It's interesting to note, if you look VERY closely at the valleys in this video and others previously posted, that the natural rivers are easily visible, even if there's no water there. It was one of the first things I noticed when I started testing Outerra. You can call up the Google maps, and literally follow all the tributaries etc.
To me, that alone speaks an amazing volume of conversation to the Outerra engine. You can see this stuff, just itching to be included. Forget the clouds and environments ... to see the Mississippi or Colorado rivers flowing, despite them not currently being in the engine, is amazing. They are there.