Answer: Kohaku River
When Haku and Chihiro are flying through the sky, Chihiro remembers back to when she first met Haku. She tells Haku her story of how she dropped her shoe into the river and the water carried her to shore. When Haku changes to his person form, he says, "You did it Chihiro! I remember, I was the spirit of the Kohaku River. My name is the Kohaku River. I remember you..."