moc gnihsilbuperwihskwrebw moc … · ers. china, the united states, and india are the chief dam...

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Dams and Reservoirs

Nearly 60 percent of the world’s rivers have large dams with reservoirs, which provide water for agriculture and drinking, hydropower, flood control, and other benefits. But dam construction affects surrounding communities, and reservoirs cause pollution and environmental prob-lems. Their sustainability requires a new approach to water resources development that promotes ecosystem preservation and water quality to the same extent as eco-nomic development and social well-being.

While any barrier across a stream can be called a dam, large dams are those that the International

Commission on Large Dams (ICOLD) defines as being at least 15 meters high. Of the 800,000 dams in the world, around 50,000 qualify as large. These dams have provided the water that enabled the development of large urban cen-ters, the growth of industry and jobs, and the expansion of agriculture into arid regions. Without them, it is difficult to envision the quality of life that many of us take for granted. Yet large dams also produce short- and long-term environ-mental and social consequences and may be unsustainable in the long run. Managing dams and the reservoirs behind them thus becomes an exercise in reconciling modern eco-nomic and social reality with good science and stewardship.

Large dams block nearly 60 percent of the world’s riv-ers. China, the United States, and India are the chief dam builders, with, respectively, 46 percent (22,000), 14 per-cent (6,600), and 9 percent (4,300) of the world’s large dams. Two-thirds of the large dams have a single purpose, either irrigation or hydropower (see Figure 1). By far, the majority, including most of the large dams in Africa and Asia, serve irrigation. In contrast, hydropower dominates single-purpose dam use in Europe and South America. Nearly 20 percent of the world’s electricity supply comes from hydropower.

The remaining one-third of the large dams around the world are multipurpose, providing two or more benefits, including flood control, navigation, water supply, irriga-tion, hydropower, and, increasingly, recreation. For exam-ple, many large hydropower dams in the United States and Canada also provide flood control and water supply. Multipurpose dams pose difficult management problems because their benefits can conflict with one another. We store water for irrigation and hydroelectric power, but we reduce water levels or even empty reservoirs to accommo-date floodwater. To ensure efficiency and satisfy different customers, water levels must be adjusted regularly.

Behind the dams are the reservoirs. They inundate nearly 500,000 square kilometers of land worldwide—approxi-mately the area of Spain—and are capable of storing 6,000 cubic kilometers of water: roughly half the volume of Lake Superior. Scientists report that this massive movement of freshwater is responsible for a small yet measurable increase in the speed of the Earth’s rotation and for a slightly altered inclination of the Earth’s axis. These reservoirs are some-times called lakes, and many vacationers think of them as lakes until the dam manager lowers or raises the water level to accommodate different needs. In fact, natural lakes dif-fer from reservoirs. Reservoirs generally have more shore-line than lakes, and the residence time of a reservoir (how long on average a drop of water remains in the reservoir) is usually shorter than a lake’s. In one study, Kansas reservoirs were compared with similar-sized Michigan lakes, and sci-entists found that the residence time was about fourteen months in the reservoirs and nearly four and a half years in the lakes (Pielou, 207).

A more significant difference between lakes and reser-voirs relates to chemical composition. Plant material under reservoir water provides habitat for bacteria, which thrive and multiply. As they do, they absorb mercury and convert it into methyl-mercury, about a hundred times more toxic

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dams and reserviors • 3

100%90%

80%70%

60%50%

40%

30%20%

10%0%

Flood Control Irrigation Water Supply Other Single Purpose MultipurposeHydropower

Africa NorthAmerica

SouthAmerica

Asia Austral-Asia Europe

6%

2%

19%

20%

52%10%

11%

24%

30%

11%

13%1%

13%

26%2%

19% 2%

31%

16%

25%

3%

44%

2%

19%

3%

14% 23%

7%

1%

65%

2%

4%

25%

15%

17%

24%

than the original mercury. By the time fish ingest the sub-stance, the concentration may have increased a millionfold. Fish introduce the substance into the food chain. People who eat enough toxic fish may suffer damage to the brain, liver, and kidneys and may eventually die. Controversy con-tinues over how much mercury can cause organ failure or other severe afflictions, but studies show that in some cases, local populations dependent on fish from reservoirs have mercury levels that exceed the limit recommended by the World Health Organization. The decomposing vegetation can also release into the atmosphere large amounts of two major greenhouse gases, carbon dioxide and methane. Some data suggest that a reservoir supplying water to a hydro-electric plant may yield as much greenhouse gas as a coal-fired generator producing the same amount of electricity. This challenges a long-prevailing theory that hydroelectric power does not harm the atmosphere. Estimates, however, vary widely on the degree to which reservoirs contribute to greenhouse gas pollution, and far more study is needed to provide precise data. Preliminary findings indicate that shallow, warm reservoirs in the tropics are more likely to emit greenhouse gases than deep, cold boreal reservoirs.

Dam Costs

The contributions of dams to economic development and social well-being come at a cost, and scientific investiga-tions over the last fifty years have revealed to what extent. First, dam construction has displaced millions of people. How many exactly is impossible to determine, but the World Bank estimates that between 1986 and 1993 alone approximately 40 million were displaced, while the World

Commission on Dams (WCD; 2000) concludes that per-haps as many as 80 million people have been resettled because of dam construction. The majority of people moved from dam sites reside in India and China, two countries with huge populations and aggressive dam-building pro-grams. Forced shifts in population can cause strife and hardship, as people resettle in communities that are not always able or willing to accept them. Incomes often decline, and those resettled may feel powerless. The Three Gorges Dam on the Yangzi (Chang) River in China epito-mizes the problem. Its construction required the relocation of about 1.25 million people, the inundation of thirteen cit-ies, and the whole or partial submergence of 1,352 villages. Despite the promises of politicians and others, displaced populations rarely enjoy the same standard of living they had before their removal—nor do they always have access to or enjoy the benefits of the dams.

Second, dams ecologically “disconnect” rivers. They alter the flow and the temperature of the water. Both high and low flows determine the physical characteristics of a river and how much habitat space is available for aquatic organisms. When dams flatten these flow variations, they significantly change the environment, and the reservoirs behind dams can destroy aquatic and terrestrial ecosys-tems. Modified habitats may attract non-native plant and animal species, which often flourish at the expense of the natives. Rivers that once flowed freely flow more slowly as they enter reservoirs, causing sediment to settle, creating narrower and shallower channels and making upstream flooding more likely. Sediment carried into the reser-voirs reduces storage capacity by about 1 percent annu-ally and, in China, with its highly erodible soil, reservoirs

Distribution of existing large dams by region and purpose. Source: Adapted from ICOLD, 1998.



4 • Berkshire encyclopedia of sustainaBility: natural resources and sustainaBility

annually lose over 2 percent of their storage capacity. This reduced capacity can seriously affect dam operations and lessen the benefits that justified the project in the first place. Downstream impacts can be equally significant. Dams affect fish populations, particularly anadromous fish that swim to the sea but return upstream to spawn. Also, water plunging from a dam may carry entrained bubbles that can dis-solve in the water below, releasing air into the water. Fish swimming in this supersaturated area may die of gas-bubble disease, or what we call the bends when applied to humans. Also, without the sediment from upstream, down-stream beaches and backwaters slowly disappear, reducing habitat for aquatic species and waterfowl.

The disruption of the natu-ral rhythm of a stream—its f low variations over a year’s time—not only separates the river above and below the dam but alienates the river from the floodplain around it. High flows decrease or are eliminated. In consequence, the river deposits less gravel in spawning areas, transports less organic material from land into the water (reduc-ing food for fish), disrupts the life cycles of certain insects that depend on floods, and endangers fish that rely on floods to transport their eggs until they hatch. In the absence of higher flows, vegetation encroaches on the channel, reducing space for aquatic species. Low flows often lead to fish kills and harm fish populations sensitive to higher temperatures and lower oxygen levels. This, in turn, may affect wildlife dependent on fish for food. Groundwater tables may also fall if not recharged by the river, threat-ening both vegetation and human settlements that may depend on pumped water. Even if the groundwater table does not fall significantly, increased salt levels in the sur-rounding floodplain can infiltrate into the groundwater and degrade water supplies. In short, where large dams are constructed, forests and wildlife habitats are lost, ter-restrial and especially aquatic biodiversity declines, and inundated villages, rich agricultural land, and enthralling scenery become receding memories.

Dams also increase pollution. If the river can no lon-ger spread out over the floodplain, floodplain plant com-munities can no longer cleanse its nutrient load. Instead,

the river carries these pollutants downstream, where they threaten both rural and urban areas. In some cases, the pollutants make their way to the sea. For instance, the nitrogen load at the Mississippi’s delta in Louisiana is esti-mated to be up to three times what it was before human development (Postel and Richter, 25). This results from Midwest farmers’ extensive use of fertilizer. Transported all the way to the Mississippi’s mouth, the nitrogen load contributes to algal blooms in the Gulf of Mexico and

resulting “dead zones” where low oxygen supply kills most aquatic life.

Aside from their contribution to greenhouse gases, dams and reservoirs

apparently affect the atmosphere in other ways. For decades,

scientists have debated whether dam projects can affect precipitation pat-terns, and there appears to be a growing belief that it does. Irrigation and urban-ization apparently can trig-

ger heavy rainfall patterns. At the same time, evaporation from large reservoirs can mod-ify fluxes of moisture and heat in

the air, thus serving as a catalyst for precipitation. While more study

is needed, the results of scientific inves-tigations to date suggest that future dam

designers should consider the atmospheric variations resulting from their projects.

Dams, Reservoirs, and Sustainability

The World Commission on Dams (2000, 2) reports that “the end of any dam project must be the sustainable improvement of human welfare.” This means, according to the authors, that human development must be on a basis that is “economically viable, socially equitable, and environ-mentally sustainable” (WCD 2000, 2). The concept is easy to describe, but much more difficult to implement. Until the late twentieth century, exploitation, not sustainability, defined water resources development. Developers sought to provide water for agriculture, industry, urban areas, and navigation; to produce hydropower; and, generally, to raise standards of living. They also built dams to protect people and property from devastating floods. Preserving ecosys-tems and ecological benefits did not obtain similar support. The approach seemed to work until people discovered the detrimental effects of disconnected ecosystems, silt-laden reservoirs, and pollution.



dams and reserviors • 5

Experts agree that sustainability requires a new approach to water resources development and use, one that elevates the preservation of ecosystems and the protection of good-quality water to the same level as the promotion of eco-nomic development and social well-being. Toward the end of the twentieth century, the term that emerged to describe this approach was integrated water resources management (IWRM). The approach recognizes that healthy ecosys-tems provide valuable benefits to humans, and that river flows must be adjusted to ensure the quality and quantity of water necessary to protect ecosystems. IWRM emphasizes managing the entire watershed, understands the linkage between rivers and land, and integrates social and physical phenomena to accurately describe the physical constraints as well as human needs. Then planners attempt to provide a sustainable and practical solution. Good conservation prac-tices, improved maintenance of water infrastructure, the elimination of water subsidies, and greater decentralization of water supplies can all be used to reduce water waste and prevent the construction of dams with marginal benefits. The most challenging aspect of this approach is convincing people that limiting the extent to which humans modify rivers in the long run provides a better quality of life for present and future generations.

Experiments to mimic natural flow have been attempted on numerous rivers in the United States. The Missouri River, a staircase of dams reaching from southeastern Nebraska to eastern Montana, has generated many con-troversial attempts to regulate the water to more closely resemble natural flow patterns, helping both aquatic spe-cies and waterfowl. Ecologists also call for the restoration of some of the river’s natural meanders. Such efforts raise objections from navigation, hydropower, and agricultural interests, who fear that changes in flow releases from dams or the restoration of meanders might damage the economy. Meanwhile, environmental organizations call for the out-right removal of dams that damage ecosystems and pro-vide minimal economic and social benefits. In the United States, over seven hundred dams have been removed since the late 1990s. Many were removed primarily because they no longer served any useful purpose. Others, however, were torn down to restore flows and enable the return of aquatic species. Most of these dams were not large, but calls to tear down some of the large dams, such as Glen Canyon on the

Colorado River in Arizona, have also emerged. It is too early to judge the success of many of these efforts.

Dams and reservoirs will remain essential tools of water management into the foreseeable future. Yet they can be operated to respond to both ecological and human needs and, while those needs will not be fully satisfied, a reason-able compromise can be reached that promises sustainable growth. Future water projects can be designed and con-structed to eliminate dams wherever possible in favor of less environmentally damaging structures—or, indeed, no structures at all—and with more emphasis on conserva-tion, floodplain management, and protection of the Earth’s resources.

Martin REUSSUnited States Army Corps of Engineers (Retired)

See also Aquifers; Irrigation; Nitrogen; Oceans and Seas; Recreation, Outdoor; Rivers; Water Energy

Further readingBlack, Maggie, & King, Jannet. (2009). The atlas of water: Mapping the

world’s most critical resource. Berkeley: University of California Press.Gleick, Peter H. (1998). The world’s water 1998–1999: The biennial report

on freshwater resources. Washington, DC: Island Press.Gleick, Peter H. (2000). The world’s water 2000–2001: The biennial report

on freshwater resources. Washington, DC: Island Press.Gleick, Peter H. (2002). The world’s water 2002–2003: The biennial report

on freshwater resources. Washington, DC: Island Press.Goldsmith, Edward, & Hildyard, Nicholas. (1986). The social and envi-

ronmental consequences of large dams. Cornwall, UK: Wadebridge Ecological Centre.

Hossain, Faisal, & Jeyachandran, Indumathi. (2009). Have large dams altered extreme precipitation patterns? EOS, 90(48), 1–2.

Lohan, Tara. (Ed.). (2008). Water consciousness: How we all have to change to protect our most critical resource. San Francisco: AlterNet Books.

McCully, Patrick. (1996). Silenced rivers: The ecology and politics of large dams. London: Zed Books.

Pielou, E. C. (1998). Fresh water. Chicago: University of Chicago Press.Postel, Sandra, & Richter, Brian. (2003). Rivers for life: Managing water

for people and nature. Washington, DC: Island Press.Wescoat, James L., Jr, & White, Gilbert F. (2003). Water for life: Water

management and environmental policy. Cambridge, UK: Cambridge University Press.

Wohl, Ellen. (2004). Disconnected rivers: Linking rivers to landscapes. New Haven, CT: Yale University Press.

World Commission on Dams. (2000, November). Dams and develop-ment: A new framework for decision-making/The report of the world com-mission on dams. London: Earthscan Publications.



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