Yeah, some of these rivers freeze up a bit too. These gator/crocs may have some refuge in some power plant cooling water discharges, or perhaps they're migrating somehow. Conventional wisdom would say these creatures were pet releases, but who knows?

In my neighborhood pond, which catches run-off from the streets, I feel like I'm seeing a Pacu-like fish hitting feed. That would have to be introduced by a human in my opinion and could be very probable. I also have fresh water clams in this pond; how did they get in there??

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."