According to an article in today's Denver Post it is time for the Skwala Stonefly hatch on the Bitterroot.
"A large stonefly that appears in late March on certain rivers of the northern Rockies, the skwala is a bug shrouded in considerable mystery. It receives little mention in angling literature and is even less known to most anglers outside the area."
While you contemplate the stonefly hatch you should check out the website of the Monte L. Bean Life Science Museum at Brigham Young University. They have an entire section devoted to everything "Stonefly"