Some will tell you that the end of Decemeber is early for Steelies in the southfork of the C. Typically, the run is beginning at the middle to end of feb/first of march and peaks through out March and some of April BUT, the advantage to Decemember is there will be no crowds......if you think the Boise is bad (when they plant Steelies) it does not hold a candle to the SF when the word is out that there are fish in the river........by March.....it is fun to catch them for C & R, not keeping because they are boots and are barely worthy of the smoker...let alone for any table type of meals.
Got off track there for a second..In the end of the December there will be a fish or two in the system. I would reccommend fishing between Stites and Kooskia. I would also say that the section of river which is the Middle Fork of the C between Kamiah and Kooskia would have more fish in it and has plenty of good water for throwing a fly. Coming from Clarkston, you can go to the SF by driving through Orofino, Kamiah and Kooskia, and see a ton more river to fish verses taking 95 to Grangeville and then to the SF and missing all of that river.
One last word of caution...those rivers in that area have been known to ice up pretty good depending on the winter and time of year.....December might be ok where I have seen in years past in middle January or early feb...covered with ice.
Just some other ideas. Good luck!
gg