Nice photos, Jess! Don't know how I missed this thread.
The first one looks like Rhithrogena undulata
(Western Red Quill), of the flatheaded clinger family Heptageniidae. As Gonzo intimated, "Mahogany Dun" is the western name for the little three tailed duns of the Paraleptophlebia
species of prong-gilled crawlers. R. undulata
is one of the better western hatches of this family, rivaled only by the much earlier hatching R. morrisoni
or Western March Brown (especially on the Coast) and a few of the Epeorus
species in some locales. R. hageni
(Western Black Quill) is a third common and even darker species of this genus that hatches between the two seasonally. It usually doesn't rival the other two as its May/June emergence is often right in the middle of runoff. The Western Red Quill is one of my favorite hatches as water is usually low and fishable while the air can be cool enough to keep the duns on the water awhile. It's also one of the few where standard Catskill style patterns are still as effective as anything else most of the time. Did you get the chance to fish it much?
I believe you're right about the big ephemerellid. It is Timpanoga hecuba hecuba
, the inland subspecies. Notice the thick body, spots along the sides, and dark markings on the sterna (ventral abdomen)? There is no real common name established for this species. Western Red Drake and Great Leadwing Red Quill among others have been floated by various writers but they never really took. It seems what little you hear about them in shops and guide reports simply refer to them as "hecubas", as you called it. These guys love the silt and I've had great fishing with them on the Truckee during drought years when their numbers seem to increase. Want to see the cool nymph it hatches from? Check this link out: