Yellow Sallies are the small workhorse of summer stonefly hatches in the West. June through August, sizes 12–16, with a bright yellow body and reddish abdomen tip. They flutter close to the water and get knocked into the current frequently.
The hatch is steady rather than dramatic. Even when fish aren't keyed on bigger stoneflies, a Yellow Sally pattern often produces — it's a "no other reason" attractor that fits the size profile of opportunistic feeding.