Even though I didnt like the physX approach, thinking about the ball you said pushed my mind in the direction to the solution to my rivers!
And analyzing how that would be a problem because it would fall into the vallys for the lakes gave me a great idea for the lakes and borders between lakes and rivers!
So even though I am not going to use PhysX, thanks alot for pushing me in the right direction!

Even when discarding ideas, having someone else take a look at problems from the outside always brings a fresh point of view when I am stuck! Never discard a concept you dont like before considering ramifications of that idea!