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.
Probably Callibaetis ferrugineus.
This mayfly was collected from Flathead Lake in Montana on May 25th, 2005 and added to Troutnut.com by Bnewell on June 27th, 2011.