Ah, the whoel dam argument.....an ocean liner isn't natural....it wouldn't arise out of nothing, while a dam across a river (constructed of trees) could be considered natural, since natural dams are caused all the time by downed trees and floods. So you could say the beaver is just mimiking a nature.

Plus, beaver dams create habitat and species diversity, while all humans (usually) do is cause monolithic species communities with little species diversity.

(and as a bologist, wouldn't you think that diversity is good? Heck, my example of the Rocky Mnt forests is a perfect example why monolithic species communities is bad)