Pizzle, are you asking about the big t? Gannon's whining about his poor life choices has gotten this thread all off track.
I don't know much about the lower section, but I think that you need about 500 cfs or more. You put in below Cannon shot and paddle to wherever you feel like. There is like a dam or something... and two or three rapids... maybe a bridge? A bridge and a rapid together? Sort of fuzzy, really. I remember it being sketchy because of a lack of eddies. People seem to like it, though.
The upper section (the Gnar) runs from about 120 on up. 250 is a very nice flow, although still manky. By the time it gets up to about 300 and above, it is a lot less manky, but it is also a lot more serious. Seriousness increases with flow. I think I tap out at about 450-500. I have no idea how high it's been run. Scout extensively.