4 years ago I was camping along the river quite a few miles up the North Branch from my usual spots. It was already fall. I saw a hatch and was dumbfounded to see they were the White Flies indeed. Caught a few bass and was working up river when the insects just seemed to disappear.
When I turned around I saw a wall of white flies heading up river but stopping and were turning around to go back down, it was the end of their migration up river for the day, very strange sight to see.