The sentence that best supports the inference that the rapids of Sixty Mile River are unpredictable and dangerous but passable is:
"Round and round they dashed for some four hours, and then, by a peculiar freak of the waters, were whirled on down the canyon, finally escaping with no damage done save to their nerves."
This sentence indicates that although the experience was chaotic and unpredictable, the travelers ultimately managed to pass through the rapids without physical harm, highlighting both the danger and the passability of the rapids.