I just saw on TV that an experienced kayaker drowned on the Clackamus today.   The news footage showed a raft being lowered towards what looked like a huge (possibly splatable) rock midstream.  The kayak and kayaker were still pinned but were not visible despite the low flow.     Looks like a broach on the upstream face is likely.  There was a pillow but the upstream face could be overhung at the bottom or have a log pinned against it or something.

There were boaters in the background, so I imagine they have aided the rescue squad to recover the man by now.     I think a boater lowered down from ON the rock would be successful.  
Condolences,  
Clay