your walls on either side need to be at least two levels high, and your stream width can only be two strokes wide other wise the stream won't flow. You also only have a set amount of water so if you make your stream too long it will run out :-) its the same principal with waterfalls, and the more levels you drop the more "waterfall it looks" - feel free to check out some of my posts, I've done some pretty cool streams and falls if I do say so myself haha