The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
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.
Costa: The costa (denoted C) is the first (anterior) major longitudinal wing vein. In mayflies, it forms most or all of the front edge of the forewing. It is a ridge vein (+).
These other words reference the same concept: costal, costal vein, vein C, C vein, C.