The San Juan Worm gets dismissed as "barely a fly," but trout don't care. Aquatic worms (annelids) live in soft river bottoms and get rooted out constantly; after rain, terrestrial worms also wash in from the banks. The pattern — a single strand of red or pink chenille tied to a curved hook — covers both meals.
It's named for the San Juan River in New Mexico, where guides figured out in the 1970s that trout on tailwater silt beds ate worms more reliably than insects most days.
Fish it deep, dead-drifted under an indicator or as the trailing fly on a Euro rig. After a heavy rain, the San Juan is often the only fly worth tying on for the first day of high, dirty water. Hot pink for stained water, brown for clear.