<!--intro-->Many dams are failing to live up to expectations. Instead they make flooding worse, and cause ecological havoc and social conflict, says a report by the World Commission on Dams due out this week.<!--/intro--> <center ><a target="_new" HREF='http://www.exosci.com/portal.php?url=http%3A%2F%2Fwww.newscientist.com%2F&what=link&item=20001115202740695'><img SRC='http://www.newscientist.com/ads/people_why.gif' BORDER=0></a></center> The report is the first to assess the world's dams--the biggest drain on aid budgets for the past 50 years, costing $4 billion a year in the 1980s. So far dam building has driven up to 80 million people from their homes. Yet one of the most disturbing findings, the commission says, is that few dams have ever been looked at to see if the benefits outweigh the costs. A quarter of dams built to supply water deliver less than half the intended amount, says the report. In a tenth of old reservoirs, the build-up of silt has more than halved the storage capacity. What's more, by stopping the flow of silt downstream, dams reduce the fertility of flood plains and "invariably" cause erosion of coastal deltas. It's not all bad news, though. Dams irrigate fields that provide up to a sixth of world food production, while hydroelectric dams power many homes and factories. Too often, though, the rural poor don't benefit at all--only the urban and well-off. The report also concludes that some dams designed to prevent flooding actually exacerbate it. Such problems will worsen with climate change, it says. The commission, which is sponsored by the World Bank, backs warnings that most reservoirs emit greenhouse gases (New Scientist, 3 June, p 4). And it says dam construction is one of the major reasons for freshwater fish going extinct and bird species vanishing from flood plains. The largest dam project ever--the $20 billion Three Gorges dam on the River Yangtze in China --is mired in controversy over allegations that much of the money allocated to resettling a quarter of a million people has gone missing. Meanwhile, managers of the $4 billion Xiaolangdi dam on China's Yellow River, part-funded by the World Bank, reported last month that they could not find buyers for the electricity in China's newly liberalised market for power. Fred Pearce From New Scientist magazine, 18 November 2000.