Marya Zimmet

Marya Zimmet, who played Robert Duvall's daughter Rosalie, is shown on IMDB as having just this one movie to her credit. However, she also played the role of Robin Clemens on the soap opera "The Secret Storm" in the 1968-69 season (the same season when Joan Crawford briefly stepped in for Christina Crawford, as recounted in Mommie Dearest).

(EDIT: I made the correction to Zimmet's page, but since IMDB only lists 5 episodes for that 1968-69 season, those are the only episodes for which she can be credited. It's likely that this was a season-long role.)

Zimmet is now a jazz singer and while her own website makes no mention of The Rain People, I found another page where she did acknowledge that credit.

http://maryazimmet.com/

http://metropolitanroom.com/event.cfm?id=70924

http://www.angelfire.com/80s/cultsoaps/sst60s.html