The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
This female looks very much like a male I collected a few hundred miles away a few days later, so I'm guessing it's the same species, which I believe is Maccaffertium mediopunctatum.
This mayfly was collected from the Little Juniata River in Pennsylvania on May 23rd, 2007 and added to Troutnut.com by Troutnut on June 4th, 2007.