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.
I'm guessing this female is of the same species as this male dun, because they came from the same pool at the same time and the size matches, although the males and females would look very different in this case.
This mayfly was collected from the South Fork Sauk River in Washington on July 5th, 2017 and added to Troutnut.com by Troutnut on July 6th, 2017.