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.
Here's a Baetid nymph close to hatching, with darkened wingpads. It's got dark bands on the tail at the middle and tip, a rounded, oval 7th gill, and gill veinlets which are visible but not conspicuous.