Ancients of days - Paul McAuley

Confluence is riven by a civil war fired by the heresies of the last humans, the Ancients of Days. And the Great River, lifeblood of the inhabited part of the world, is failing.
Yama was found as a baby on the breast of a dead woman in a boat on the Great River. He has two ambitions: to fight against the heretics, and to solve the mystery of his birth.
Yama has the ability to control the machines which maintain the fabric of the world. As he journeys, so his powers increase, but his understanding of them is still limited and he finds himself becoming the unwilling focus of a dispute in the ancient and gigantic Palace of the Memory of the People.