Salmonflies hatch on Western freestone rivers in mid-May through late June, depending on the watershed. The naturals are huge: orange-and-black, sizes 4–8 in commercial patterns. Trout that have been eating midges all winter switch overnight to ambushing six-inch helicopters off banks and overhanging foliage.
The hatch is fickle — peak windows last 7–14 days per river, and missing them by three days can mean the whole season's gone. When they're on, every fish in the river feeds; when they're off, the bigger imitations spook fish that aren't yet locked in.