I'd suggest, so far as the rivers go, that you do them in Wilbur when you make your height map. They tend to turn out rather nicely there, and following that portion of Arsheesh's tut would go a long way to tying all the water features together.
It kinda looks like you may have mixed and matched a couple tutorials to make this map, which is fine, but it does leave you with some rather bright portions of the map.
All in all though, it is a good effort and I look forward to seeing more of your work in the future!