This specimen keys to the Epeorus albertae group of species. Of the five species in that group, the two known in Washington state are Epeorus albertae and Epeorus dulciana. Of the two, albertae has been collected in vastly more locations in Washington than dulciana, suggesting it is far more common. On that basis alone I'm tentatively putting this nymph in albertae, with the large caveat that there's no real information to rule out dulciana.
These stoneflies, known for their roach-like shape, are not really mentioned in books about angling. They are a major component of the invertebrate fauna in a few streams, which are presumably few and far between. Since their robust shape is so distinct, I speculate that a special nymph pattern would pay off on the rare waters where they are major insects.
The adult stoneflies of this family look surprisingly normal given the strange shape of the nymphs. I tried to identify it beyond family level 15 years after collecting it, based on my photos, which are not ideal. It's difficult even to tell the gender for sure because I don't have a good picture of the terminalia, but the body size suggests it's a male, and the species ranges suggest either Peltoperla arcuata or a male Tallaperla maria. One distinctive feature of the original description of this species is that the antennae are paler for a short distance beyond the first two thick, dark segments. That can be seen on this specimen and on a specimen on BugGuide identified as Tallaperla maria.
I'm guessing this is an immature Peltoperla, because it lacks the well-developed setal fringe on the hind femora that would indicate Tallaperla, but it also lacks the dark spots that should indicate a mature Peltoperla nymph.