Hold Back the River
Arrangement for Sops:

Bar 50 to most of 52: Hold back the River let me look in your eyes: Sops join the Altos (and this is the original Sop part an octave down)

Bar 52 to most of 58: Hold back the river so I... to hold back: Sops move up a third(?) from the Altos to sing parallel with them, starting on A. Easy to find.

Bar 58 to most of 60: Hold back the River let me look in your eyes: Sops jump back down join the Altos again (and Tenors are there too).

Bar 60 to to 66: Hold back the river so I... to hold back: Sops move up to start on A again - same as above and carry on up there to the end of the section.