Just read what I wrote before, there's quite a bit of explanation in there. About painted shaders and such. The water is actually quite straightforward, a little extra displacement and foam here and there (thru extra surface layers, attached to water shader-lake), blended by some more painted shaders. The underground (riverbed up the fall) is the same painted shader as the waterfall itself, just a little lower level. Feed this layer into a lake as well, and dump the water shader from this second lake.
Getting the painted shaders just right is the hardest; get the brush size right, paint carefully after pausing a 100% preview (I changed the POV temporarily, for a better overview of where to paint), it's not a perfect (PS-like) way of painting, but it works. The water was falling on some ground at the right side, so I made another painted shader and displaced the ground down some 10 meters at that particular spot, so there would be lake there instead of ground. If you chosse the right spot for a waterfall that kind of straightening out little mistakes would perhaps not be neccessary.
--- Dune
