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.
On certain rivers in late summer the Ephoron mayflies gives new meaning to the words "blizzard-like hatch," because their large white bodies give a true snowstorm appearance to their enveloping swarms. This is the most intense aquatic insect hatch of the year in places, and sometimes the flies are so thick that it's hard to get a trout to find one's imitation among the carpet of real insects on the water.
Ephoron leukon is most important species in the East and Ephoron album in the West. They overlap in the Midwest. These are the only two mayflies of this genus recognized in the United States, but Caucci and Nastasi in Hatches II report inspecting specimens which did not fit the description of either species.
See the main Ephoron page for details about this species, which is not known to differ in any important ways (besides location) from the other species.
On page 243 of Hatches II there is a passage from an 1802 speech before the American Philosophical Society in which this mayfly was introduced to science. It was the first mayfly species described in the United States, so it is ironic that it went unnoticed through so many of the early decades of our sport.
Caucci, Al and Nastasi, Bob. 2004. Hatches II. The Lyons Press.