Do NOT, I reapeat NOT, put them in the river! That will do one of two things,
1. The fish, balas, will be killed by the local fish,
2. The balas will destroy the natural eycosystem. They will eat any small fish, etc. They may even breed, though that is a long shot, and the water ways will be taken over with the balas.
I believe you should go on a rescue mission and take the fish to a trusty lfs, and tell the parents that they have jumped out do to over stocking and bad water qualities, because some fish, such as killies, will jump out of tanks with water that they do not like.