Salmonflies
Pteronarcys californica
The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
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This one seems to lead to Couplet 35 of the Key to Genera of Perlodidae Nymphs and the genus Isoperla, but I'm skeptical that's correct based on the general look. I need to get it under the microscope to review several choices in the key, and it'll probably end up a different Perlodidae.
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Yakima Springflies
This common name refers to only one species. Click its scientific name to learn more.
Stonefly Species Osobenus yakimae
These are very rarely called Yakima Springflies.
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.