Goldfish are members of the cyprinidae family, which means they are cousins of carp and minnow. They are accustomed to low-oxygen environments like the bottom of a pond, so they should have no problem surviving in a river, as rivers are often much more oxygen rich. The only question is if your species can tolerate near-freezing temperatures in winter. Most goldfish can, but I think there are some tropical varieties that would struggle.
But yeah, as the Guardian says, it's probably a bad idea for other reasons.
