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.
Isidro on Mar 27, 2008March 27th, 2008, 1:09 am EDT
I think that this one can be impossible...
Sabiñanigo, NE Spain. Fresh mountain-climate, about 900 meters. River: with many trees and hedges in the sides. Contaminated, slow, with some lime and big stones, without water weeds, not deep. Mayfly: about 11-12 mm. wingspan, Three cerci, of very clear colour, very dark body, turbinate eyes.
Taxon on Mar 27, 2008March 27th, 2008, 3:09 am EDT
Isidro-
My guess would be a species in family Leptophlebiidae. The fact that the median filament is longer than the cerci should identify it rather easily for someone familiar with European mayflies. I recommend you ask Dr. Nikita J. Kluge of St. Petersburg University.
Edit: Now that I look at the photo again, I suspect the tails may actually be of equal length, and the angle of the photo makes it appear they are not. In any event, that impacts neither my guess as to family, nor my recommendation for consultation.