The meager benefits of Third World dams and large-scale water projects are outstripped by disastrous side effects. Promises of greater crop yields and limitless hydroelectric power are seldom fulfilled. Many projects destroy flood plains and fisheries, cause salinization of croplands, and send millions of peasants to urban shantytowns. From an ecological, agricultural, humanitarian and economic standpoint, big dams make little sense.

Environmentalists in Brazil, China, India and Thailand would each read that as a summary of their own national water projects. But in The Dammed, Fred Pearce, a former staff writer for New Scientist magazine, arrives at these conclusions after scrutinizing the impact of big dams all over the world.

These are some of the unforeseen consequences he discovered:

*China's Sanmenxia Gorge Dam was supposed to operate for 50 years when it opened in 1960s. Within two years, silt knocked it out of commission. It has since been dynamited and rebuilt twice, yet now produces only one-quarter of the electricity originally promised.