#12 Whitefish Dynamite
The Willow River has a lot of mountain whitefish up to 16-inches and small rainbow trout in the 7 to 11 inch range. You may find occasionally the odd rainbow trout that is around 14-inches, but they are few and far between.
A 9-foot 4-weight fly rod is a good general purpose fly rod for this river. You probably will do a lot of nymphing on this river and then a 10- to 11-foot 3-weight fly rod would be good.
Flies to start with
#6 Orange Stimulator