A few improvements

I realized that my game code contained an unnecessary for-loop, by removing it I was able to add:

1. An increased play-area, the area went from 126 tiles to 224 tiles (a 78% increase!)
2. Acceleration, the river now picks up speed as the game progresses.

Yeah, that's pretty much it...
The changes are pretty small, but for such a tiny game, I think they add a lot.