Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This nymph keys out to Osobenus yakimae, and a comparison of the markings with a specimen on flyfishingentomology.com provides a good verification. The size is about right, as that species has been described as 12 mm. The markings generally fit the original description of the nymph of that species (Jewett 1955) perfectly.