This is ace. Wolves were introduced back to Yellowstone National Park 70 years after they were last there. As a result the whole ecosystem was altered.
We often bang on in lessons about linking ideas together and how in real life there are so many knock on effects. Well have a look at this video and see for yourself
More wolves means less deer (especially in the valley floors where they are easier to hunt down).
Less deer means less grazing. Less grazing means vegetation can grow more.
More vegetation means less soil erosion. Less soil erosion means less material in the rivers.
But more vegetation on the valley floors also means more stable river banks. More stable river banks means straighter rivers in places.
The animals the vegetation the ecosystem and the rivers are all interlinked