Randy nailed it.
With "tidal" action due to releases from the dam above, you may encounter daily flows ranging anywhere from 5-8k to 15-20k cfs. When I was down there last Fall, a steady three-day rain event raised the river 5-6 feet overnite at our Ledges camp and kept it high -- significantly changing the rapids below us the next day. Many places (Horn comes to mind) look quite a bit different even from morning to afternoon, because of when the "tide" arrives.
So lines vary dramatically when you are working with that kind of volume range, and guides are really only that. Relying on scouting and your ability to read water are what really gets things done.