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 female dun emerged from a creek loaded with mature Cinygmula nymphs including this specimen, so they're probably the same species. Unfortunately there aren't species keys for either stage.
This mayfly was collected from Green Lake Outlet in Idaho on August 4th, 2020 and added to Troutnut.com by Troutnut on August 20th, 2020.