If you're fishing mtn streams, I wouldn't evern go if I didn't have an assortment of Adams dry flies (16,12, 10) and Elk Hair Caddis (size 12) Pale yellow to match the Yellow Sallies.
You can get more technical if you're trying to chase the Green Drakes, early Black stones, PMDs or BWOs, but those two flies will bring a lot of trout to the surface for you.
Travis