Patricia A. McKillip 
Moon-Flash [EPUB ebook] 

Kyreol’s small world begins at the Face, a high rock cliff, and ends at Fourteen Falls, a series of rapids. Each year, her people celebrate Moon-Flash-a spark of light that seems to come from and go into the moon, a symbol of life and joy. When a mysterious stranger arrives, Kyreol wants to know more about him, as well as the Moon-Flash, and soon she and her childhood friend Terje leave their home to look for answers. Those answers will pluck Kyreol from Riverworld and transform her life forever-by fast-forwarding her into a future she can barely comprehend.