Interesting project!
Though, as a geographer, I can't let this uncommented:
There are no such things as "natural borders" (even if you call them barriers)!
For example, let's have a look at the Mississippi and other rivers in the MM-system: You used them as borders, not only between counties, even between states! Why?
Think about it: Do the left abutters of the river have more in common with the right abutters of the same river, or with some guys that live a two hours drive away from the river? Wouldn't projects like flood protections, harbour areas and the like be easier if there was only one authority in charge and not two competing and disputing ones?
Therefore, wouldn't watersheds make better borders?
Sorry about the rant, but the term "natural border" just annoys me.
I know, it's and old problem and still used in contemporary geopolitical work, but it's mostly crap. Rivers (and to a lesser extent, mountain ranges) may have some use as an international border if you aren't really at peace with your neighbour on the other side, but once inside a county, it turns into a problem.