The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
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.