In the ancient world, waterfalls, like fountains, were often sacred places where humans could feel the presence of the deity. Hence, they were often shaped by men through their devotion and by means of care for the grounds and the site. We see this behavior to this day at such sites as Niagra Falls even where there are no overt religous connections.
Similar to fountains, waterfalls were originally very natural in design, but today urbanization has also impacted their design. Today, waterfall design can range from attempts to imitate nature to geometric abstractions of the essence of a waterfall with many variations in between. Some of the worst attempts can be the use of prefabricated faux stone in a natural manner. The spirit of the place or genius loci is murdered by such attempts. Always, use materials honestly and design at a scale and in a style which is appropriate for the site.