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.