The Waterborn by Greg Keyes

In this world, the River, a jealous and powerful god, runs in the blood of the nobility of the city of Nhol, including in that of a bright and independent princess, Hezhi. Ten years old, Hezhi wishes to understand the world she was born into and why her cousin and best friend, Den, was taken away to the River by the priests.

Meanwhile, Perkar, a young warrior, has fallen in love with a Stream Goddess who fears being eaten by the River. Wishing to save his beloved, Perkar vows to kill the River. In interweaving story lines, both protagonists make mistakes but unravel mysteries and, aided by many strong secondary characters, come of age.

It was a good entertaining book.