Three weeks or so ago, I launched from PO around 5:45 am. The water was falling, maybe 1' higher than low/low. The pool between PO and the bend was covered in Hex's. I kindly expected they were spinners, but each one I got close to was a fresh dun. Big and hulking. Some drifted motionless like sailboats, while others fluttered on the surface a bit. Many of them were eaten. We easily spotted over five hundred as we drifted straight down river and could only see bout 30'. We caught 20 fish on a straight dead drift through them. We didn't see any more after we reached the shoal.
I've seen them in low numbers many times. First time I've ever seen them in those numbers.