The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I'm guessing this one is Rhithrogena virilis based on collection of a male spinner of similar size and body configuration during the same trip.
This mayfly was collected from the South Fork Snoqualmie River in Washington on June 16th, 2018 and added to Troutnut.com by Troutnut on June 23rd, 2018.