Well the short answer is No, not all rafts are made for rivers. I am no expert, not even close, you might want to checkout this site:
and ask questions there. That site is kind of geared toward kayaking, but lots of rafters are on there too and I'm sure you can get a ton of help.
DO NOT buy one of those Sevylor (spelling?) rafts and put it in a river, or any walmart, sports authority, or Dick's sporting goods rafts, that would be asking for trouble. There are numerous brands that you could stick to, like Aire (again not sure of spelling) and Hyside come to mind for instance.
This is something I would not skimp on personally, get a quality raft.