Our local opera company performed "Samson and Delilah" last year, and the only piece I was familiar with (besides the ever-popular Bacchanale) going into seeing the performance was Delilah's aria "Mon coeur s'ouvre a ta voix" (which is a famous mezzo soprano aria), but only because I played French horn in a chorus/orchestra job which featured opera arias and choruses. I'm not sure if this is the "waterfall" music that you referred to but it is indeed a very beautiful aria. Here are some examples from YouTube:
Jessye Norman - Samson and Delilah
1994 concert performance at Avery Fisher Hall, Orchestra of St. Luke's
Placido Domingo Olga Borodina 'Mon coeur s'ouvre a ta voix'
2002 La Scala production (the beginning is cut off, unfortunately)
Denyce Graves - Samson et Dalila
1992 concert performance with Placido Domingo, backed by the Orchestra of the Teatro Municipal de Rio de Janeiro
All three are different, but have that full-throated mezzo sound that I like (Denyce Graves having the lighter sound of all three).
Liz