I'm more than a day late in getting back into this; and I'm not at all sure that I should get back into this; but my recollection from the books is the following:
In the Third Age, Elrond was renowned more for wisdom than for much else; but, in addition to wisdom, he also had a smidgen of "foresight" power (or "precognition," or "foreknowledge," or "second sight"). [And in one of the movies, Arwen complains to him that, "You have the power of foresight. You have foreseen my son. Why didn't you tell me?"]
That being so, he had plenty of time to unleash the flash flood at the ford of the Bruinen ahead of time to insure that it arrived downstream from Rivendell at the ford at exactly the right moment. He didn't have to delegate that to the rider carrying Frodo, because he had plenty of time to arrange it before Frodo got carried across the river. (And he did it from the comfort of home, from Rivendell, with all of the help he had available there.)
(Or am I misremembering this?)