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.
Here's a Baetid nymph close to hatching, with darkened wingpads. It's got dark bands on the tail at the middle and tip, a rounded, oval 7th gill, and gill veinlets which are visible but not conspicuous.