"The biggest problem associated with small hydropower concerns their sheer numbers," says Thiago Couto, an aquatic ecologist and postdoctoral associate at the University of Miami. He has studied the effects of small hydropower plants around the world, including in countries such as Brazil. "Today, for every large hydropower dam operating there are other 11 "small" ones, and their numbers are expected to continue rising into the future. The cumulative impact of all these small dams on hydrology, fish migration and water quality is an issue of high concern, especially in rivers that accommodate multiple dams."