Humanx Commonwealth #11: Voyage to the City of the Dead by Alan Dean Foster

The planet Horseye had the most spectacular river chasm in the whole Humanx Commonwealth, and Lyra and Etienne Redowl intended to explore its length. The Mai, traders from the river delta, were prepared to help, but had their own agenda for doing so. For it was rumored that at the head of the river was the City of the Dead and a great treasure.