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.