I recently came across a tutorial for making a fake time-exposed shot of a river...maybe it might help you... Basically, create a layer that only cover the reigon where you want the waterfall. Filter>Clouds in that region, than filter noise, than filter motion blur with a 90 degree rotation (so it looks like its going up and down)then mess around with luquifying, perspective changing, etc to get the effect you want. Use the dodge tool to make highlights where froth should be...just something to try ;)