Split it. Have a kayaking forum and a rafting forum, and an option to view both combined, that way, everyone could view the way they want. I would not make a general forum, as that is vague, will be redundant, and would add needless complexity. If you view both combined, you get the current site, if you view kayak, only posts in the kayak section will show up etc.
My observation is that many of the long time kayakers who put the buzz on the map don't use the buzz now, because its a full of raft chatter that they have no interest in, and they can't easily filter out.
BS to the comments about losing info. If you set it up so you can view all, that is eliminated. I'd say 80-90% of the time the posts will easily fit into either rafting or kayaking.
I'd love to see this change, and I've lobbied for it for years. My guess is that most of the kayakers would like to be able to see kayak only threads as well.
If it were my site... I would have only 4 forums... kayak, raft, gear swap, and trip planner....
Keep it simple, make a kayaking forum.