Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
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.
Described by Baumann, R.W. & Gaufin, A.R. (1972) The Amphinemura venusta complex of Western North America (Plecoptera: Nemouridae). Contributions to Science, Los Angeles 226, 1–16.