The family ID on this one was a little bit tricky. Just going by the size, shape, and color, it looks like
Chloroperlidae. However, the second anal vein of the forewing is does not appear to be forked, and the emergence emergence segment is close to the length of the emergence segment, both of which rule out that family. The position of the emergence relative to the emergence in the forewing -- touching it in this case -- indicates
Perlidae (and it really doesn't have the "look" of
Perlidae at all), but other characteristics, such as the metathorastic emergence and lack of gill remnants, point to
Perlodidae. That's the right answer. Moving on to
Perlodidae, the key characteristics in Merritt & Cummins lead straightforwarly to
Isoperla, and the species key in Jewett 1959 (The Stoneflies of the Pacific Northwest) leads to
Isoperla fusca.
There is one caveat: That source does suggest a May-July emergence, whereas this one was collected in mid-September.