My friend Matt Pederson loves log jams. He calls it Mining. He does it 10-15 minutes under each jam. Big heavy bunny leeches downstream are the ticket. He loses lots of flies but he says it goes with the territory. He bounces the big bunnies right at the edge of the log jams to get the big dogs to come out.
He uses a stout rod....he doesn't like going way under them because they get tangled easily. You need some strong fluorocarbon and you need to get their heads up quickly. This black bunny leech has dumbbell eyes and is a great mining fly.
If you are in a pinch you could use a Hornberg. This would require a split shot at the fly or a split shot about 12 inch above the fly. The 12 inches above works even better because the fly flutters in front of the log jam and teases the big dogs and they come out to play.
Coneheaded Turkey Leeches also work well.
I met Mike eons ago at the Spring Creek Festival that Nohr Chapter use to put on. We were talking about log jams and I suggested to him to use the down stream mining tactic. I even told him exactly were to go. Mike is from Iowa but the next season he was non the Pine River using my Mining suggestion. He fished exactly where I told him to. The log jam directly behind him is where he caught this football. His biggest EVER brown!!!