The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
Your latter two educated guesses are good ones.
Methinks it's not Epeorus, Roger. (I'll reserve my hunch about what it is for now.)
From: FlyFishing Entomology [mailto:FlyfishingEntomology@comcast.net]
Sent: Saturday, March 28, 2009 7:11 PM
Subject: RE: What is this bug?
... The bug in your attached photo is a male mayfly spinner, and I believe it to be Cinygmula, perhaps C. reticulata.