This is a very interesting tale of two scientists who take a boat journey up a giant river in an interestingly populated world. There are actually three sentient species in the world, and they all occupy different ecological niches. ADF crams in a lot of culture and world building in to a rather small book, and at times the novel can read like an ethnographical study. I didn't mind this at all, as I found the species quite interesting. The main tale itself if pretty good, and the ending comes out of nowhere, catching you quite off guard.I believe the book is out of print, but is well worth picking up if you are an Alan Dean Foster fan. I've read it 3-4 times over the last 25 years, and each time enjoyed it a little more. I felt that the book was a set up for a sequel, but if it was, I never found the sequel or he never wrote it. Anyway, very fun, and worth seeking out.