Down go the dams - Many dams are being torn down these days, allowing rivers and the ecosystem they support to rebound. But ecological risks abound as well. Can they be averted?

Some 800,000 dams exist around the world, but small ones are increasingly being removed so rivers and streams recover. Ecologists are learning, however, that removing or lowering dams take a great deal of careful planning and active intervention because sometimes the dams confer environmental benefits, such as holding back toxic sediments or blocking the progress of invasive species. Before decommissioning a dam on Fossil Creek in Arizona, managers poisoned exotic fish and airlifted native species to safety. Such strategies could prove key to the success of future dam removal projects.