Means "Arroyo of the Alder Trees or Bears." This is probably the same arroyo that Portolá's men named the "Cañada de los Osos" (Canyon of the Bears) on September 7, 1769, because of the excitement they had hunting the numerous bears that were there.

An "arroyo" is a waterway (gully, or wash) which, in the desert, is usually dry, except after a heavy rainstorm.