For the river can you use chocolate fudge sauce? If you think that will be too sweet, the Wilton Piping gel works great for rivers. Just color it brown.
Good luck and I can't wait to see the pic.

I'd thought about using chocolate pudding but fudge sauce might even be better. The stickiness of it would make it more stable than runny pudding, I think. I'll have to experiment with both and see.

Well, here it is...

It didn't turn out as I'd imagined but it looked ok and I learned a lot in the process namely, never to use pudding on top of fondant! The cake looked great before I decided to use pudding for the chocolate river. It congealed into chunks overnight (weird!) and the liquid dripped down the sides of the cake. It looked so bad. I fixed it up as best I could but it looked pretty gross! Oh well, at least it tasted good!