In 1805, Lieutenant Gabriel Moraga was ordered by the Spanish Governor of California to explore the Great Central Valley and to (re)name everything he found.
One day Gabe came across a river the banks of which were littered with skulls. No one knows how the skulls came to be there. Perhaps an ancient battle. Or perhaps a really great party gone horribly wrong.
Whatever. Old Gabe, being a true master of the obvious, named this river "El Rio De Las Calaveras" or in english, "The River of Skulls."