http://5newsonline.com/2015/05/03/piranha-caught-in-lake-bentonville-woman-says/Army Corps Engineer Alan Bland said they find piranhas in local lakes often. He said people will dump their aquariums when they are tired of taking care of the fish.
Maybe they won't attack swimmers because of the cold but I think they might be wrong about piranhas not being able to survive in colder water for very long. There have been tests done and seems they can survive down to 50 degree water, albeit not well. If a lake is spring fed then that would provide warm enough water for them to survive in.
Wish people would stop dumping their aquariums. If you are tired of them give them to someone or kill them.