Looking for a fantasy novel clearly inspired by "Heart of Darkness"

Hello, everybody;

I'm trying to find a book I read sometime over a decade ago and haven't been able to find since. I do not remember the novel's author nor its title, but I remember its basic plot beats because they were essentially all the same as Joseph Conrad's "Heart of Darkness."

The book featured a boat going up a river, perhaps the Mississippi, in a fantasy version of North America during the colonial period. The crew of the ship is made up of the human and dwarf employees of a particular trade company who wield firearms the spit hellfire, and whose ammunition is the soul of the wielder, burned away an inch at a time with every shot. The protagonist is a dwarf employee who I think worked as a cleric, and is noted for his refusal to use one of the guns and whose grandmother practiced "the old ways" of a pre-modern magic. The natives, encountered as the boat party gleefully opens fire on a herd of bison, are elves of a savage and primal sort, who are all but unseen until near the end of the book but present a constant menace to the company party.

As mentioned, I don't know the title or the author, but I really want to re-read this book to see if it holds up to my memories. Thank you very much in advance for your help in rediscovering this title.