Female Rhithrogena Mayfly Spinner Pictures

I didn't even recognize it as a member of that family at first, but it seems to be the only option. The hind tarsus is distinctly 5-segmented, which I verified under the microscope because you can't see it very well on my photos. However, I because it's a female spinner I can't key it to genus using the Merrit & Cummins keys. Does anyone just recognize it?

I found it drifting, nearly dead on the surface at about 5pm on a hot afternoon.