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 fairly sure this is a specimen of Rhithrogena virilis based on closeup examination of the reproductive anatomy under the microscope (not shown in photos). The other other species of Rhithrogena this large is Rhithrogena flavianula, but the key in Needham's Biology of Mayflies mentions annulation in the abdomen (visible in some images on bugguide.net) more distinct than that on this specimen.
The body and front wing were both about 15.5 mm long, while the cerci were 40 mm long.
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.