The decommissioning of aging dams in the United States—thousands of which now have expired design licenses—is proving a mixed blessing for the environment, says geomorphologist Martin W. Doyle of the University of Wisconsin. On the one hand, sediment that is released when a dam is removed can disrupt ecosystems downstream, gravely affecting salmon and other aquatic wildlife. Removing dams built in areas of industrial activity can also release accumulated toxic chemicals.
On the other hand, dam decommissioning converts reservoirs back to fast-flowing rivers, restoring river species to their natural habitats over the long run. Wisconsin has removed 50 dams since 1967, the most of any state, with good and bad results. Fish and invertebrates adapted to slow-moving water were replaced by river species within a year after the Woolen Mills Dam was torn down. But the removal of the Oak Street Dam exposed thick sediments that were overrun with invasive reed canary grass within two years.