Very nice for your first try!

I am just learning GIMP from Robs tut also. Though I have not tried it yet, you may be able to use the same technique used to create the "Dirt" layer to selectivly show/hide parts of your river flowing through the forest by creating "clearnings". Again, I have not tried it yet, but you may want to give the approach a try.

Also, some of the mountains have a hard line transition to another land type. I have found that selecting the layer mask on the mountain layer(the right most icon on that row) and running pixel spread and a 'tiny" bit of blur make the areas merge better. Some people like this technique, some don't but it's there if you want to give it a try. Note that it will make the border somewhat "transparent", so play with it if you want.

Joe