Which type of border shows the division between sonora and chihuahua? national border state border physical boundary natural boundary

I believe that the border between Sonora and Chihuahua is a natural boundary.
As far as I can tell, what divides these two locations is a desert, which is obviously a naturally-created boundary. It isn't a national border because it doesn't separate two countries, it isn't a state border because it doesn't separate two states, and it isn't a physical boundary because it isn't man-made.