Are freshwater snails in rivers

i don't know but its happened in my tank but the story is that i had bought twelve tetras a bit before but i already had one of the same type in my tank and the baby gold Inca snails were born and twelve tetras were dead and one was alive(i think it ws my old one) and ive heard someone on Yahoo answers said it happened to them and a answer to that question said that they can bring in diseases but ins unlikeley that the snails killed them but have heard of it happening before so i guess it could happen-hope i helped