 When Arwen escapes from the Black Riders through the river by  flooding them, the spell she speaks isnt subtitled. According to  the Encyclopedy of Arda (see External Links: Miscellaneous # 58),  she says: "Nîn o Chithaeglir lasto beth daer; rimmo nín Bruinen dan  in Ulaer", which means roughly "Waters of the Hithaeglir, hear the  word of power, rush, waters of Bruinen, against the Ringwraiths"