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. (Old Gabe was probably one of the original red shirt people to beam down to the planet....)
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."