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.
I've been staring at this one for quite some time. My first impression was lycorias (Boreal Stone, Giant Brown Stonefly). I'm pretty familiar with this species, but I don't know of a good key to Acroneuria at the species level.
The lack of a row of spinules along the occiput of the head and presence of postocular and pronotal fringes all say Acroneuria for sure.