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.
This family contains the largest species of stoneflies in North America, with many female specimens exceeding 50mm. Unlike the large and active predaceous Perlidae Golden Stones, they are primarily detritivores that move about slowly. The most important species is Pteronarcys californica. See its hatch page for details. It takes them three years to develop to maturity, so it is useful for the angler to carry a variety of nymph sizes.