Depends on you skills - how about 7 pounds on 4x in still water?
http://www.youtube.com/watch?v=rlE9aRdxu5s
I agree that the dynamics of a big fish in moving water is the wild card.
Keep the fish on the butt of your rod.
Let the tippet do it's job.
Haul 'em in.
This is a custom rod, but it will give you an idea of what is possible in terms of brute force -
http://www.youtube.com/watch?v=ZT0wsMWLpSg
Notice that the top 2/3 of the rod is straight - the force applied with the last 1/3 of the rod.
If you try a "high stick" landing you Will break your rod.