This is one of the most amazing videos I have seen in a while. It is about how wolves have changed the physical geography of Yellowstone national park in America where they re-introduced wolves in 1995 after 70 years. The wolves changed the behaviour of the deer that moved out of the river valleys which allowed the trees and bushes to grow again. This brought back other species of birds and mammals. This is called a Trophic Cascade.
But this is the interesting part… you will have to watch the video to find out what happens. How wolves change rivers