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.