El Rio is a small unincorporated census-designated town in Ventura County, California, USA, on the northeast side of the 101 Freeway and Oxnard, and south of the Santa Clara River. The town was founded in 1875 and named New Jerusalem, California by the owner of general store who became the first postmaster in 1882. In 1885, a school and Santa Clara Catholic Church were built. Located near where the coastal railroad line crosses the Santa Clara River, the town's name was changed to El Rio around 1900. The former wagon river crossing route became the Ventura Freeway and separates the community from the commercial developments that grew up on the southwest side of the freeway. That portion of the historic community and sections closer to the river have been annexed to the city of Oxnard. Vineyard Avenue (State Route 232) is the central business district for the largely residential community.