Have you planned the campaign setting already?
I ask because I've got a campaign setting that could do with some play testing (the link is in the signature). It's a low-fantasy setting, magic being present but rare rather than all-pervasive. I've done maps and outlines of the regions on my website. The world may fit what you're looking for in terms of terrain. I don't have the huge river, but I do have two continents separated by a sea.