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 pretty clearly keys to Kogotus, but it also looks fairly different from specimens I caught in the same creek about a month later in the year. With only one species of the genus known in Washington, I'm not sure about the answer to this ID.
Radius: The radius (denoted R) is the third major longitudinal wing vein in insects. In stoneflies, its principal branch R1 is often the strongest, thickest vein, running parallel to the front wing edge for most of its length and appearing to be the second vein in places where the subcosta does not reach.
These other words reference the same concept: radial, first radial vein, vein R1, R1 vein, R1, R, vein R, R vein.
