The Hexes ( intentional mispelling) have always been around on the Clinch, not in any great numbers but enough that I have seen more then one fish eat them.
I would call it an extremely sparse population, and I have seen them as the dark brown speckled spinners, and the big yellow freshly emerged adults.
Usually I have seen more of the big brown ones, and like Rod said early early in the morning in the fog. Usually late July and August. If you see one drift past you in the fog listen up and most times they get eaten before they get too far.
Madison, I would love to see what you collected, and it is possible they could show up anytime. It has always surprised me that there seems to be zero Baetis in the river. I personally have never seen them, but that doesn't mean that something doesn't exist.
I also forgot about a hatch we had for a few years in the late 90's that was some sort of mahogany dun looking mayfly. They were about a 16, maybe a 18 with slate gray wings and a redish brown body. We only saw them for a year or two, and only at 61 bridge. The hatch was super spotty and brief, but they were there for a short time. Haven't heard about any in a very long time however.