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 (Cercus: The left and right "tails" of an insect are known as the cerci or caudal cerci. The middle tail of a three-tailed insect is not.) were 40 mm long.
This mayfly was collected from the South Fork Sauk River on July 5th, 2017 and added to Troutnut.com on July 6th, 2017.
There's a pale amber tinge to the anterior areas of the wings, which doesn't show up all that well against the blue background.
Tick marks are 1/16". The body and front wing were both about 15.5 mm long.
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