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These guys were coming off strong from 3pm to 7pm. Moving water. 2 tails. Looks like a Hex but only 15mm. Wing and forewing completely opaque. No coloration in the wing. Even on the leading edge. Small veins on tips of wings not present. Pic #2 Notice the markings on the noticibly lighter top of the throax. Segments on back are brown but under body just pale yellow(no dark markings)Are there Nixe's this big?

Where did you collect them?

Probably Siphlonurus.

Those look a lot like one damaged specimen I collected two years ago. Surely they're closely related, although the dorsal markings seem sufficiently different that they're probably different species. I would love to have one of the real entomologists weigh in here. I suspect they're some sort of Heptageniid but I can't say for sure.

I really doubt Siphlonurus. I've never heard of them having yellow bodies.

These are from a tailwater in eastern oregon. Lake supports warmwater species. Tailwater below is about 3,000 ft ele. Bugs hatched in faster water just above slow moving pools. Nymphs have 3 tails, duns two. I thought marking on top would be a dead give away for someone. Which specie of Siph gets to about 15mm or 5/8in w/o tail.
Thanks in advance

Forget to mention the body is fairly robust unlike a brown drake. The bottom of the abdomen is very flat...

this flat bottom of the abdomen: is is all pale, or is it colored & marked in some way?

all pale.

In doing some further discovery I may have them. Let me know what you guys think. Ephemeroptera/Heptageniidae/Epeorus/Hesperus.
Listed as Endangered, no real common name but Yellow quill.
The description and geo sure fits. Water type cold moving water (this is a tailwater). All patterns note the need to match the unusually flat body both in the nypmph and adult. Coloration is right on olive nymph pale dun. They are found of the Makenzie river not too far away from the location in question. Size of the sub-image about 5/8".
Unique characterists of the hatch are that they break their shucks @ the bottom and don't float very long before flight. This would correspond to the trouts behavior of splashy rises. Also noted in the info. Was the lack of any witnessed spinnerfall a mystery. That would be consistent with what I have observed over the last three weeks.The data says that they hatch typically between Memorial day and the 4th of July. Anybody familar with this hatch/bug?

Two (perhaps pertinent) items:

The Biology Of mayflies lists the adult body length of this species as being 10 mm. Also, Epeorus would have 2 tails in their nymphal life stage.