It’s a caddis fly larva.
According to Mr. Gordon Ramel at Earthlife.net,
The Trichoptera have been known to fishermen since they advent of fly-fishing and to the entomological for a longer time. Mouffet the author of the first English book on entomology (the ‘Theatrum Insectorum’) writes in 1658 of the great variety of ‘cados worms’ to be found in rivers and streams. The name possibly arises from the ancient name for a travelling cloth salesmen who pinned samples of their wares to their coat, they were known as ‘cadice men’ and it is possible the name ‘Caddis fly’ is a reference to the cases many Caddis-fly larvae build from bits of debris. The Latin name ‘Trichoptera’ comes from the Greek ‘Trichos’ = a hair and ‘Pteron’ = a wing, meaning hairy winged which is a good description of the adult or imago forms.
There are about 7 000 named species world-wide of which over 400 occur in Europe and about 190 in Britain. Fossil Caddis flies have been found as far back as the Cretaceous.
There are dozens of those species in California, but my best guess based on fish and wildlife maps is Diplectrona californica. This nerd’s curiosity is sated.