Actually, it was Arwen, and she only summoned the flood in the movie. In the books, it was Glorfindel, although i believe Gandalf did say it was his addition of the horses, but that just goes back to simply effects and nothing major, he didn't summon the flood, he just made it look pretty. Actually on second thought It may have been Elrond completely who summoned the flood at the Ford of Bruinen....not sure if Glorfindel just provided a distraction to keep the Nazgul in the center of the river....been a while since I read them...