Wolves vanished from Yellowstone by 1926. Deer exploded, grazed the valleys bare, and stripped the young trees.
In 1995, wolves were reintroduced — just a handful. They killed some deer. That wasn't the point. The deer moved. They stopped loitering in open valleys and gorges where they could be caught.
Those places regenerated: aspen, willow and cottonwood grew back fast. Birds returned. Beavers came for the willow, and their dams made pools for otters, ducks and fish. Bears returned for the berries. Ravens and eagles fed on wolf leftovers.
And the rivers changed too. Regenerated forests stabilized the banks. Erosion dropped. Channels narrowed. The rivers physically changed course — because someone added a predator.
Scientists debate how much the wolves alone caused. But the core cascade is real: a few wolves, and an entire landscape rearranged.