Nearly 500 dams in the U.S. have been taken down over the past 15 years, opening up fish runs and flushing out rivers. Campaigns to remove large-scale dams in the West garner most of the media attention, but Steve Higgs of American Rivers says more progress is being made knocking down smaller dams in the East that are either defunct or no longer generate a significant amount of electricity. As a consequence, fish are returning to stretches of rivers that they hadn’t visited in scores of years. However, lest you get too excited about this good news, consider this: In some cases, taking out the old dams can destroy wetland habitat used by migrating birds and send tons of polluted sediment rushing downstream. Bummer.