Figure One: Grand Coulee Dam; Washington, U.S.A.
The most pervasive environmental impacts of a mega dam occur during the decades the mega dam is in use. These impacts do not just remain on-site, rather, the river is impacted upstream and downstream, and waterways are affected on a global scale. In addition, active dams have the potential to fail, which has dire consequences on the surrounding environment.
The Aswan High Dam of Aswan, Egypt illustrates many of the issues caused by an active dam.
I think there are many issues to consider with the use of dams. Although dams do affect waterways, there are many positives, including using them as a source of power.