The lack of a darkened
humeral crossvein rules out
Epeorus albertae and
Epeorus dulciana. The lack of a dark
macula on the
forefemora rules out
Epeorus longimanus. The small size rules out
Epeorus grandis and
Epeorus permagnus. That leaves as the only possibility known in Washington state
Epeorus deceptivus. It is a small species, although not reportedly quite as small as this specimen. I couldn't find anything in the species description in Traver (1935) to definitively confirm or rule out the species ID, given that I don't have the preserved specimen to check under a microscope, but it has to be either
deceptivus or something not yet reported in Washington.
It was collected at the same time as a
similar-sized female dun.