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.
A trout, and steelhead guide for many years in the Seattle, WA area. I've taught fly fishing schools for 30 yrs., and was the fly fishing technical advisor, and worked many days on the set for the very popular tv show, Northern Exposure.