and not Maccaffertium. What's the distinction between these genera?

I am unable to rule out that suggestion, nor to provide a useful answer to your question, as distinctions in published scientific papers refer to male imagoes, rather than female imagoes. Therefore, this image should probably be elevated to placement in the Guide at family level.

Do you know what species occur in Alberta?

Yes, Maccaffertium terminatum terminatum and Maccaffertium vicarium are known to occur in Alberta.

So there could be a few unknown species, whether they are range extensions or otherwise...What are the differences between those two species you mentioned, assuming they don't require a microscope?

Most characters (useful for species differentiation) only apply to male imagoes. As a result, mayfly taxonomists don't generally attempt to identify females to species, even under a microscope. However, in the case of Maccaffertium, due to the visibility of the dark cross veins in their fore wings, the degree of cross vein crowding near the bulla is a (somewhat) useful character for species differentiation for both males and females.

So, getting back to your question, the degree of cross vein crowding near the bulla (for the two species in question) is described in their book, The Biology Of Mayflies (by Needham, Traver, and Hsu), as follows:

Stenonema terminatum - very slight
Stenonema vicarium - moderate

Hope this helps.