The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I suspect this is an Epeorus vitreus nymph. At the current instar, it's tough to make out the pair of dots on the tergites which signify that species, but with zooming and contrast enhancement they become evident.
This mayfly was collected from the Namekagon River in Wisconsin on January 13th, 2004 and added to Troutnut.com by Troutnut on January 25th, 2006.