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.
On the way home from deer hunting somewhere in Idaho, I had just a couple hours to check out the South Fork Boise River. I drove up and down the canyon for a bit, looking at the water, trying to decide where to stop to fish. Eventually I found some fish rising to some little Baetid duns, and I waded into a bouldery tailout to try for them. They stopped rising pretty quickly, so I switched to my Euro nymphing rig. I hooked two nice rainbows in quick succession, landing a snaky, geriatric-looking 19.5" rainbow and losing one that was a bit shorter but a lot stronger.