Doug,
The question is what do you mean by a river or sport kayak? There are many touring or sea kayak plans that work well in rivers. In fact almost any touring boat will work on a channeled river.

If however, you are talking about building a white water kayak..... Then your right you don't see much on that for the simple reason that wooden boats covered with glass are not well suited to being cut hacked and slashed by rocks. This is not to say such boats haven't been built. They have but they require a lot of fix it later work, and that means sanding below the glass to fix structure at the wood level and then reglassing.

As one who both sea kayaks and who white water kayaks. The simple truth is its hard to spend a year building a boat that will be beat up to the point of structural failure. When there are other materials plastic kevlar glass layups that are better adapted to surviving the pounding grinding and cutting that a white water boat gets.

When I am playing in white water jumping water falls and slamming up against cucumber rock... falling off of hydrolics onto knife edge rocks..That I do in composite layups.
Hope this helps
!RUSS