Here is a blurb for the book:
Waterfall is the first in the River of Time trilogy by Lisa T. Bergren. The Bentarrini girls have spent every summer of their lives in Italy with their archeologist parents, but this summer is most definitely different. In a bizarre moment, the girls catapult through time and end up in medieval Italy, in the midst of a battle between knights. And after that, every day becomes a battle to get home...and becomes a terrible choice about if they want to get home at all. Action, romance, and heroes and heroines every reader will fall for, this is teen fiction at its best.