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).
Alameda County is governed by an elected Board of Supervisors.
The county's law enforcement is overseen by an elected county sheriff and an elected district attorney. The sheriff supervises a force of deputies whose primary responsibilities include policing unincorporated areas of the county, and providing security for county buildings including courthouses, the county jail and other properties. The County Sheriff also serves various court writs and warrants.
The San Francisco Bay borders the county on the west. The crest of the Berkeley Hills form part of the northeastern boundary, and reach into the center of the county. A coastal plain several miles wide lines the bay, being home to Oakland and the most populated regions. Livermore Valley lies in the eastern part of the county.
The Hayward Fault, a major branch of the San Andreas Fault to the west, runs through the most populated parts of Alameda County, while the Calaveras Fault runs through the southeastern part of the county.
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In 2005 Alameda County had a population that was 38.0% non-Hispanic whites. African-Americans constituted 13.8% of the population. Asians were 24.2% of the population. Hispanics came in at 20.8%, while both Native Americans and Pacific Islanders came in at 0.7% of the population.
In 2000 there were 523,366 households, out of which 32.60% had children under the age of 18 living within them, 47.00% married couples living together, 13.00% had a female householder with no husband present, and 35.20% were non-families. 26.00% of all households were made up of individuals and 7.30% had someone living alone who was 65 years of age or older. The average household size was 2.71 and the average family size was 3.31.
In the county the population was spread out with 24.60% under the age of 18, 9.60% from 18 to 24, 33.90% from 25 to 44, 21.70% from 45 to 64, and 10.20% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 96.60 males. For every 100 females age 18 and over, there were 94.00 males.
The median income for a household in the county was $55,946, and the median income for a family was $65,857. Males had a median income of $47,425 versus $36,921 for females. The per capita income for the county was $26,680. About 7.70% of families and 11.00% of the population were below the poverty line, including 13.50% of those under age 18 and 8.10% of those age 65 or over.
There is a major Portuguese community, from where Tom Hanks' mother came and where Lyndsy Fonseca was raised for some time.
In the State Assembly, parts of the 14th and 15th districts, all of the 16th and 18th districts, and most of the 20th district are in the county. Four of its five legislators are Democrats: Loni Hancock of the 14th district, Sandré Swanson of the 16th district, Mary Hayashi of the 18th district, and Alberto Torrico of the 20th district. The lone Republican that represents Alameda County is Guy Houston of the 15th district. In the State Senate, most of the 9th and 10th districts are in the county and are both represented by Democrats: Don Perata and Ellen Corbett, respectively.
According to the California Secretary of State, there are 709,414 registered voters in Alameda County. 401,847 (56.6%) are registered Democrats, 116,864 (16.5%) are registered Republicans, 33,689 (4.8%) are registered to minor political parties, and 157,014 (22.1%) declined to answer. This means Democrats have a 40.1% registration advantage over Republicans. Every city, town, and unincorporated area in Alameda County has more registered Democrats than Republicans.
| Year | DEM | GOP | Others |
|---|---|---|---|
| 2004 | 75.2% 422,585 | 23.3% 130,911 | 1.5% 8,594 |
| 2000 | 69.4% 342,889 | 24.1% 119,279 | 6.5% 32,168 |
| 1996 | 65.8% 303,903 | 23.1% 106,581 | 11.2% 51,560 |
| 1992 | 63.0% 334,224 | 20.6% 109,292 | 16.3% 86,629 |
| 1988 | 64.8% 310,283 | 34.0% 162,815 | 1.2% 5,899 |
| 1984 | 58.7% 282,041 | 40.0% 192,408 | 1.3% 6,425 |
| 1980 | 48.3% 201,720 | 38.0% 158,531 | 13.7% 57,366 |
| 1976 | 57.9% 235,988 | 38.1% 155,280 | 4.0% 16,413 |
| 1972 | 55.0% 259,254 | 42.8% 201,862 | 2.1% 10,079 |
| 1968 | 53.9% 219,545 | 37.6% 153,285 | 8.5% 34,519 |
| 1964 | 66.4% 283,833 | 33.5% 142,998 | 0.1% 509 |
| 1960 | 54.0% 217,172 | 45.6% 183,354 | 0.4% 1,474 |