As Maurice points out-stoneflys crawl to shore and emerge.
In the West the best known stonefly is the Salmon fly.
The reason size 12 Royal Wullf is so deadly in fast water,especially on cloudy days,is because Salmon flies emerge all summer long,a few at a time,and not just during the main hatch.
Try a yellow bodied Wullf at dusk or on a cloudy day in the fast water and see what happens.I know stones have down wings but in fast water the fish are keying on the body and you can see the wings.Thats dry flyfishing excitement when they hit them or hoppers.