So... You ARE a Bugmeister! Out of the closet now, huh?:)
I also agree it's a Leptophlebid male imago. Besides lacking the eye bands of the Siphlonuridae, the segment count of the hind tarsi preclude Heptageniidae (not enough of 'em). The segment ratio of the visible foretarsus is also a useful indicator. Other visual aids are the light rear legs and dark forelegs and overall look/shape of the body. This family seems to have issues with hanging on to their caudal filaments so you were right to ignore that.
Too bad the wings are covered, but it's most likely Leptophlebia cupida
(Black Quill, Borcher Drake). It is possible to be nebulosa
or perhaps even a few others. Out West we also have the species pacifica
that lookes very similar. Perhaps a better look at the nasty bits and wings would help to know which species for sure but regardless, they're all Black Quills.
These are often hard to find as nymphs unless you scoop the slower margins with a net (cut banks are good) in late winter before they start to school up, which leads many to think they are far less important than they can actually be at times. There are pretty good populations of them in waters I fish often, the upper Williamson R. in Oregon for example.
Both are slower water types, but the Siphlonurus takes the prize: The ones I found were in a shallow ditch in a hay field (!) -a good half mile from the nearest stream.
Wow, are you right about that! I dread high water that floods the marshes of the upper Williamson. You won't find a Gray Drake nymph anywhere near feeding fish, yet the duns and spinners fill the meadows. Very frustrating...