There is something to be said about fall on the river. The temperatures are cool, the crowds have thinned out, and the fish are hungry.
The Warm RIver to Ashton section of the Henry's Fork produced well on the last days of September, with fish eating the dropper section of a dry dropper rig and streamers.
Use caution on this seciton of the river as flows are at a bear minimum as water regulators begin storing all the water possible in Island Park Reservoir. Your boat will be beat up so prepare accordingly.