Someone's confused here and I don't think it's me . . . I didn't mention rivers specifically . . . my advice would be to make the overall map smaller to begin with by eliminating a good deal of the barren green space and also add a greater variety of topography i.e. forest, rivers, hills, and mountains where they obviously should be. This will enhance the visual quality of the world at large, add terrain variety for what it's worth in terms of battlefields and movement speeds, and lead to more frequent encounters among combatants.