The county was formed on March 25, 1853 from a large portion of Contra Costa County and a smaller portion of Santa Clara County.
The word 'alameda' means 'a place where poplar trees grow', a name which originally was given to the Arroyo de la Alameda (Poplar Grove Creek). The willow and sycamore trees along the banks of the river reminded the early explorers of a road lined with trees, also known as an 'alameda'.
The county seat at the time it was formed was located at Alvarado; it was moved to San Leandro in 1856 where the county courthouse was destroyed by the devastating 1868 quake on the Hayward Fault. The county seat was then re-established in the town of Brooklyn from 1872-1875. Brooklyn is now part of Oakland, which has been the county seat since 1873.
Much of what is now considered an intensively urban region, with major cities, was developed as a trolley car suburb of San Francisco in the late 19th and early 20th centuries. The historical progression from native American tribal lands to Spanish, then Mexican ranches, thence to farms, ranches, and orchards, suburbs and eventually cities, is shared with the adjacent Contra Costa County (see that article for an extensive history applicable to this county).
The County of Contra Costa, California (the "County") was incorporated in 1850 as one of the original 27 counties of the State of California (the "State"), with the City of Martinez as the County Seat. It is one of the nine counties in the San Francisco-Oakland Bay Area. The County is the ninth most populous county in California, with its population reaching approximately 1,051,677 as of January 1, 2008.
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