I saw some mahagony duns at Nance's last year so I tied some just in case.
Hex usually live in ponds; maybe there are some small ponds near the spot on the Clinch where you see them?
Kudo's to Shawn for all his nymph work; identifying them is tedious.... so much to look at on the nymph that needs to be compared to the book sata ub order to confirm one may fly from another.
Historical Bug Question:
I wonder.... before the COLD water days (meaning tailwaters) did the Clinch, SoHo and other places have the hatches they do now?