The tide turns on nature's 'master'

Deliverance or disaster? Fred Pearce casts a critical eye over dams.

The World Bank does not often put up its hand to say sorry. And it did not this time. But the report of the World Commission on Dams, a body initiated and promoted by the bank, comes pretty damned close.

The bank has been the biggest funder of large dams round the world over the past 50 years - a time when so many barrages have been built across the world's great rivers that they have headed off several millimetres of sea-level rise. The bank has not been alone. Dams, whether for hydroelectricity or for irrigation and water supply, have swallowed more aid dollars than any other technology. But the bank has been the driving force behind the construction of some of the most spectacular of the world's 45,000 large dams.

Since 1900, the world has built one large dam every day, on average. They barricade 61 per cent of the world's rivers. Their reservoirs cover an area about six times the size of Britain and hold as much water as the North Sea. Yet until the commission began its work three years ago, there had been no overall assessment of the costs and benefits of these behemoths.

Dams have great symbolic power as totems of modernism, of mankind's mastery over nature and of national independence. While most of the early large dams were built in the United States and Soviet Russia, they swiftly became emblems of modernisation in the third world. A string of newly independent African nations enthusiastically embarked on giant hydroelectric projects in the 1960s and 1970s. Almost everyone supported them: engineers and economists, politicians and even environmentalists. For a time, especially after the successful developments of the Tennessee Valley Authority in the US, they were symbols of the New Deal and of environmentalism. They offered the prospect of endless supplies of cheap, renewable energy rather as wind power does today.

Only in recent years has their downside become more apparent. Greens led the turn-round. The opposition began in earnest when engineers tried to flood the Grand Canyon in the American West. But the case against dams swiftly developed from the scenic to a wider social and environmental agenda, first crystallised in an analysis by two British greens, Nicholas Hildyard and Edward Goldsmith, 17 years ago. By the mid-1990s, even the head of the Bureau of Reclamation, once the US's foremost dam builders, was arguing the case for tearing down US dams.

Dams may generate hydroelectricity, prevent floods, irrigate farms and supply water to industry and cities. But they wreck ecosystems and impoverish millions of people who lose land to reservoirs or see dams destroy their fisheries.

As the commission puts it: "The true profitability of these schemes remains elusive. There have been precious few, if any, comprehensive independent analyses as to why dams came about, how they perform and whether we are getting a fair return on our investment." It is an extraordinary state of affairs for what, during the 1980s, was one of the world's largest construction industries.

The report is an attempt to find common ground between those who build and prosper from dams and those who oppose them on environmental grounds or who have become victims of efforts to "tame" rivers. It is hard to imagine that the world would have been better without certain large dams. Both the High Aswan Dam in Egypt and the Hoover Dam in the US have brought gains that probably compensate for the damage they have caused. But so strong is the case against dams in many places that this report sometimes reads like a tract from anti-dam group the International Rivers Network. The long-delayed audit is, in many ways, a searing indictment of these structures and of the exaggerated claims made for them. One in four dams irrigates less than 35 per cent of the land it was supposed to. Average construction cost overruns are 56 per cent. Two-thirds of dams deliver less water to cities than promised. A quarter deliver less than half the promised amount. Some flood-control dams "have increased the vulnerability of river communities to floods". Many dams have clogged up with silt far faster than expected. Every year an extra 1 per cent of the world's reservoir capacity is taken up with silt. In some of the worst cases, the reservoirs become useless in less than 30 years. Even the claim that the hydroelectric power from dams is pollution-free "green" electricity has been undermined. It seems that in many cases, the rotting vegetation in their still waters is adding to atmospheric concentrations of methane, a greenhouse gas 20 times more potent than carbon dioxide. The commission concludes that somewhere between 1 and 28 per cent of all artificial greenhouse-gas emissions worldwide could be bubbling out of reservoirs.

It is not all bad news. Dams irrigate up to a sixth of the world's food production. And they generate about a fifth of our electricity. The problem is often as much about who gains and who loses. When the High Aswan flooded, it drove 100,000 people from their homes; many remain refugees. Across Africa, rural farmers have lost the free irrigation service provided by the annual flood of rivers because the water is kept back to supply electricity for city-dwellers. In North America, farmers get cheap water supplies for growing surplus crops, while the barricaded rivers become useless for salmon.

Water disputes abound, as when Turkey dammed the headwaters of the Euphrates river, leaving intakes empty downstream in Syria and Iraq. And freshwater ecology usually suffers. Across the US, "dam construction is one of the major causes of freshwater species extinctions". Fish passes rarely work effectively. Large dams can even give fish "the bends" when water in spillways becomes supersaturated with oxygen. On the heavily dammed River Colorado - home of the Hoover Dam - "5-14 per cent of the adult salmon are killed at each of the eight large dams they pass while swimming up river". In Russia, dams have stopped sturgeon migrations from the Caspian Sea. In West Africa, the Manantali Dam on the River Senegal caused fish catches on the river to collapse. And by stopping the flow of silt into the sea, dams have caused coastal erosion. The Nile delta is retreating by a kilometre every four years, coastal lagoons are being washed away along the West African coast. The Camargue, the French wetland at the mouth of the Rhone famous for its flamingos, is disappearing because of dams upstream.

This report is important because of its authorship. It is also the most compelling analysis available of the up and down sides of large dams. Governments and aid agencies cannot ignore the growing body of evidence. Hopefully, even the World Bank's financial controllers will read it.

Fred Pearce is a freelance writer on the environment and author of The Dammed .