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03/01/2005 08:00:00

Negative Impacts of Dam Construction on Human Populations Can Be Reduced, Author Says

PASADENA, Calif.--Despite the adverse impacts of large dam construction on ecosystems and human settlements, more and more dams are likely to be built in the 21st century wherever there is a need to store water for irrigated agriculture, urban water supplies, and power generation. But world societies and governments would do well to evaluate the consequences of dam construction as an integral part of the planning process, a leading authority writes in a new book.

The book, The Future of Large Dams, is the latest work by California Institute of Technology anthropologist Thayer Scudder, who is arguably the world's foremost expert on the impact of dam construction on human societies living along major world rivers. Published by Earthscan, the book argues that the early analysis by affected stakeholders of the impact of a dam's proposed construction is a worthwhile undertaking. And not only is it worthwhile, but also is quite possible to accomplish with established research techniques.

According to Scudder, large dams are a "flawed yet still necessary development option." Flaws include both the shortcomings of the dam itself as well as ecological and social impacts. In terms of the former, Scudder says that dams, on the average, can be expected to get clogged with sediment at a rate of about 0.5 to 1 percent per year. And in terms of the latter, changing habitat caused by the flooding of land behind and below dams is certain to change the habits of nearby humans and animals alike--if not devastate both.

"Although dams have their problems, they're unfortunately still necessary because of the growing needs of humans for water storage," says Scudder. "That's the dilemma."

Given that governments throughout the world-- the United States included--will continue to dam rivers, Scudder says it's important to take into consideration that hundreds of millions of people have been adversely affected by dams in the last century. Somewhere between 40 and 80 million people have been forcibly relocated by the flooding of the land on which they live to create the reservoirs above the dams. Furthermore, even larger numbers of people have had their lives and livelihoods disrupted by the change of the river flow below dams.

"Lots of people in many places in the world are dependent on the natural flow of rivers, and the consequences can be the sort of things you might not normally even take into account," he says. "For example, a settlement that depends on an annual flooding of agricultural land when the river rises can be wiped out if the regulated flow of the dam causes the annual flooding to cease."

Scudder, in fact, wrote his doctoral dissertation many years ago on such an instance, in which the construction of a dam obliterated the most productive component of an upstream farming system.

"But the book argues that, despite these adverse impacts, there are state-of-the-art ways of addressing them," he says. "For example, if local populations downstream have been depending on an annual inundation of an agricultural flood plain, then the authorities in charge and other stakeholders should consider a controlled release of water that recreates the flooding conditions. Experiments have been done with coordinating hydropower generation and flood recession irrigation needs with the release of 'environmental flows'--that is, releases of water to protect habitats and communities. This approach has been tried in several African countries, and research has shown in other cases that managed floods would be a 'win-win' option."

In general, the way to make dams work for humans everywhere, Scudder suggests, is to address the social and environmental impacts both downstream and upstream of any dam project before the structure is even built, and to evaluate the situations in river basins where dams have already been constructed.

Finally, the political and institutional consideration of dam construction should be addressed, Scudder says. Too often, a dam project is undertaken at a specific locale because of its political expedience, and this is not the best way to minimize the negative human and ecological impact. Restructuring governmental departments that oversee dams can also maximize negative environmental, agricultural, or other impacts.

"We should all be able to benefit from the dams that are to be built in the future rather than suffer from them," he concludes.

Review copies of the book are available from Earthscan Sales and Marketing Administrator Michael Fell by e-mailing him at or calling +44 (0)20 7121 3154.


Written by Robert Tindol