Whatever name you give it that is certainly a nice big chunk of a Stonefly.
I don't know how to identify stoneflies very well. I know a Salmonfly when I see one because it is ridiculously big and always has bright orange markings. The little Yellow Sallie is pretty easy as well. Otherwise, I don't know.
This spring I came across some stones hatching on the lower Bitterroot when it was supposedly still to early for Skwalas. But they were pretty big, about #10 so larger than the earlier hatching "Nemouras" but pretty dark. All the patterns you see for Skwalas are green, and I've seen some green bugs for sure, but also black and large brown ones. Unless of course there are different species of stoneflies that all hatch at the same time in the spring and we just call them all the same name. I'm fine with that, just don't be fooled into making all your "Skwalas" green! I've also encountered a hatch on a small stream during what would have been Salmonfly time, and they were big, but all tan in color. Goldens? Maybe. Luckily I had some tan elk hair and hare's ear tarantulas that did the trick!
My point is that there are a lot of stoneflies and they are very important to fish. Just match up the size, then approximate the color. I personally think it's better to just have some tan, dark brown and black ones tied up.
Get out there and fish some of those see what happens!
Short of reading entomology text books, it's difficult to come by factual info about bugs that interest fisherman because of the sheer number of hits you get on the innernets when searching for common names of "flies." Anyway, Troutnut.com does a pretty good job in my opinion. Here is the spring stonefly discussion, if you are interested. But I think you're correct, probably a Skwala hatch you've got there on the other Rock Crick! Lucky you!