Norway is gorgeous. It has the two main ingredients you need for white water: lots of rain and big mountains. But, a big problem is that there are so few people there that there are few good roads and getting information may be sketchy. I think the most action will be a few hours north of Oslo and closer to Bergen, so you are a little out of the way in Kristiansand. There is a big river nearby, so you may be in luck.
The ocean towns just west of Kristiansand are spectacular. Checking them out may be higher priority than finding white water. Sea kayaking may be something to try.
Here are two good links for white water:
Who would have thought you'd get Norway info on the buzz?