I first read it about ten years ago. The basic concept is that the course of human history is controlled by flow of time, which is represented as a higher plane that takes the form of a river that flows from the past to the future.
Humans have the ability to ascend to this plane, where they assume the form of serpent or fish-like beings that can influence the course of human events by splashing around in the waters and occasionally fighting each other at various points along the stream.
Throughout history, various cultures develop to the point where they can ascend to the higher plane, where they naturally seek to advance the interests of their own culture, and do battle with their rivals. Various hijinks, bootstrap, and grandfather paradoxes ensue.