Ken,
I used silicone latex caulk - the kitchen/bath type - not the paintable type that starts out white and turns clear.
Cut some clear plastic bag material to the size and shape for the waterfall.
Apply parallel beads of caulk the length of the plastic.
Holding three toothpicks or other small pointed pieces of wood together, spread the beads so they merge into a single stream.
Use a whipping motion to add "froth" to the stream. The more you work it, the whiter and less transparent it will become.
When the caulk is set, attach the completed falls to the cliff using more caulk at top and bottom. Use straight pins to tack it in place until the caulk sets.
Add some polyfiber material at the bottom and wherever else splashing will occur to represent spray.