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.
This nymph was one of several mature Cinygmula nymphs collected at the same time as this female dun emerged, so they're probably the same species. Unfortunately there aren't species keys for either stage.
This mayfly was collected from Green Lake Outlet in Idaho on August 4th, 2020 and added to Troutnut.com by Troutnut on August 20th, 2020.