Contra Costa County; Contra Costa, Spanish for “Opposite Coast” is a county located in the U.S. state of California, in the East Bay of the San Francisco Bay Area. As of the 2020 United States Census, the population was 1,165,927. The county seat is Martinez. It occupies the northern portion of the East Bay region of the San Francisco Bay Area and is primarily suburban. The county’s name refers to its position on the other side of the bay from San Francisco. Contra Costa County is included in the San Francisco–Oakland–Berkeley, CA Metropolitan Statistical Area.

Contra Costa County was one of the original 27 counties of California, created in 1850 at the time of statehood. The county was originally to be called Mt. Diablo County, but the name was changed prior to incorporation as a county. The county’s Spanish language name means opposite coast, because of its location opposite San Francisco, in an easterly direction, on San Francisco Bay. Southern portions of the county’s territory, including all of the bayside portions opposite San Francisco and northern portions of Santa Clara County, were given up to form Alameda County effective March 25, 1853.

The land titles in Contra Costa County may be traced to multiple subdivisions of a few original land grants. The grantee’s family names live on in a few city and town names such as Martinez, Pacheco and Moraga and in the names of streets, residential subdivisions, and business parks. A few mansions from the more prosperous farms have been preserved as museums and cultural centers and one of the more rustic examples has been preserved as a working demonstration ranch, Borges Ranch.

In the 1860 United States Census, Contra Costa County had a population of 4,381.

According to the U.S. Census Bureau, the county has a total area of 804 square miles, of which 716 square miles is land and 88 square miles (11%) is water.

Contra Costa County’s physical geography is dominated by the bayside alluvial plain, the Oakland Hills–Berkeley Hills, several inland valleys, and Mount Diablo, an isolated 3,849-foot (1,173 m) upthrust peak at the north end of the Diablo Range of hills. The summit of Mount Diablo is the origin of the Mount Diablo Meridian and Base Line, on which the surveys of much of California and western Nevada are based.

The Hayward Fault Zone runs through the western portion of the county, from Kensington to Richmond. The Calaveras Fault runs in the south-central portion of the county, from Alamo to San Ramon. The Concord Fault runs through part of Concord and Pacheco, and the Clayton-Marsh Creek-Greenville Fault runs from Clayton at its north end to near Livermore. These strike-slip faults and the Diablo thrust fault near Danville are all considered capable of significantly destructive earthquakes and many lesser related faults are present in the area that cross critical infrastructure such as water, natural gas, and petroleum product pipelines, roads, highways, railroads, and BART rail transit.

Sub-areas

Contra Costa County is broadly divided into three sub-areas:

• West County, including of the cities of El Cerrito, Richmond, San Pablo, Pinole, and Hercules, as well as the unincorporated communities of Kensington, El Sobrante, North Richmond, Rodeo, Crockett, and Port Costa.
  Central County, which is itself further divided into three areas:
  Lamorinda, including of the cities of Lafayette, Moraga, and Orinda (the name being a portmanteau of the three cities’ names), as well as the unincorporated area of Canyon.
• North Central County, including of the cities of Walnut Creek, Pleasant Hill, Concord, Clayton, and Martinez, as well as the unincorporated areas of areas of Pacheco, Vine Hill, Clyde, the Pleasant Hill BART station, and Saranap.
  San Ramon Valley, including the cities of Danville and San Ramon, and the unincorporated communities of Alamo, Blackhawk, and Tassajara.
• East County, including of the cities of Pittsburg, Antioch, Oakley, and Brentwood, as well as the unincorporated communities of Bay Point, Bethel Island, Knightsen, Discovery Bay, and Byron.