Dams and reservoirs

DEFINITION: Structures that obstruct the natural flow of water in rivers and streams, and the bodies of water created by the impoundment of water behind or upstream of such structures

Because dams obstruct the natural flow of water, they have significant effects on stream and river ecosystems. Although dams provide benefits such as flood control and hydroelectric power generation, they also have a number of negative environmental impacts.

Dams are designed for a number of purposes, including conservation, irrigation, flood control, hydroelectric power generation, navigation, and recreation. Not all dams create reservoirs of significant size. Low dams, or barrages, have been used to divert portions of stream flows into canals or aqueducts since human beings’ first attempts at thousands of years ago. Canals, aqueducts, and pipelines are used to change the direction of water flow from a stream to agricultural fields or areas with high concentrations.

Dams and reservoirs provide the chief, and in most cases the sole, means of storing stream flow over time. Small dams and reservoirs are capable of storing water for weeks or months, allowing water use during local dry seasons. Large dams and reservoirs have the capacity to store water for several years. As urban populations in arid regions have grown and irrigation agriculture has dramatically expanded, dams and reservoirs have increased in size in response to demand. They are frequently located hundreds of kilometers from where the water is eventually used. The construction of larger dams and reservoirs has resulted in increasingly complex environmental and social problems that have affected large numbers of people. This has been particularly true in tropical and developing nations, where most of the large construction of the last three decades of the twentieth century was concentrated.

Sizes and Purposes of Dams

Early dams and their associated reservoirs were small, and dams remained small, for the most part, until the twentieth century. The first dams were simple barrages constructed across streams to divert water into irrigation canals. Water supply for humans and animals undoubtedly benefited from these diversions, but the storage capacity of most dams was small, reflecting the limited technology of the period. The earliest dams were constructed some five thousand years ago in the Middle East, and dams became common two thousand years ago in the Mediterranean region, China, Central America, and South Asia.

The energy of falling water can be converted by water wheels into mechanical energy to perform a variety of tasks, including the grinding of grain. Dams create a higher “head” or water level, increasing the potential energy, and thus served as the earliest energy source for the beginnings of the Industrial Revolution during the nineteenth century. The most significant contribution of dams to industrialization occurred in 1882 with the development of hydroelectricity, which permitted energy to be transferred to wherever electric power lines were built rather than being confined to river banks.

During the nineteenth century, large-scale settlement of the arid regions of western North America and Asia soon exhausted the meager local supplies of water and prompted demands for both exotic supplies from distant watersheds and storage for dry years. Big dams for storage and big projects for transportation of the water were thought to be the answer. Small dam projects could be financed locally; grander schemes required the assistance of federal or national governments. To justify expenditures on larger dams, promoters of these projects touted the multiple uses for water as benefits that would offset the projects’ costs. Benefit-cost ratios thus became the tool by which potential projects were judged. To raise the ratio of benefits to costs, promoters placed increasing importance on intangible benefits—those to which it is difficult to assign universally agreeable currency values. While dams in arid regions were originally justified chiefly for irrigation, public water supply, and power, decisions to build dams in wetter areas were usually based on projected benefits from flood control, navigation, and recreation in addition to power generation and public water supply.

Complicating the equation is the fact that multiple uses are frequently conflicting uses. While all dams are built to even out the uneven flow of streams over time, flood control requires an empty reservoir to handle the largest floods; conversely, power generation requires a high level of water in the reservoir to provide the highest head. Public water supply and navigation benefit most from supplies that are manipulated in response to variable demand. Recreation, fishing, and the increasingly important factor of environmental concerns focus on in-stream uses of the water.

By the last two decades of the twentieth century, environmental costs and benefits and the issue of Native American water rights in the American West dominated decisions concerning dam projects in the United States, and few dams were constructed. Most of the best sites for the construction of large dams in the developed nations had been utilized, and the industry turned its attention to the developing nations. Most of the large dam projects of the last quarter of the twentieth century were constructed in or proposed for developing nations and the area of the former Soviet Union.

Human Impacts

Small dams have small impacts on the environment; they affect small watersheds and minor tributaries and usually have only a single purpose. Farm ponds and tanks, as they are known in many parts of the world, generally cover a fraction of 1 hectare (2.47 acres) in area and are only a few meters in height. These tiny ponds are designed to store water for livestock and occasionally for human supply. They frequently serve recreational purposes as well, such as fishing. During dry spells they become stagnant and subject to contamination by and other noxious organisms, which can threaten the health of humans and livestock. Otherwise, they have little negative impact on the or on nearby people and animals.

Large dams and reservoirs are responsible for environmental and social impacts that often appear to be roughly related to their size: The larger the dam or reservoir, the greater the impact. Geographical location is also important in assessing a project’s impact. Scenic areas in particular, or those with of plants or animals or irreplaceable cultural or archaeological features, raise more controversy and litigation if they are chosen as potential sites for dam and reservoir projects.

People in tropical regions suffer proportionately greater health-related impacts from dams than do those in corresponding nontropical areas. The large numbers of workers required for the construction of big dams and associated irrigation projects can carry diseases into unprotected populations. Stagnant or slow-moving waters in reservoirs and irrigation canals, as well as fast-moving waters downstream of dams, are associated with particularly vicious tropical health risks. Snails in slow-moving water carry schistosomiasis, a parasitic disease that infects intestinal and urinary tracts, causing general listlessness and more serious consequences, including failure of internal organs and cancer. Estimates of the numbers of people infected range into the hundreds of millions. Malaria, lymphatic filariasis (including elephantiasis), and other diseases are carried by mosquitoes that breed in water; the incidence of such insect-borne diseases dramatically expands near irrigation projects and reservoirs. River blindness, which results from the bite of black flies, is associated with fast-flowing water downstream of dams and affects hundreds of thousands of humans.

The flooding of densely populated river valleys by reservoirs displaces greater numbers of people, with attendant health problems and social impacts, than similar projects in sparsely populated areas. Population displacement in developing countries, especially those in the Tropics, causes greater health and social problems than in developed nations, where remedial measures and compensation are more likely to assuage the loss of homestead and community.

Environmental Impacts

Many of the environmental problems created by large dams are associated with rapid changes in water level below the dams or with the ponding of stream flow in the reservoirs, which replaces fast-flowing, oxygenated water with relatively stagnant conditions. Indigenous animal species, as well as some plant life, are adapted to seasonal changes in the natural stream flow and cannot adjust to the postdam regime of the stream. Consequently, the survival of these species may be threatened. In 1973 the Tennessee Valley Authority—the worldwide model for builders of many large, integrated projects—found that the potential demise of a small fish, the snail darter, stood in the way of the completion of the Tellico Dam. After considerable controversy and litigation, the dam was completed in 1979, but few large projects have been proposed in the United States since then, particularly in the humid East. Since the early 1970s, the arguments for abandoning dam projects have been more likely to be backed up by laws, regulations, and court decisions.

Construction of the Hetch Hetchy Dam in the Sierra Nevada mountain range of California in the early twentieth century sparked vigorous dissent, which is said to have led to the growth of the Sierra Club and organized environmental opposition to dam building. This opposition successfully challenged the construction of the Echo Park Dam on the Green River in Colorado in the early 1950’s but was unsuccessful in stopping the construction of the Glen Canyon Dam on the Colorado River, which was completed in 1963. Glen Canyon, however, was the last of the big dams constructed in the American West. The preservationists, whose arguments chiefly concerned scenic and wilderness values, with attendant benefits to endangered species, lost the Glen Canyon battle but won the war against big dams. The controversy surrounding the Glen Canyon Dam continued for more than three decades after its completion, pitting wilderness and scenic preservationists against powerboat recreationists, who benefit from the access accorded by the dam’s reservoir to the upstream canyonlands.

All reservoirs eventually with from upstream erosion; deltas form on their upstream ends. Heavier sediments, mainly sands, are trapped behind the dam and cannot progress downstream to the ocean. The Atlantic coastline of the southeastern United States suffers from beach and retreat because the sands are no longer replenished by the natural flow of nearby rivers. The Aswan High Dam on the Nile River in Egypt has had a similar impact on the Nile Delta. Moreover, the natural flow of downstream has historically replenished the fertility of soils during floods. To the extent that the flood-control function of a dam is successful, new fertile sediment never reaches downstream agricultural fields. While irrigation water provided by the dam may permit the expansion of cropland, this water in arid regions is often highly charged with salts, which then accumulate in the soils and eventually become toxic to plant life.

Reservoir waters methane from decaying matter into the atmosphere. Methane is a that promotes global warming, and some estimates suggest that the effect of large reservoirs is roughly equal to the greenhouse gas of large thermal-powered electrical generation plants. The weight of the water in large reservoirs has also been implicated in causing earthquakes, which may lead to the failure of a dam. Dam failure may also occur because of poor construction or poor design, or because the builders had inadequate knowledge of the geology of the site. Tens of thousands of lives have been lost as a consequence of such failures.

Climate change has affected the amount of water in reservoirs because rising temperatures lead to additional evaporation. Another problem in the 2020s was an increase in sediment. Dams block river sediment from flowing downstream. Over time, it accumulates, leaving less space for water. A study published in 2023 found that by 2050, the world's major dams will have lost more than one-quarter of their storage capacity. The United States could face a 34 percent reduction in storage capacity in its 7,469 large dams by 2050. Researchers believe that the best solution to the problem is to build a bypass to divert water flows around the dam using a separate channel.

Bibliography

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

Cech, Thomas V. “Dams.” In Principles of Water Resources: History, Development, Management, and Policy. 3d ed. New York: John Wiley & Sons, 2010.

Goldsmith, Edward, and Nicholas Hildyard. The Social and Environmental Effects of Large Dams. New York: Random House, 1984.

Leslie, Jacques. Deep Water: The Epic Struggle over Dams, Displaced People, and the Environment. New York: Farrar, Straus and Giroux, 2005.

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McCully, Patrick. Silenced Rivers: The Ecology and Politics of Large Dams. Enlarged ed. London: Zed Books, 2001.

"Reservoir." National Geographic, 21 June 2024, education.nationalgeographic.org/resource/reservoir/. Accessed 17 July 2024.

Stevens, Joseph E. Hoover Dam: An American Adventure. Norman: University of Oklahoma Press, 1988.

"What Are Dams and Reservoirs?" Alberta WaterPortal Society, 8 Sept. 2023, albertawater.com/what-are-dams-and-reservoirs/. Accessed 17 July 2024.