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 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.
Described by Nebeker, A.V. & Gaufin, A.R. (1965) The Capnia columbiana complex of North America (Capniidae: Plecoptera). Transactions of the American Entomological Society 91, 467–487.