Hosmer Lake has them in Central Oregon. I don't know about other Oregon waters. Lost lake by Mt. Hood has the Hex mayflies. That hatch is one of the main reasons to fish BC. They were hatching on Sheridan lake when I was there a few years ago and the risers sounded like beavers slapping the water with their tails or a big bass exploding on a popper!
About 10-12 years ago myself and a buddy fished a high elevation walk in lake that had rather large grey caddis hatching on it. Never did find out what they were and of course we did not touch a fish that day as they were all very well fed by this hatch. The bugs did not move much that day as it was raining pretty hard all day. I would say they were about a size 8-6 hook size all a light grey.