The Hair-Trigger Kid by Max Brand

The Kid, when he got to the bottom of the long lariat, still found that his feet dangled well above either water or ground. He looked down, but all that he could see was the white dashing of the water--not white, really, but a dusky gray in that half light. He could not tell whether the water ran directly beneath him or if there were a small ledge of rock at the side of the canyon bed.

Hanging by one strong hand, with the other, he took out a match and scratched it. It was only a single spluttering of dim light before a dash of spray put it out, but that glimpse was enough to reveal to the Kid a raging inferno of waters. And, beneath him, a narrow, slippery ledge of rock, hardly a single foot wide.