This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
This species is not known to be important to anglers. It is noteworthy for its relatively recent discovery, its large size, and the striking coloration of its nymphs and duns. They are sometimes called Tiger Mayflies.
This is the largest species of Heptagenia on the continent, and it's also one of the largest in the entire Heptageniidae family. Nymphs and adult females have been collected with bodies up to a size of 19.5mm, a little over 3/4" long.
Troutnut on Oct 20, 2006October 20th, 2006, 2:35 pm EDT
Many thanks to user Softhackle for digging up this link. I knew about the thread from back when it started, but I wasn't able to find it when I went back to look last night. Good work!