Somebody call the River Police, we've got a situation 10-33 here.
Apart from the river violations the only suggestion I have is to put some drop shadows on those forests to make them stand out. A bit more bump-mapping in general would do some good as well.
As for your fluff: nanotechnology is not magic. It could make for more interesting plot possibilities if there was some sort of facility at the center that maintained the temperature and the food chain, especially if you're going with a medieval tech level and having the people who live there now be ignorant of how the science actually works (except for the people who run the facility, of course). That could open up the possibility of the nanotech having become obsolete but continuing to work, making the edges of the area dangerous (dangerous creatures mutated by malfunctioning nanotech, a thin film of gray goo on the ice that slowly eats away at anything it touches, etc.) But it's your campaign. Do with these suggestions as you will.