ in the air and re-releasing it as CH₄ with greatly increased greenhouse warming effect.

In 2005, Philip Fearnside, one of the most-cited researchers on methane production by dams, found that in 1990, 13 years after its lake filled, the Curuá-Una Dam in Brazil emitted 3.6 times more greenhouse gas than oil-burning plants generating the same amount of electricity would have emitted. That is, the dam released enough methane to cause 3.5 times as much global warming as the CO₂ released by the oil plants would have.

Ivan Lima and colleagues, among other scientists, have suggested that at least some of this methane could be collected and burned to produce electricity. This would both extract useful energy and convert the methane (CH₄)to CO₂, greatly reducing its greenhouse potential. Most dams allow deep, high-pressure, methane-rich water to run through their turbines. This causes the release of much of the methane in the water and may account for up to 70% of methane emissions from dam lakes (the rest is emitted from the lake surface). Some or most of this methane might be collected for burning. As of 2007, however, such technologies had not yet been demonstrated on a practical scale.

There are other relationships between dams and climate change. Dams in high latitudes (far from the equator) do not produce large amounts of methane, and so can indeed produce electricity with less greenhouse impact than comparable fuel-burning plants would. This does not reduce their other environmental impacts, however. Counting all dams worldwide, the greenhouse impact of dams is less than half that of the least-polluting fuel-burning alternatives.

Dams are also linked to climate change because they may serve as storage reservoirs in places where precipitation patterns and flood risk may be altered by climate change. Thus, some dams may help with mitigation of and adaptation to the effects of climate change. Finally, some dams may have to contend with decreased rainfall and water flow from changing climate, which might reduce water and energy benefits from those dams.

Impacts and Issues

Reports in the 1990s of significant methane emissions from dams led to intense debate on the climate-change mitigation merits of large dams. In 1997, a workshop was convened by the International Union for the Conservation of Nature and Natural Resources (an 83-state organization headquartered in Geneva, Switzerland) and the World Bank to discuss the question. The two groups established the World Commission on Dams to investigate the effectiveness and costs of dams, including their relationship to climate change.

WORDS TO KNOW

ANAEROBIC BACTERIA: Single-celled creatures that thrive in anaerobic environments, that is, environments lacking free molecular oxygen (O₂). They digest organic matter and release methane, a greenhouse gas.

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

CLEAN DEVELOPMENT MECHANISM: One of the three mechanisms set up by the Kyoto Protocol to, in theory, allow reductions in greenhouse-gas emissions to be implemented where they are most economical. Under the Clean Development Mechanism, polluters in wealthy countries can obtain carbon credits (greenhouse pollution rights) by funding reductions in greenhouse emissions in developing countries.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

HYDROELECTRICITY: Electricity generated by causing water to flow downhill through turbines, fan-like devices that turn when fluid flows through them. The rotary mechanical motion of each turbine is used to turn an electrical generator.

METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH₄. It is the simplest hydro-carbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.

WORLD BANK: International bank formed in 1944 to aid in reconstruction of Europe after World War II, now officially devoted to the eradication of world poverty through funding of development projects. Although ostensibly independent, the bank is always headed by a person appointed by the President of the United States.

In November 2000, the commission released its report. Although stating that “dams have made an important and significant contribution to human development, and benefits derived from them have been considerable,” the report also said that “in too many cases an unacceptable and often unnecessary price has been paid to secure those benefits, especially in social and environmental terms, by people displaced, by communities downstream, by taxpayers and by the natural environment.” The commission estimated that 40–80 million people had been displaced by dams and many species driven to extinction; it also stated that most of the benefits from 1% to 28% of anthropogenic (human-caused) greenhouse emissions might come from dams. A study by Ivan Lima and colleagues stated that 4% of global emissions are from dams.

The World Commission on Dams made a number of recommendations for how proposed dam projects should be judged, including whether people who would be displaced consent to the project. Its findings have been controversial because one of the commission's founders, the World Bank, is the world's largest funder of dam projects, having provided a total of $75 billion (1998 dollars) for 538 large dams in 92 countries since its inception in 1945. The World Bank has not accepted the commission's recommended international standards for dam-building. Europe's two largest public banks, the European Investment Bank and the European Bank for Reconstruction and Development, announced in 2005 that they would abide by the commission's standards. A directive of the European Union mandates that carbon-trading hydropower projects under the Clean Development Mechanism must abide by the commission's standards.

See Also Clean Development Mechanism; Energy Contributions; Methane.

BIBLIOGRAPHY

Periodicals

Fearnside, Philip M. “Do Hydroelectric Dams Mitigate Global Warming? The Case of Brazil's Curuá-Una Dam.” Mitigation and Adaptation Strategies for Global Change 10 (2005): 675–691.

Lima, Ivan B. T., et al. “Methane Emissions from Large Dams as Renewable Energy Resources: A Developing Nation Perspective.” Mitigation and Adaptation Strategies for Global Change (February 2007).

Yardley, William. “Climate Change Adds Twist to Debate Over Dams.” New York Times (April 23, 2007).

Web Sites

“Citizen's Guide to the World Commission on Dams.” International Rivers Network, 2002. < http://www.irn.org/wcd/wcdguide.pdf> (accessed November 13, 2007).

“Climate Change and Dams: An Analysis of the Linkages Between the UNFCCC Legal Regime and Dams.” United Nations Environment Programme, November 2000. < http://www.dams.org/docs/kbase/contrib/env253.pdf> (accessed November 13, 2007).

“Dams and Development: A New Framework for Decision-Making.” World Commission on Dams, June 22, 2007. < http://www.dams.org//docs/report/wcdreport.pdf> (accessed November 13, 2007).

Grahame-Rowe, Duncan. “Hydroelectric Power's Dirty Secret Revealed.” New Scientist, February 24, 2005. < http://www.newscientist.com/article/dn7046.html> (accessed November 13, 2007).

Larry Gilman

Dams

Dams are structures designed to restrict the flow of a stream or river, thus forming a pond, lake , or reservoir behind the wall. Dams are used for flood control, for production of hydroelectric power, to store and distribute water for agriculture and human populations, and as recreation sites.

Classification of dams

Dams may be classified according to the general purpose for which they are designed. These include storage, diversion, and detention.

Storage dams are built to provide a reliable source of water for short or long periods of time . Small dams, for example, are often built to capture spring runoff for use by livestock in the dry summer months. Storage dams can be further classified by the specific purpose for which the water is being stored, such as municipal water supply, recreation, hydroelectric power generation, or irrigation .

Diversion dams are typically designed to raise the elevation of a water body to allow that water to be conveyed to another location for use. The most common applications of diversion dams are supplying irrigation canals and transferring water to a storage reservoir for municipal or industrial use.

Detention dams are constructed to minimize the impact of flooding and to restrict the flow rate of a particular channel. In some cases, the water trapped by a detention dam is held in place to recharge the subsurface groundwater system. Other detention dams, called debris dams, are designed to trap sediment.

Large dams frequently serve more than one of these purposes and many combine aspects of each of the three main categories. Operation of such multipurpose dams is complicated by sometimes opposing needs. In order to be most effective, storage behind a flood control dam should be maintained at the lowest level possible. After a detention event occurs, water should be released as quickly as possible, within the capacity of the downstream channel. Conversely, the efficient and economic operation of storage and diversion dams requires that water levels be maintained at the highest possible levels. Releases from these reservoirs should be limited to the intended user only, such as the power generating turbines or the municipal water user. Operators of multipurpose dams must balance the conflicting needs of the various purposes to maintain the reliability, safety, and economic integrity of the dam. Operators must use a variety of information to predict the needs of the users, the expected supply, and the likelihood of any abnormal conditions that might impact the users or the dam itself. Failure to do so can threaten even the largest of dams.

During the El Niño of 1983, climate and hydrologic forecasts failed to predict abnormally heavy spring runoff in the Rocky Mountains. Dam operators along the Colorado River maintained high water storage levels, failing to prepare for the potential of the flooding. By the time operators began to react, water was bypassing the dams via their spillways and wreaking havoc throughout the system. Ultimately, the Glen Canyon dam in Arizona was heavily impacted with flood flows eroding large volumes of rock from within the canyon walls that support the dam. Fortunately, the flooding peaked and control was regained before the dam was breached.

Dam construction

There are four main types of dams: arch, buttress, gravity, and embankment dams. The type of construction for each dam is determined by the proposed use of the structure, qualities of the intended location, quantity of water to be retained by the structure, materials available for construction, and funding limitations.

Arch dams use an upstream-facing arch to help resist the force of the water. They are typically built in narrow canyons and are usually made of concrete . Good contact between the concrete and the bedrock are required to prevent leakage and ensure stability. A dome dam is a special variant with curves on the vertical and horizontal planes, while the arch dam is only curved on the horizontal plane . In addition, dome dams are much thinner than arch dams.

A buttress dam is characterized by a set of angled supports on the downstream side that help to oppose the force of the water. This design can be employed in wide valleys where a solid bedrock foundation is not available. Because of the steel framework and associated labor needed for construction, these dams are no longer economically viable.

The gravity dam withstands the force of the water behind it with its weight. Made of cement or masonry, this type of dam normally utilizes a solid rock foundation but can be situated over unconsolidated material if provisions are made to prevent the flow of water beneath the structure. The solid, stable nature of this dam is favored by many and often incorporated into the spillway designs of embankment dams.

An embankment dam uses the locally available material (rocks , gravel, sand , clay, etc.) in construction. Just as with gravity dams, the weight of embankment dams is used to resist the force of the water. The permeability of the materials that make up these dams allows water to flow into and through the dam. An impervious membrane or clay core must be built into them to counteract the flow and protect the integrity of the structure. Because the materials are locally available and the construction of these dams is relatively simple, the cost of construction for this type of dam is much lower than the other types. Embankment dams are the most common.

Impact of dams

Dams have long been acknowledged for providing electricity without the pollution of other methods, for flood protection, and for making water available for agriculture and human needs. Within recent decades, however, the environmental impacts of dams have been debated. While dams do perform important functions, their effects can be damaging to the environment. People have begun to question whether the positive contributions of some dams are outweighed by those negative effects.

The damming of a river will have dramatic consequences on the nature of the environment both upstream and downstream of the dam. The magnitude of these effects are usually directly related to the size of the dam.

Prior to dam construction, most natural rivers have a flow rate that varies widely throughout the year in response to varying conditions. Of course once constructed, the flow rate of the river below a dam is restricted. The dam itself and the need to control water releases for the various purposes of the particular dam result in a flow rate that has a smaller range of values and peaks that occur at times related to need rather than the dictates of nature. In cases where the entire flow has been diverted for other uses, there may no longer be any flow in the original channel below the dam.

Because water is held behind the dam and often released from some depth, the temperature of the water below the dam is usually lower than it would be prior to dam emplacement. The temperature of the water flow is often constant, not reflecting the natural seasonal variations that would have been the case in the free-flowing river. Similarly, the chemistry of the water may be altered. Water exiting the lake may be higher in dissolved salts or have lower oxygen levels than would be the case for a free-flowing river.

Impoundments increase the potential for evaporation from the river. Because the surface area of a lake is so great when compared to the river that supplies it, the loss of water to evaporation must be considered. In some desert areas, potential annual evaporation can be greater than 7 ft (2.1 m), meaning that over the course of one year, if no water flowed into or out of the system, the reservoir would drop in elevation by 7 ft (2.1 m). At Lake Mead on the Colorado River in Arizona and Nevada, evaporation losses in one year can be as great as 350 billion gal (1.3 trillion l).

The impoundment of water behind a dam causes the velocity of the water to drop. Sediment carried by the river is dropped in the still water at the head of the lake. Below the dam, the river water flows from the clear water directly behind the dam. Because the river no longer carries any sediment, the erosive potential of the river is increased. Erosion of the channel and banks of the river below the dam will ensue. Even further downstream, sediment deprivation affects shoreline processes and biological productivity of coastal regions.

This problem has occurred within the Grand Canyon below Glen Canyon Dam. After the construction of the dam was completed in 1963, erosion of the sediment along the beaches began because of the lack of incoming sediment. By the early 1990's, many beaches were in danger of disappearing. In the spring of 1996, an experimental controlled flood of the river below Glen Canyon Dam was undertaken to attempt to redistribute existing sediments along the sides of the channel. While many of the beaches were temporarily rebuilt, this redistribution of sediments was short lived. Research on this issue is continuing, however, the fundamental problem of the lack of input sediment for the river downstream of the dam remains unresolved.

The environmental changes described above create a new environment in which native species may or may not be able to survive. New species frequently invade such localities, further disrupting the system. Early photographs of rivers in the southwest desert illustrate the dramatic modern invasion of non-native plants. Entire lengths of these rivers and streams have been transformed from native desert plants to a dense riparian environment. Native species that formerly lived in this zone have been replaced as a result of the changes in river flow patterns. The most commonly cited species affected by the presence of dams is the salmon . Salmon have been isolated from their spawning streams by impassable dams. The situation has been addressed through the use of fish ladders and by the use of barges to transport the fish around the obstacles, but with only limited success.

David B. Goings

Resources

books

Dunar, Andrew J., and Dennis McBride. Building HooverDam: An Oral History of the Great Depression. New York: Twayne Pub, 1993.

Fradkin, Philip L. A River No More: The Colorado River and the West. Berkeley: University of California Press, 1996.

High Country News. Western Water Made Simple. Washington, DC: Island Press, 1987.

Keller, Edward. Environmental Geology. Upper Saddle River: Prentice-Hall, Inc., 2000.

Pearce, Fred. The Dammed: Rivers, Dams, and the ComingWorld Water Crisis. London: Bodley Head, 1992.

Reisner, Marc. Cadillac Desert: The American West and its Disappearing Water. New York: Penguin Books Ltd., 1993.

periodicals

"Water: The Power, Promise, and Turmoil of North America's Fresh Water." National Geographic Special Edition. November 1993.

other

"Colorado River Watershed." Arizona Department of Water Resources. [cited October 16, 2002.] <http://www.adwr.state.az.us/AZWaterInfo/OutsideAMAs/UpperColoradoRiver/Watersheds/coloradoriver.html>.

"Lower Colorado River Operations." United States Bureau of Reclamation. [cited October 16, 2002]. <http://www.lc.usbr.gov/lcrivops.html>.

"Controlled Flooding of the Colorado River in Grand Canyon." United States Geological Survey. February 14, 1999 [cited October 16, 2002]. <http://az.water.usgs.gov/flood.html>.

KEY TERMS

Arch dam

—A thin concrete dam that is curved in the upstream direction.

Buttress dam

—A dam constructed of concrete and steel that is characterized by angled supports on the downstream side.

El Niño

—The phase of the Southern Oscillation characterized by increased sea water temperatures and rainfall in the eastern Pacific, with weakening trade winds and decreased rain along the western Pacific.

Embankment dam

—A simple dam constructed of earth materials.

Gravity dam

—A massive concrete or masonry dam that resists the force of the water by its own weight.

Impoundment

—The body of water, a pond or lake, that forms behind a dam.

Permeability

—The capacity of a geologic material to transmit a fluid, such as water.

Spillway

—A passage for water to flow around a dam.

DAMS

Dams, barriers to alter flowing bodies of water, are among the most ancient and powerful examples of the proclivity of humans to alter nature for their own benefit. (Dams are also a type of construction shared with other animals, that is, beavers.) Before the advent of written history, dams were already being built to provide water storage and irrigation. An earthen dam in the Orontes Valley in Syria was ancient when visited by the Greek geographer Strabo around the beginning of the Common Era. The oldest large dam of which traces survive today is at Sadd-el-Kafara, near Cairo. Ninety-eight meters long, there are indications that it was intended to stand 125 meters high. It is estimated that this structure was built around 2500 b.c.e.

Dam Engineering

Despite their ubiquity and importance, dams are a stepchild of traditional engineering. Premodern treatises on construction such as Vitruvius's De architectura (first century b.c.e.) do not mention dams, although Roman dam achievements were not to be matched for 1,500 years. The scientific engineering of dams begins in the 1800s and was one of the early achievements of civil engineering as it replaced trial-and-error intuition with empirical rules of thumb for dam design.

In terms of function, dams primarily supply water for irrigation or urban use, or serve as sources of power. In conjunction with closely related structures called dikes, dams may also protect from flooding and/or facilitate transportation by creating navigable bodies of water such as canals.

In terms of design, dams are of two basic types: earth- or rock-filled gravity embankment dams and masonry or concrete dams. The former take the general shape of a large-based equilateral triangle with sloping embankments facing both upstream and downstream; the latter have more the shape of a right-angle triangle with a perpendicular upstream face and a sloping downstream face.

It was not until the mid-1800s that French engineers designed the first dams using scientific procedures to determine such issues as the slope of repose for embankments. At the same time engineers began to consider the geological structures on which various types of dams might rest and to analyze the internal stresses of masonry and concrete dams. Such analyses promoted the design of arch dams, in which a vertical upstream face is given a convex horizontal curve to help transfer forces from the impounded water into the walls of a canyon. The engineering of auxiliary structures such as spillways, locks, and power conversion systems also became part of dam design.

Progressive demands for water and power together with advances in dam engineering led in the first half of the twentieth century to what may be called the golden age of dam construction. But the second half of the twentieth century witnessed a technical reassessment of dam engineering in terms of safety and ecology, social and natural.

Dam Debates

For most of human history, dams were conceived and built with an eye only to the task to be accomplished, such as water storage, irrigation, or more recently, promotion of tourism, and without much concern for other implications, such as the impact on local populations or the environment. Of all major rivers in the United States, only the Salmon and Yellowstone are without dams. Half of the American wetlands that existed in 1790 have been flooded and destroyed by dam projects—up to 80 percent in river states such as Missouri, where one-third of all the water in the Missouri River is stored behind dams.

At the same time some experts argue that dams are often inefficient mechanisms for water storage, spreading water out over large areas in hot, dry desert climates where it evaporates. As much as 8 percent of Colorado River water may be lost to evaporation behind the Glen Canyon Dam in northern Arizona. Dams, by promoting water use, also contribute to the eventual depletion of aquifers.

In the modern world dams nevertheless continue to be seen as important symbols of human domination of the environment, sometimes outweighing all other issues. China's Three Gorges Dam, which will flood thousands of acres of agricultural land and displace more than one million people, is nevertheless viewed by the Chinese government as a powerful symbol of mastery and progress.

DAM SAFETY AND FAILURES. Like other huge, complex human technology projects, dams can fail if ill-designed or negligently maintained. The most famous failure in the United States was that of the South Fork Dam in Johnstown, Pennsylvania, in May 1889. Over the years, successive owners of the dam made dangerous modifications, eliminating outlet pipes, reducing its height, and narrowing the spillway. During an unprecedented rainfall, the water rose 3 meters (10 feet) above the usual lake level, breaking the dam and inundating Johnstown, with the loss of almost 3,000 lives.

RELOCATING PEOPLE. Dam projects have often involved the removal of the populations least able to defend themselves politically. Most often the groups forced to relocate are poor members of minority groups, subsisting on small-scale agriculture.

In June 1957 Congress voted the creation of Kinzua Dam in western New York, flooding half of a Seneca Indian reservation. More than 500 Seneca were forcibly moved in the dead of winter to trailer camps. Without access to hunting grounds, and denied compensation for their homes, these already poor individuals were, according to the sociologist Joy A. Bilharz (1998), driven into greater poverty, which lasted for decades.

Organized political opposition to large dam projects was pioneered in India, where in the late 1940s important projects backed by the prime minister, Jawaharlal Nehru, made little provision for the relocation of affected villages. Large demonstrations and other opposition increased the costs unacceptably, causing the government to back away from some of these projects.

ENVIRONMENTAL CONCERNS. During the twentieth century, the environmental movement advanced the argument that natural beauty was a factor to be taken into account in dam construction. John Muir led an early campaign against the O'Shaughnessy Dam in Yosemite National Park's Hetch Hetchy Valley on the grounds that it would destroy a unique environment. Later came the related idea that wild species themselves had interests worthy of protection, interests that might be harmed by dam construction. Environmentalists went to court to end construction of the Tellico Dam on the Little Tennessee River, on the grounds that it would destroy the remaining population of snail darters, an endangered fish. In response, federal courts halted construction of a dam already 80 percent completed. In 1978 the U.S. Supreme Court affirmed the court order halting construction, stating that the Endangered Species Act unambiguously bars projects that threaten the continued existence of a listed species. Congress, however, later passed legislation exempting Tellico from the Endangered Species Act, and the dam was completed.

Egypt's Aswan High Dam has been argued to have caused an environmental disaster, starving the Mediterranean of nutrients, making croplands excessively salty, and creating a reservoir in one of the highest evaporation zones on Earth.

DAM REMOVALS. Because of changing views of the utility of dams and the relative importance of environmental considerations, more than 500 dam removal projects were undertaken in the United States during the last decades of the twentieth century. The first dam removed for purely environmental reasons was the Quaker Neck Dam on the Neuse River in North Carolina. Built in 1952 to provide cooling water for a steam-driven electrical generating plant owned by Carolina Power & Light Company, the dam prevented shad from migrating upstream. The shad catch, 318,000 kilograms (700,000 pounds) in 1951, was only 11,400 kilograms (25,000 pounds) by 1996.

Carolina Power & Light was glad to get rid of Quaker Neck. The dam was expensive to maintain and also created litter and liability problems. Instead of the dam, a canal between two channels of the river now provides cooling water. More than 1,600 kilometers (1,000 miles) of local rivers have since been reopened to fish.

As the political and psychological importance of dams has faded and other considerations have come to the fore, Americans have stopped building dams. Since the mid-1970s, there has not been a single major dam construction project commenced in the United States.

JONATHAN WALLACE

SEE ALSO Bridges;Environmental Ethics;Three Gorges Dam;Water.

BIBLIOGRAPHY

Bilharz, Joy A. (1998). The Allegany Senecas and Kinzua Dam: Forced Relocation through Two Generations. Lincoln: University of Nebraska Press. Examines impacts on the Seneca nation of forced relocation to accommodate the Kinzua Dam project.

Devine, Robert S. (1995). "The Trouble with Dams." Atlantic Monthly 276(2): 64–74. Influential article arguing that most dams serve no significant purpose.

Goldsmith, Edward, and Nicholas Hildyard. (1984). The Social and Environmental Effects of Large Dams. San Francisco: Sierra Club Books. Representative of arguments critically reassessing dams during the last half of the twentieth century.

Jackson, Donald C. (1995). Building the Ultimate Dam: John S. Eastwood and the Control of Water in the West. Lawrence: University Press of Kansas. A biography of Eastwood (1857–1924), who from 1906 until his death designed more than sixty dam projects in the western United States, that stresses the psychological attractiveness of the multiple arch dam.

Jackson, Donald C., ed. (1997). Dams. Brookfield, VT: Ashgate. Collects seventeen previously published articles highlighting technical and social developments.

Khagram, Sanjeev. (2004). Dams and Development: Transnational Struggles for Water and Power. Ithaca, NY: Cornell University Press.

Levy, Matthys, and Mario Salvadori. (2002). Why Buildings Fall Down, rev. edition. New York: Norton. Analyzes dam collapses from a structural engineering standpoint.

Petersen, Shannon. (2002). Acting for Endangered Species: The Statutory Ark. Lawrence: University Press of Kansas. Contains an account of the snail darter and Tellico Dam.

Schnitter, Nicholas J. (1994). A History of Dams: The Useful Pyramids. Rotterdam, Netherlands: A. A. Balkema. A technical history with some comments on social context that takes its subtitle from the praise of dams by the Roman Sextus Julius Frontinus (c. 35–c. 103 c.e.).

Smith, Norman. (1971). A History of Dams. London: Peter Davies. The first general history of dams.

World Commission on Dams. (2000). Dams and Development: A New Framework for Decision-Making. London: Earthscan. A report that grew out of a 1997 World Bank workshop, arguing for a critical assessment of large-scale dam projects in terms of technical, economic, environmental, and social impacts.