Damming evidence calls for planning.

• Position: Hydropower - Social and environmental impacts of dams

Despite the adverse impacts of large dam construction on ecosystems and human settlements, 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. However, world societies and governments would do well to evaluate the consequences of such construction as an integral part of the planning process, advises California Institute of Technology, Pasadena, anthropologist Thayer Scudder, author of The Future of Large Dams.

According to Scudder, huge dams are a "flawed yet still necessary development option." Flaws include the shortcomings of the dam itself as well as ecologic and social impacts, In terms of the former, Scudder says that dams, on average, can be expected to get clogged with sediment at a rate of about 0.5 to one percent per year. 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. That's the dilemma."

Somewhere between 40-80,000,000 people forcibly have been relocated by the flooding of the land on which they live to create the reservoirs above dams. Even larger numbers 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 the sort things you might not normally...