(All mileage estimates in the following post are based on 2 year old memory)
I've only been there once, so I'm not an expert of the area, so take this for what it's worth, but I do have one loooong weekend experience with the area.
I would say that only a few miles of the upstream area are safely runnable. A few "creeks" enter at the upper part. Some of these creeks run in the thousands of cfs. The biggest probably is whitecap creek. So, by the time you get 10 + miles upstream, it's reletively small, (compared to it's downstream size), and gets fairly choked with wood. The trees in the area are huge, over 100 feet tall, so when one (or several) of those babies falls into the river, it's often from bank to bank in that upper area.
A couple years ago I was on a permit for the Selway... IT was WAY too high, none of the rafts would put-on, and we sat at the put-in for 3 or 4 days while permit after permit after permit cancelled. We had payed for an "over the snow" shuttle, the pass was still frozen over, so after our day came and went, we planned to take-over a cancelled permit at that point, we had the rangers blessing. The water level never came down to a runnable level, but durring those days, me and the only other kayaker had fun. We hiked up and ran a few miles of whitecap creek (running several thousand cfs) and part of that section of the Selway above the permit section. We only ran about 5-10 miles, I forget ecxactly, but it went by fast. Above where we got in the water, there where tree after log after tree after log crossing the river. Even a few on our little section that had to be negotiated....
The other kayak was a glass boat, failed to get over one of them, slid back off on the upstreamside, got sideways against the log, rolled under, and a branch slid into his cockpit. Somehow he rolled upright, but his boat was held under the surface still with that damned branch in his cockpit holding him in the boat. He was stiing upright in the boat, on the downstream side of the log with that branch in the boat down between his legs. It had punched through his skirt.
The river was huge so it had taken me about 100 yards, a long time to get out of the river to get back up to him , and luckily he was okay. stuck, but stable and okay. It took us a long ass time to get him out.... I recall it took awhile of trial and error for 10 minutes or so, we had to roll the boat (with him still in it) to get it off the branch so he could exit. He seemed a bit shook up... and very happy to see his wife back at the rafts/camp/put-in.
There where log jams on almost every outside bend too. Scarry stuff.
Aside from the mishap, it was a action-packed float with some awsome waves, but the danger level is pretty high in that upper area with all the wood, at least at the top of peak high water.
"So in two seconds, away we went, a sliding down the river, and it did seem so good to be free again and all by ourselves on the big river and nobody to bother us." -Mark Twain (The Adventures of Huckleberry Finn)