The San Juan worm is another fly that looks too simple to work, a short length of soft red chenille on a hook, and works anyway. Aquatic worms are a real and abundant trout food, and they get washed loose in numbers when the water rises, which is exactly when this fly earns its keep.
What it imitates
It imitates an aquatic worm (an annelid, like an underwater version of an earthworm), usually tied in red, pink or brown. When high or stained water dislodges worms from the streambed, trout feed on them heavily, and few things drift as convincingly as a soft chenille worm.
How to fish it
Fish it dead-drift along the bottom like a nymph, especially after a rain when the river is up and off-color. It is a superb high-water fly and a great searching pattern, often fished as a dropper with an egg or a bead-head nymph.
Tip Reach for the San Juan worm when the river comes up and colors after a rain. That is when the naturals wash free and trout expect an easy worm drifting by, so a red worm bounced along the bottom in high water is a proven producer.