"As a result of damming and development, major rivers worldwide have experienced dramatic changes in flow, reducing their natural ability to adjust to and absorb disturbances," said lead study author Margaret Palmer, director of the University of Maryland Center for Environmental Science Chesapeake Biological Laboratory. "Given expected changes in global climate and water needs, this could lead to serious problems for both ecosystems and people."

The researchers project river discharge under different climate and water withdrawal scenarios and combine that with data on the impact of dams on large river basins. The projections indicate every populated basin in the world will experience changes in river discharge -- some in which there won't be enough water to meet human needs.

The study also finds nearly 1 billion people live in areas likely to require action and approximately 365 million people live in basins almost certain to require action.

The findings appear in the online version of the journal Frontiers in Ecology and the Environment.