assive dams are much more than simply machines to generate electricity and store water. They are concrete, rock and earth expressions of the dominant ideology of the technological age: icons of economic development and scientific progress to match nuclear bombs and motor cars.
More than 400,000 square kilometers (155,000 square miles)—the area of California—have been inundated by the world's 40,000 large dams. Freshwater resources, because of a host of human assaults, but especially because of dams, are the most degraded of the Earth's major ecosystems. Dams are the main reason why one-fifth of the world's freshwater fish are now either endangered or extinct.
The number of people flooded off their lands by dams is in the tens of millions—30 million would be a conservative estimate, 60 million more likely. Very few of these people ever recover from the ordeal, either economically or psychologically. And dams kill people, because they spread diseases such as malaria and because they break.
Now the future of every dam on Earth is threatened—not by environmental protests or economic constraints—but by the Greenhouse Effect and the world's changing climate.

Static Dams, Changing Climate

Dam designers work on the assumption that historic hydrological data such as average annual flow, annual variability of flow and seasonal distribution of flow are a reliable guide to the future. As global warming takes hold, however, there are likely to be significant changes in seasonal and annual rainfall patterns and other factors affecting streamflow, such as the rate and timing of snowpack melt and the nature of watershed vegetation.
Historical and geological evidence over past millennia indicate that even small changes in climate can cause major changes in the size of floods. Reservoir sedimentation can be affected significantly. In arid areas, an increase in average annual precipitation of only 10 percent can double the volume of sediment washed into rivers.
Calculations of the amount of water available to turn turbines, the maximum flood that spillways will have to discharge and the rate at which reservoirs fill with sediment will thus become more unreliable as global warming takes hold.
Insurers increasingly are convinced that global warming is to blame for the greater frequency and severity of violent storms, floods and droughts since the late 1980s. This extreme weather already has resulted in burst dams, increased sedimentation and reduced hydropower capacity.
A 1991 report from the United Nations Intergovernmental Panel on Climate Change noted that “increased runoff due to climate change could potentially pose a severe threat to the safety of existing dams with design deficiencies. Design criteria for dams may require re-evaluation to incorporate the effects of climate change.”

Political Hydrology

Just as dam builders often skimp on geological surveys, so have they shown themselves willing to begin construction with seriously inadequate hydrological data. When there is not enough water to turn a dam's turbines or fill its canals (or so much water that the dam is threatened with breaching), an “act of God”—drought or flood—invariably will be blamed for the ensuing electricity shortages or inundation. However, an “act of dam builder” may be the likeliest place to lay the blame.
Hydrologists cannot predict exactly how much water will flow into a planned reservoir. To make a “best guess,” they project past streamflow data into the future. A lack of reliable hydrological data, however, frequently does not stop dam builders, who often build first and then hope for the hydrological best. The dam-building fraternity has shown a pattern of overestimating annual flows and underestimating peak floods.
Overestimates of average flows mean that many dams fail to yield as much power and water as predicted. The huge Buendia-Entrepenas reservoir in central Spain, built in the late 1950s during General Franco's decade-and-a-half dam-building binge, has never been able to supply more than half the capacity of the aqueduct built to transfer water to the Mediterranean coast. In early 1994, the reservoir contained just 17 percent of its capacity.
In Thailand, with lower-than-expected rainfall and higher-than-expected leakage through its limestone bed, the nation's largest-volume reservoir, Srinakharin, completed in 1977, has never filled. During 1991, Thailand's 25 largest dams held a total of just under half of their combined usable capacity. By 1992, this figure fell to just over one-third. Bhumibhol and Sirikit, World Bank-funded dams that impound the second-and third-largest reservoirs in Thailand, contained only 7 percent of their total usable volume in March 1994.
Authorities building India's Sardar Sarovar dam refused to accept the overwhelming evidence that much less water is likely to be available than was assumed when the project was planned in the 1970s. Engineers designed Sardar Sarovar with the assumption that more than 27 million acre-feet of water flowed down the Narmada River in three out of every four years. Yet in 1990, the 42 years of flow data then available recorded an average flow of just 22.7 million acre-feet. More recent figures indicate the flow may be even lower.

Mud Against Dams

All rivers contain sediments. A river can be considered a body of flowing sediments as much as one of flowing water. When a river is stilled behind a dam, its sediments sink to the bottom. Every reservoir loses storage to sedimentation, although the rate at which this happens varies widely. Despite more than six decades of research, sedimentation still may be the most serious technical problem faced by the dam industry.
In a 1987 World Bank study, Professor Khalid Mahmood of George Washington University in Washington, DC, “roughly estimated” that around 50 cubic kilometers (1.8 million cubic feet) of sediment—nearly 1 percent of global reservoir storage capacity—is trapped behind the world's dams every year. Mahmood calculated that around 11,100 cubic kilometers (39 million cubic feet) of sediment had accumulated in the world's reservoirs by 1986, consuming almost one-fifth of global storage capacity.
In the US, large reservoirs lose storage capacity at an average rate of around 0.2 percent per year, with regional variations ranging from 0.5 percent per year in the Pacific states to just 0.1 percent in the Northeast. Major reservoirs in China lose capacity at an annual rate of 2.3 percent.
By far the world's muddiest river is the Yellow, which flows through the easily eroded, loess soil of north-central China. The average concentration of sediment in the Yellow is nine times grater than any other major river. Soil scientist Daniel J. Hillel describes it as a “rippling tide of liquefied mud, resembling thick lentil soup.” The record of reservoirs built on the Yellow is, not surprisingly, atrocious.
Engineers built Sanmenxia (Three Gates Gorge) chiefly for flood control on the lower Yellow, with technical assistance from the Soviet Union. Construction began in 1957. Chinese hydrologists who protested that the reservoir soon would fill with mud were accused of being rightists and silenced. Within just three years of reservoir impoundment in 1960, the river had deposited more than 50 billion tons of sediment at its upper end, raising the riverbed by several meters and threatening upstream areas—including the ancient capital Xian—with serious flooding.
The Sanmenxia fiasco was repeated at other reservoirs constructed on the upper Yellow River in the late 1950s. The 57-meter-high (187 feet) Yangouxia Dam lost almost one-third of its storage capacity before it even began operation. By 1966, three-quarters of Yangouxia's reservoir had filled with sediment.
In India, government statistics on 11 of the country's reservoirs with capacities greater than one cubic kilometer (35,000 cubic feet) show that all are filling with sediment 130 to 1,650 percent faster than expected. In 1993, the US Army Corps of Engineers concluded that sedimentation could reduce the life of El Salvador's 135-megawatt Cerron Grande Dam to 30 years—as compared to the pre-construction prediction of 350 years.

Sedimental Journey

The amount of sediment carried into a reservoir is at its highest during floods. In the US, for example, half of a river's annual sediment load commonly is transported during only five to 10 days' flow. During and after a particularly violent storm, a river may carry as much sediment as it would in several “normal” years. Mudslides caused by earthquakes and volcanoes also can have a dramatic and unpredictable effect on reservoir sedimentation. Global warming, which experts predict will cause more intense storms, likely will increase both the unpredictability and rate of reservoir sedimentation.
In July 1993, the sediment scoured off upstream mountainsides during a single 30-hour stormburst cut the storage capacity of Nepal's Kulekhani hydrodam by nearly one-tenth. When completed in 1981, Kulekhani had a predicted life of 75 to 100 years. But sediments could put the 114-meter-high (374 ft) dam out of operation around the turn of the century.
Despite all the uncertainties surrounding reservoir sedimentation, authorities very rarely stop planned projects due to a lack of adequate sediment data. In fact, time and again, dam planners have made hugely over-optimistic predictions that reservoirs will fill much more slowly than they actually do.
Chixoy is one of a number of very expensive hydrodams built in Central America during the 1970s and 1980s with loans from the World Bank and InterAmerican Development Bank, despite the very high and accelerating rates of erosion in their watersheds.
Around the world, dams are rapidly filling with sediment, leaving small, impoverished countries like Guatemala, Honduras and Costa Rica with huge debts and a desperate need to build new power plants to reduce their dependence on their white elephant dams.
Dams open up remote areas to road builders, developers, loggers, farmers and miners—accelerating deforestation and soil loss. When insufficient resettlement land is available, ousted farming families may have no choice but to clear land further up the valley or hillside.
Deforestation and soil erosion both are increasing rapidly around the world and, it should be assumed, that when dams are built, soil erosion in their watersheds will increase over the projected economic life of the reservoir.
It is difficult to find any examples of the successful implementation of watershed anti-erosion measures in the tropics and subtropics. While these schemes may be recommended in project plans, they rarely are implemented. Dam-building agencies are usually more interested in putting their funds toward building dams than planting trees and digging field terraces.

Excerpted from Silenced Rivers: The Ecology and Politics of Large Dams by Patrick McCully