I vaguely remember reading a story, I think it was Louis L'Amour, where the characters rode thru a creek (maybe even a waterfall) to a hidden spot.
There was a cave that you could ride thru, but you had to turn at just the right moment or you would fall to your death.
I think that this may have be a Milo Talon story?
My memory just isn't helping me enough to recall it exactly.

Thanks