It's nearly the end of October. The cold autumn nights have brought a thin crust of ice to a beaver's pond. Unlike in the river valleys below, up here, there are not many tall cottonwood trees, so this beaver has built his dam from the shoots of young willows sprouting all along the side of his pond.

 

But he is something of a novelty. Even by the time Yellowstone was made a national park, beavers had been virtually hunted to extinction by fur tappers. They only began to reappear here in 1995, the year the wolves came back.

 

Now wolves are chasing elk again. Elk have had less time to eat willows, so willows are sprouting everywhere. Today, as winter approaches, all over Yellowstone, beavers are using those willows to put the finishing touches to a dam-building renaissance