Awesome! You absolutely have to keep me posted on that project! I really want a program that can do this sort of thing… What platform would your program run on?

As for the sea streams - well to be completely honest this world isn't a planet. The shape is that of a cone and this continent is a little bit, well, big - therefore this giant stream running from top to bottom is a bit hard to do… I think it's best if I show you, it's always so hard to explain:
Click image for larger version. 

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Ok so this is the general shape of my world and very roughly the size of the continent in comparison. The other continents are a lot, lot smaller this one is special. The balls are temperature-suns and sit half-way inside the hollow cone. Anyway, you see why a stream that strong from top to bottom would pose a problem?
So the part about erosion, well to be honest the whole shape problem, is really up to me. I could just go and say "the gods wanted it like this!" but that's not the way I build my world. It does not run on normal physics or laws of nature or anything, but it does have internal logic and its own physics and laws - and it just so happens that this equivalent to water I have on my world works similar to real water.
So. yeah.
What was I actually talking about?