The Salmonfly hatch is probably the west's best known hatch and brings people from all over the world to fish Montana, Idaho, Wyoming, Oregon and Washington State. The adults can be up to three inches long and their clumsy nature makes them easy meals for trout who will gorge themselves on both the nymphs and adults. Like all stoneflies, they have incomplete metamorphism. Meaning that they spend most of their lives (up to three years) as nymphs, crawl to the banks when the water temperature and conditions are right, split their nymphal husks and emerge as adults. They then will live for a few days to about a week where they find a mate and the females will deposit their eggs back into the river, and then die.
This hatch generally happens around the end of June to early July here in Southwestern Montana. This can be a rather tricky hatch to completely figure out because the hatch moves up stream at various speeds. If the weather gets hot, the hatch can explode and move quickly upstream. If it cools off, the hatch may even stall and slow way down. Like mentioned before, the trout can gorge themselves on these flies and a section of river that fished well the day before, may fish poorly because the trout are stuffed. Head too far upstream ahead of the hatch and the trout won’t recognize the Salmonflies yet.
There are many factors to consider, and if you would like some shortcuts to figure out this hatch I recommend booking a day with one of seasoned guides.