Big-dam construction is on the rise.

As he era of big dams fades in North America, construction is increasing in Asia, fueled by growing demand for electricity and irrigation. Worldwide, the number of dams under construction in 1993 (the latest year for which global data are available) rose by more than 9 percent, to at least 1,240, after a much smaller increase in 1992. The accompanying chart shows that China accounts for more than one-fourth of the big dams under construction, while China, Japan, South Korea and India together account for more than half. The increases follow a decline in the 1980s, when construction worldwide averaged less than half that of the preceding 25 years.

Data for the early 1990s, though incomplete, indicate a shift toward larger dams, with concomitant greater social and economic costs. In 1992, 60 percent of the dams being built were more than 30 meters high, compared with only 21 percent of existing dams in 1986. Construction of dams higher than 100 meters rose by some 27 percent between 1991 and 1993; half of these large structures were built by just three countries - Japan, China, and Turkey.

Dams are a symbol of modernity and a source of national prestige, partly because they are a multipurpose tool of development. They generated more than 18 percent of the world's electricity in 1992, and reservoir water irrigates millions of hectares of land around the world, raising agricultural yields two to three times over those of dryland farming, and in many cases bringing agriculture to regions that could not otherwise support it. Areas with a high percentage of irrigated agriculture, such as China, India, and the western United States, rely heavily on reservoirs for water. Irrigation is increasingly listed as the primary purpose of new dams, possibly reflecting growing food needs and water scarcity in many developing countries.

Dam reservoirs also protect societies against drought. Water in the world's reservoirs effectively increases the normal supply from rivers by some 30 percent. This insurance can be invaluable: Egyptians maintain that the Aswan Dam saved Egypt from catastrophe during the drought of 1979-88.

In the past two decades, however, as it became clear that dams have serious costs as well as benefits, dam construction began to decline. Loss of land to reservoirs can be significant. The Narmada Sagar Project in India is expected to drive 31 species of plant to extinction because of habitat changes caused by reservoir creation. Decomposing plant life at reservoir bottoms can release as much methane and carbon dioxide both greenhouse gases - as a coal-fired plant with the same electricity-generating capacity, according to the Freshwater Institute in Canada.

Dams also restrict stream flow with often disastrous biological and economic consequences. Anadromous fish populations and fisheries have been nearly eliminated in many places worldwide, as on the Columbia River in the United States, where once plentiful salmon are now endangered species, and valuable fisheries have collapsed (see cover story, page 10). Changes in temperature and water alter habitats of fish and other wildlife. Sediment that would normally enrich floodplain soils gets trapped behind dams. This last problem, siltation, can shorten a dam's useful life: in an extreme case, the turbines of China's Sanmenxia Dam were shut down in 1964 after only four years of operation when the dam's reservoir filled with sediment.

By restricting stream flows, dams can also contribute to the decline of local economies. One example is Indian diversion of the Ganges River before it reaches Bangladesh, which has caused $25 million in losses for an agricultural project in northwest Bangladesh, and increased salination of fishing areas at the Bay of Bengal, the river's natural outlet.

Dams exact a high human toll as well. Millions of people were resettled in the past half-century to make room for dam reservoirs. China's Three Gorges Dam alone may well displace more than 1 million people. Those affected have had little say in their resettlement, and most displaced citizens wind up in worse situations than before their move.

Dams can increase health risks. Waterborne diseases, including malaria and river blindness, have been introduced and spread as reservoirs are created. Large-scale water projects were a major contributor to the 75-percent global increase in cases of schistosomiasis, a sometimes fatal disease, between 1947 and the early 1980s.

Advocates of dams acknowledge these shortcomings, but argue that these effects can be mitigated, or that the alternatives to dams would be worse. Others argue that evaluation of dams involves widely differing environmental, social, and political contexts, and a project-by-project analysis of each dam is needed.

Leading Builders of Big Dams
(Higher than 10 meters)(1)

Country           New Starts(2)       Under
                                   Construction
                    (number)         (number)

China                  85             311
Turkey                 84             190
Japan                  11             140
South Korea             2             125
India                  48              76
United States          30              55
Spain                  16              53
Romania                 0              39
Italy                   0              37
Tunisia                16              28
Algeria                 6              27
Iran                    1              76
Thailand                7              17
Greece                  3              14
France                  8              12
Brazil                  4              12

1. As of 1992, 60 percent were higher than 30 meters, and some
higher than 100 meters.

2. Data are for 1993, the latest year available at the time of
writing.

Source: International Commission on Large Dams, Paris

The World Bank - long a strong supporter of big dams - appears to have slowed its involvement. The Bank was involved with an average of 18 dam projects a year between 1980 and 1985, but with only 6 a year between 1986 and 1993. In fact, the Bank established a commission in 1994 to hear complaints from parties affected by proposed dam projects, the first of whom were people affected by the Arun III dam in Nepal.

The future of dams is unclear. Proponents note that only a fraction of the technically usable hydropower potential in developing countries has been harnessed. But the discussion has moved beyond technical considerations. Wide-ranging development, conservation, energy use and population considerations are - or should be - part of every proposed dam's calculus. It remains to be seen whether current and future big dams can meet the increasingly stringent environmental and social standards expected of them, and whether development needs can be met in other ways.