Bob at Bear-ytales.net, the original unofficial Knott’s Berry Farm website, has an extensive photo restrospective of the 1991-1992 construction/installation of the Disneyland Fantasmic! show, which is the original edition of the show. That’s because Bob, who runs the site, is also a longtime Disneyland fan.
The construction involved extensive changes to the southern tip of Tom Sawyer Island, and some changes to the mainland as well. Further construction on the mainland for crowd control purposes came later. For the first season, we could sit on grass-covered gentle slopes to a nice, comfortable view of the show.
Also, in more recent years, further construction on the Rivers of America gave the once clay-and-dirt waterway a concrete lip for further erosion control.