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 associated this female with a male I collected one night later. For a great and informative discussion of all Epeorus species and other mayflies be sure and read, WESTERN MAYFLY HATCHES by R. Hafele and D. Hughes. They rate all of the western mayflies and they rate the hatch of this genus a 25 out of a possible 45 points based on several rating criterion. They provide insect photos, possible fly patterns and fly presentations. This is a great book.
Edit by Troutnut: Bob generally knows his IDs better than I do, but I think this one was misidentified as Epeorus longimanus. The darkened humeral crossvein is not supposed to be present in longimanus. I'm moving it up to Epeorus.
This mayfly was collected from the Touchet River in Washington on July 24th, 2011 and added to Troutnut.com by Bnewell on July 25th, 2011.