'Dams damage environment'

The proposed Ilisu dam in Turkey has come under fire

A major investigation into the impact of dams is reported to conclude that they cause ecological damage and exacerbate flooding, and that many deliver less than half the amount of water expected.

The report by the World Commission on Dams (WCD), to be published on Thursday, alleges that "few dams have ever been looked at to see if the benefits outweigh the costs", according to the UK's New Scientist magazine.

Those costs include social upheaval, increased flooding, damage to farmland and the extinction of freshwater fish species, the weekly said.

Author Arundhati Roy has campaigned against dams

On the positive side, the WCD study reportedly concludes that dams irrigate fields which provide about one-sixth of world food production, and also help power many homes and factories.

But there is a sting in the tail of even those positive comments - the benefits of dams go more often to urban and wealthy people than to poor rural ones.

World Bank backing

The World Bank is one of the prime sponsors of the study. It is also among the leading funders of dams worldwide.

Award-winning author Arundhati Roy, who has campaigned against the planned Narmada dam in India, welcomed the report.

But she also expressed concern that governments would dismiss it if its findings did not suit them.

The report is due to be published at 1300 GMT.