Around here, they keep talking about removing dams from the Brandywine River to allow analogous fish access to traditional spawning waters. One of the biggest problems they encounter is the release of chemicals from sediments that are up to hundreds of years old. Mercury from paper mills, PCBs from electricity suppliers etc. These would all be released into the ecosystem if the dams were removed.
Certainly makes one wonder why those companies are not required to provide clean up. I know in some cases, the companies no longer exist. That should be where the EPA comes into play.