I found it!
And looking down Tuor saw a great marvel, as it seemed to him; for a wild flood came up the narrows and strove with the river that would still press on, and a wave like a wall rose up almost to the cliff-top, crowned with foam-crests flying in the wind. Then the river was thrust back, and the incoming flood swept roaring up the channel, drowning it in deep water, and the rolling of the boulders was like thunder as it passed.