Big-dam construction is on the rise.

• Author: Gardner, Gary

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...