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Here is the relevant portion of Tolkien's text:

"Who made the flood?" asked Frodo.
"Elrond commanded it," answered Gandalf. "The river of this valley is under his power, and it will rise in anger when he has great need to bar the Ford. As soon as the captain of the Ringwraiths rode into the water the flood was released. If I may say so, I added a few touches of my own: you may not have noticed, but some of the waves took the form of great white horses with shining white riders; and there were many rolling and grinding boulders. For a moment I was afraid that we had let loose too fierce a wrath, and the flood would get out of hand and wash you all away. There is great vigour in the waters that come down from the snows of the Misty Mountains."

But, if you are such a Tolkien scholar, you'll also remember that Gandalf was a maia (Angel like being) that was sent to middle earth to oppose Sauron. You'll also remember from the Silmarillon that the Elves were created very much like the Valar and the Maia. Men had no power of their own and lusted for it. That is where the rings came in and where the sad commentary of the story goes.

Thanks, excellent!

So I was right that it wasn't Glorfindel/Legolas/Arwen who caused the river to rise, and that Gandalf did the white horses; I got my other idea from remembering "it will rise in anger." But Restorer was right that Elrond was ultimately behind it.

Dan