The short version: what is the cleverest riddle whose answer is "river" or "a river"?
I'm preparing to run the D&D module "White Plume Mountain". At one point in that adventure there's an encounter with a sphinx who asks each character a riddle. Unbeknownst to the party, the answer ("river") is also the password to disarm a nearby magical trap, so that the trap does not attack that character. So the sphinx needs to get each character to say the word "river" before they continue.
Using the scenario as written, the sphinx would ask this riddle to each character in turn:
I have a mouth but never speak. I have a bed but never sleep. I run smoother than any rhyme. I love to fall but cannot climb.
But this is silly. What self-respecting sphinx would ask the same riddle five times in a row?
So: what's a clever riddle whose answer is "river"?
Difficulty of the riddle is not super important since the adventurers will know from context that the answer is probably "river". It's better if the riddles avoid anachronisms since this is a fantasy setting.