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 appears to be of the same species as this one collected in the same spot two months earlier. The identification of both is tentative. This one suffered some physical damage before being photographed, too, so the colors aren't totally natural. I was mostly photographing it to test out some new camera setting idea, which worked really well for a couple of closeups.
Mayo, V.K. 1952. New Western Ephemeroptera, IV, With Notes. The Pan-Pacific Entomologist 28(4): 179-186.
Needham, James G., Jay R. Traver, and Yin-Chi Hsu. 1935. The Biology of Mayflies. Comstock Publishing Company, Inc.
Slater, J. and Kondratieff, B.C. 2004. Review of the Mayfly Genus Cinygmula McDunnough (Ephemeroptera: Heptageniidae) in Colorado. Journal of the Kansas Entomological Society 77(2): 121-126.