The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
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.
This stonefly was collected from the South Fork Snoqualmie River in Washington on June 13th, 2019 and added to Troutnut.com by Troutnut on June 14th, 2019.