If I were to suggest anything: first I'd taper the rivers instead of them being the same width all the way to the end. Minor thing, happens all the time. Second, might throw in some other colors like rust, orange, dark brown - everything's green, light green, or tan. But those colors have to be tied to the terrain and since the terrain isn't all that defined then it's hard to place the colors correctly. This is also a minor thing and, for this style, probably useless so you can ignore it Third, the mountains are all the same - they look more like dunes or hills so I'd definitely get some more definition into the mountains. This is the only thing that I think that needs attention. So you can throw out the first and second things, if you want, as they are more pointers than anything but the mountains need a bit o work to bring them out more. All in all, as I said earlier, it looks really nice and easy on the eyes.