I think it was
@addy1 who used a preformed as an "upper pond", drilled 2 holes in the side near the top and attached PVC for a waterfall outlet spout, then had it dumping into a "lower pond" and hid the entire thing with rocks piled to give a waterfall cascade. You just have to situate the Upper so it is higher up, and I would also use a piece of liner behind the camouflaging rocks so any drips would just slide down into the lower preform.
That being said, i'm terrified of punching holes in pondy things, be it a preform or a rubber liner, mostly because i've never done it before and don't want to mess up >.< Good luck!