Because we recently discussed this subject, Roger is probably expecting a post from me, and I wouldn't want to disappoint him by not taking the bait. Although I agree with Roger that this does look like a Hexagenia limbata spinner, I think it could also be H. rigida.
I lived in the Harrisburg area for about half of my life, and a surprising number of burrowing mayflies are found right in that immediate area. There are four Hexagenia species (limbata, rigida, atrocaudata, and bilineata) as well as Green and Yellow Drakes (Ephemera guttulata and varia). Although I haven't been able to confirm it personally, another big burrower, Litobrancha recurvata, is said to be found in at least one stream flowing into the Susquehanna in the vicinity of H-burg. (Though recurvata probably would not be found in the river.)
Of the Hexagenia species, atrocaudata and bilineata can be easily ruled out. That leaves limbata and rigida. The problem is that rigida is so similar in appearance to one or two of the limbata varieties ("morphs") that they can only be conclusively identified by the distinctive genitalia of the males or the chorionic egg structures of the females. In other words, it is probably impossible to ID your specimen with any certainty based on the photo.
It is my impression (and only an impression) that rigida might be the more common Hex species found in that area. Further suggestive (though not at all conclusive) evidence comes from a very nice study of Hexagenia that was done by Dr. Spieth back in the forties. In that study, he lists records of three of the four Hex (limbata, rigida, and bilineata) collected right at Harrisburg, and the fourth (atrocaudata) at Chambersburg. The interesting thing is that the Harrisburg limbata record is of the morph that was described in his study as as a subspecies of what was then considered to be a separate species, Hexagenia munda affiliata. This is now considered to be a synonym of limbata. Spieth's description of the habitat of the affiliata morph ("As yet I know it to be taken only from clear, clean, usually small streams and small lakes") would lead me to wonder if the population in the Susquehanna might be rather sparse. The dorsal markings of that morph are described as being rather different than your specimen--darker and of a different configuration--and the markings on your specimen appear to be fairly consistent with rigida. However, the catch is that such distinctions are much harder to make with female specimens of these species. So, all I can really say is that both limbata and rigida are probably found in the river, both emerge at about the same time, and your specimen could be either one. (From a fly-fishing/imitation standpoint the distinction between these species makes virtually no difference.)
Although Roger could be right about this being limbata, as Dr. Spieth says in his study, "This species has always been the dispair of ephemeropterists."
PS--I just noticed your latest post, Chris, and my answers would be yes, yes, and yes. I would normally imitate Hex with nymphs and dries on trout streams, but it is hard to argue against swinging a big wet after dark, even when Hex are not hatching.