Lloyd Gonzales has made the following comment relative to the identity of this mayfly on my flyfishing entomology forum:

I've been looking at this specimen for quite some time, and I don't believe this is P. praepedita. That species is distinguished by penial lobes that are as long or longer than the 1st segment of the claspers and lacks the basal enlargement of the claspers. The genitalia and basal enlargement of the claspers suggests that this is either P. moerens or P. debilis. I somewhat favor debilis based on the collection date, but both species may have multiple broods or cohorts.

thank you Roger for your input - I was eagerly waiting for someone more knowledgeable than me on this order to give a thought on this one, as, had it indeed been an Oligoneuriidae, this would have been a provincial first for the whole family, according to the (admittedly outdated) documentation I have.

After browsing the web to find more information on the species, it seems this northern town is a likely place to collect it and it seems that morphologically, it's indeed a good match. I had focused on the wings that seemed to have a quite reduced venation, as in Oligoneuriidae, but the adult pictured here (http://www.boldsystems.org/views/taxbrowser.php?taxid=159101) has most veins so weak that they could pass for absent on a damaged specimen like mine.

And, after all, it's a new species for the guide. :-)