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 specimen is really strange, very different in form from any other mayfly I've seen. Unfortunately it was found alone crippled in an eddy and in pretty bad shape, and I couldn't find any others like it.
I don't recognize this fly at all, and the major features are too damaged for me to follow my key to families. I'm guessing maybe it's some large and obscure species of Heptageniid, but it could be from some rare family of mayflies too.