This one seems to lead to Couplet 35 of the Key to Genera of Perlodidae Nymphs and the genus Isoperla, but I'm skeptical that's correct based on the general look. I need to get it under the microscope to review several choices in the key, and it'll probably end up a different Perlodidae.
I found a couple small (one or two dozen flies) clouds of these male spinners dancing low over the riffly parts of a pool in this fast river draining out of the Cascades.