This Skwala nymph still has a couple months left to go before hatching, but it's still a good representative of its species, which was extremely abundant in my sample for a stonefly of this size. It's obvious why the Yakima is known for its Skwala hatch.
My wife and I spent a weekend out at Chena Hot Springs, which included doing a loop on a nearby ski trail that runs up nearby Mammoth Creek, a tributary of the Chena River. I don't know if anyone ever fishes Mammoth Creek itself, but it has been the locale of some studies of local aquatic insects.