I'm no expert, but I would say that you have experienced the daily blackfly hatch at the weir dam. It is unbelievable. The fish just stack up there waiting on these bugs to come through the grates.....and they always do. These fish can be tough to catch because they see so very many flies at the weir. They can afford to be very picky when it comes to which ones they actually put in their mouth. Believe me, it can be quite maddening watching those fish feed right at your feet and then reject your offering.
"Many men go fishing all of their lives without knowing that it is not fish they are after."~Henry David Thoreau