This one pretty clearly keys to Kogotus, but it also looks fairly different from specimens I caught in the same creek about a month later in the year. With only one species of the genus known in Washington, I'm not sure about the answer to this ID.
These are pretty much always called Little Sooty Olives.
This locally important species is rarely mentioned in fly fishing literature, and what little information is given is identical to that for Teloganopsis deficiens. Knopp and Cormier say both species can produce good hatches.