I think it's an optical illusion caused by the forest texture, LW. I just spent a while staring at it, and it really does kinda look like there's a drop shadow outside the rivers in places, but if you look a long time you can tell that there really isn't, it's just the forest texture. I agree with a poster above that the whole issue could be fixed by breaking the forest around the rivers in places (and breaking the river in the rest because it's under the trees). Your map though