Now he leaves the hillock and goes down its side toward a low valley in which runs a brook.

Black Rifle is thirsty and will drink deep." "That you can't possibly know, Tayoga." "But I do know it, Red Coat." "You don't even know a brook is near." "I know it, because I have seen it.

My eyes are trained to the forest, and I caught the gleam of running water through the leaves to the west.

Running water, of course, means a brook.

Black Rifle's trail now leads toward it, and I assume that he was thirsty because he had just eaten well.