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.
These little green stoneflies were the most common bug on a June day fishing the North Fork Couer d'Alene, although the trout never rose more than sporadically to them or anything else.
This specimen was completely green when I collected it from among many others gathered on a midstream rock along with their nymphal shucks early in the morning. There was also a yellow one with them, which I assumed was a different species. Now that I've seen how this one started changing from green to yellow, I have to wonder if they weren't the same species and the yellow one was just older.