This specimen resembled several others of around the same size and perhaps the same species, which were pretty common in my February sample from the upper Yakima. Unfortunately, I misplaced the specimen before I could get it under a microscope for a definitive ID.
Described by Nelson, C.R. & Baumann, R.W. (1987) New winter stoneflies of the genus Capnia with notes and an annotated checklist of the Capniidae of California (Plecoptera: Capniidae). Entomography 5, 485–521.