This interactive site shows how a healthy free-flowing river looks. It also illustrates how a well-run and poorly run hydroelectric ecosystem should look. You can also remove the dam. Personally, I do not believe there is ever a well-run dam. There will always be an environmental impact no matter what. The construction of the dam already impacted the ecosystem before it was even put in place. The geologic, historical, environmental, biological, and cultural impacts can never be replaced once the dam is in place.
aFish are impacted immensely by dams due to changes involving their migration, temperature, and habitat to name a few. However, all wildlife is impacted by dam construction. Habitats change or are taken away endangering lives and possibly causing species extinctions.