Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
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.