Don't allow the water to flow under the rock by cementing the crevices. Create a cave behind the falls to echo the sound of the splash. Line the walls of the cave with rock. Stack rock behind the falls to hide the liner and create a natural appearance. Watch the splash.
Water escapes the pond via splashing quite easily.
Simply moving a rock can often correct the splashing. Formal Statuary Fountains Cement or bronze fountains add elegance to a pond as long as it's not overdone. Most statuary fountains are not designed for the amount of water flow coming from this type of system so the main water return must be split and only a portion of the water sent to the fountain via a reduced water line. Since the plumbing on this system has been run outside the pond for filtration reasons it makes it more difficult to install one of these types of fountains. It can be done with a little planning by running the return line back to the pond and creating a manifold for the fountain.