The green drake hatch on the Yak does exist. It is sporadic, not very big, usually confined to discrete sections of the river (and who knows where from one day to the next) and short. (I do think it is more common in the sections of river above the canyon.) Catching it is often a matter of very good luck, or being on the river as often as possible in June. Even then it could turn out to have gone off like the 4th of July between the fruit stand and KOA while you were fishing the upper canyon, seeing not a bug. Given the amount of high water and unfishable days we've had this spring, it could have come and gone without anyone getting much of a shot at it. About all you can do is carry some para-drakes or #10 parachute Adams' if you're going to be on the river this time of year, and hope for the best.