Post Carbon Fellow Sandra Postel was quoted in this article on the damage being doing to river ecosystems by dams.
From the article:
The biggest challenge is not the science of evaluating what to save, but generating the political will needed to maintain the protections.
When rivers are dammed, we must insist on naturalistic flows to support the basic ecosystem functions of dammed rivers. So-called "environmental flows" are planned releases intended to support the basic ecosystem functions of dammed rivers. Such systems can be complex to devise and maintain, and many dams around the world currently lack the mechanisms needed to control water discharge. Therefore, as with medicine, the best approach is to "first, do no harm" - no dams unless there are no better options (and there almost always are).
But once a dam is inevitable, it is imperative that the river be maintained with as natural a flow as possible. As water expert Sandra Postel has written, "an ethic of stewardship toward fresh water and its dependent species requires that we err on the side of allocating too much water to ecosystems rather than too little."