9.3: San Francisco Bay

The San Francisco Bay Area, often referred to as simply the Bay Area, is a populous region surrounding the San Francisco, San Pablo, and Suisun Bay estuaries in Northern California. The Bay Area is defined by the Association of Bay Area Governments to include the nine counties that border the estuaries: Alameda, Contra Costa, Marin, Napa, San Mateo, Santa Clara, Solano, Sonoma, and San Francisco. The core cities of the Bay Area are San Francisco, San Jose, and Oakland.

Home to approximately 7.76 million people, Northern California's nine-county Bay Area contains many cities, towns, airports, and associated regional, state, and national parks, connected by a complex multimodal transportation network. The Bay Area is known for its natural beauty, progressive politics, world-class universities, technology companies, and affluence. The larger federal classification, the combined statistical area of the region which includes fourteen counties, is the second largest in California (after the Greater Los Angeles area), the fifth largest in the United States, and the 41st-largest urban area in the world. The Bay Area's population is ethnically diverse: roughly three-fifths of the region's residents are Hispanic, Asian, African American, or Pacific Islander (with the other two-fifths being non-Hispanic White American), all of whom have a significant presence throughout the region.

Despite its urban character, the San Francisco Bay is one of California's most ecologically important habitats, providing key ecosystem services such as filtering pollutants and sediments from the rivers and supporting several endangered species. In addition, the Bay is known for its stands of coast redwoods, many of which are protected in state and county parks. The region is additionally known for the complexity of its landforms, the result of millions of years of tectonic plate movements. Because the Bay Area is crossed by six major earthquake faults, the region is particularly exposed to hazards presented by large earthquakes. The climate is temperate and generally very mild and is conducive to outdoor recreational and athletic activities such as hiking, running, and cycling. The Bay Area is host to six professional sports teams and is a cultural center for music, theater, and the arts. It is also host to higher education institutions, including research universities such as Stanford University and the University of California, Berkeley. Home to 101 municipalities and nine counties, governance in the Bay Area involves numerous local and regional jurisdictions, often with broad and overlapping responsibilities. Over time, droughts and wildfires have increased in frequency and become less seasonal and more year-round, further straining the region's water security.

The COVID-19 pandemic resulted in many tach career-based people leaving the area to live elsewhere, while working remotely. In fact, it was suggested by some that over 116,000 people moved within the first year of the Pandemic. One of the effects is empty residences, empty professional buildings, increased property rates, and increase homelessness. As of 2024, data is suggesting that the population decline may be leveling off, but the long-term impacts will be better known after the next Census.

Figure 9.4: Location of the San Francisco Bay Region in California. California Bay Area county map by English Wikipedia User GEreikat is in the public domain.

Physical Geography

The Bay Area is in the warm-summer Mediterranean climate zone (Köppen Csb) that is a characteristic of California's coast, featuring mild to cool winters with occasional rainfall, and warm to hot, dry summers It is largely influenced by the cold California Current, which penetrates the natural mountainous barrier along the coast by traveling through various gaps. In terms of precipitation, this means that the Bay Area has pronounced seasons. The winter season, which roughly runs between November and March, is the source of about 82% of annual precipitation in the area. In the South Bay and further inland, while the winter season is cool and mild, the summer season is characterized by warm sunny days, while in San Francisco and areas closer to the Golden Gate strait, the summer season is periodically affected by fog

Due to the Bay Area's diverse topography, the result of the clashing tectonic plates, the region is home to numerous microclimates that lead to pronounced differences in climate and temperature over short distances Within the city of San Francisco, natural and artificial topographical features direct the movement of wind and fog, resulting in startlingly varied climates between city blocks. Along the Golden Gate Strait, oceanic wind and fog from the Pacific Ocean can penetrate the mountain barriers inland into the Bay Area. During the summer, rising hot air in California's interior valleys creates a low-pressure area that draws winds from the North Pacific High through the Golden Gate, which creates the city's characteristic cool winds and fog. The microclimate phenomenon is most pronounced during this time, when fog penetration is at its maximum in areas near the Golden Gate strait, while the South Bay and areas further inland are sunny and dry.

Along the San Francisco Peninsula, gaps in the Santa Cruz Mountains, one south of San Bruno Mountain and another in Crystal Springs, allow marine air into the interior, causing a cooling effect for cities along the Peninsula and even as far south as San Jose. This weather pattern is also the source for delays at San Francisco International Airport. In Marin county north of the Golden Gate strait, two gaps north of Muir Woods bring cold air across the Marin Headlands, with the cooling effect reaching as far north as Santa Rosa. Further inland, the East Bay receives the moderate effects of the ocean, that travels through the Golden Gate strait, and further diffuses that air through the Berkeley Hills, Niles Canyon and the Hayward Pass into the Livermore Valley and Altamont Pass. Here, the resulting breeze is so strong that it is home to one of the world's largest array of wind turbines. Further north, the Carquinez Strait funnels the marine air into the San Joaquin River Delta, causing a cooling effect in Stockton and Sacramento, so that these cities are also cooler than their Central Valley counterparts in the south.

Physical Geology

The Bay Area is well known for the complexity of its landforms that are the result of the forces of plate tectonics acting over millions of years, since the region is in the crux of a meeting point between two plates. Nine out of eleven distinct assemblages of rock types have been identified in a single county, Alameda. Diverse assemblages adjoin in complex arrangements due to offsets along the many faults (both active and stable) in the area. Therefore, many types of rock and soil are found in the region. The oldest rocks are metamorphic rocks that are associated with granite in the Salinian Block west of the San Andreas fault. These metamorphic rocks were formed from protolith of sedimentary rock of sandstones, limestones, and shales in uplifted seabeds. Volcanic deposits also exist in the Bay Area, displaced by the movement of the San Andreas fault, whose movement sliced a subduction plate and allowed magma to briefly flow to the surface

The region has considerable vertical relief in its landscapes, the topography, and geologic history, of the Bay Area can largely be attributed to the compressive forces between the Pacific Plate and the North American plate. The three major ridge structures in the Bay Area, part of the Pacific Coast Range, are all roughly parallel to the major faults. The Santa Cruz Mountains along the San Francisco Peninsula and the Marin Hills in Marin County follow the San Andreas fault, The Berkeley Hills, San Leandro Hills and their southern ridgeline extension through Mission Peak roughly follow the Hayward fault, and the Diablo Range, which includes Mount Diablo and Mount Hamilton and runs along the Calaveras fault.

In total, the Bay Area is traversed by seven major fault systems with hundreds of related faults, all of which are stressed by the relative motion between the Pacific Plate and the North American Plate or by compressive stresses between these plates. The fault systems include the Hayward Fault Zone, Concord-Green Valley Fault, Calaveras Fault, Clayton-Marsh Creek-Greenville Fault, Rodgers Creek Fault, and the San Gregorio Fault. Significant blind thrust faults (faults with near vertical motion and no surface ruptures) are associated with portions of the Santa Cruz Mountains and the northern reaches of the Diablo Range and Mount Diablo. These "hidden" faults, which are not as well known, pose a significant earthquake hazard Among the more well-understood faults, as of 2014, scientists estimate a 72% probability of a magnitude 6.7 earthquake occurring along either the Hayward, Rogers Creek, or San Andreas fault, with an earthquake more likely to occur in the East Bay's Hayward Fault. Two of the largest earthquakes in recent history were the 1906 San Francisco earthquake and the 1989 Loma Prieta earthquake.

Cultural Geography

The earliest archaeological evidence of human settlements in the Bay Area dates to 8000–10,000 BCE (from shell mounds in the Coyote Hills). In 1769, the Bay Area was inhabited by Ohlone people when a Spanish exploration party led by Gaspar de Portolá entered the Bay – the first documented European visit to the Bay Area. After Mexico established independence from Spain in 1821, the region was briefly controlled by the Mexican government until the United States seized the territory in 1846 during the Mexican American War. Soon after, discovery of gold in California attracted a flood of treasure seekers, many using ports in the Bay Area as an entry point. During the early years of California's statehood, state legislative business rotated between three locations in the Bay Area before a permanent state capital was established in Sacramento. A major earthquake leveled the city of San Francisco and environs in 1906, but the region quickly rebuilt in time to host the 1915 Panama-Pacific Exposition. In fact, a lot of the debris from the 1906 earthquake was pushed offshore to help create a man-made extension of the San Francisco Harbor. During World War II, the Bay Area played a major role in America's war effort in the Asiatic-Pacific Theater, with the San Francisco Port of Embarkation, of which Fort Mason was one of fourteen installations and location of the headquarters, acting as a primary embarkation point for American forces. In 1945, the United Nations Charter was signed in San Francisco, establishing the United Nations, and in 1951, the Treaty of San Francisco re-established peaceful relations between Japan and the Allied Powers. Since then, the Bay Area has experienced numerous political, cultural, and artistic movements, developing unique local genres in music and art, and establishing itself as a hotbed of progressive politics. Economically, the post-war Bay Area saw huge growth in the financial and technology industries, creating an economy with a gross domestic product of over $700 billion, and home to the third-highest concentration of Fortune 500 companies in the United States (as of 2018).

According to the 2010 United States Census, the population of the nine-county Bay Area was 7.15 million, with 49.6% male and 50.4% female. The Bay Area cities of Vallejo, Suisun City, Oakland, San Leandro, Fairfield, and Richmond are among the most ethnically diverse cities in the United States.

Non-Hispanic whites form majorities of the population in Marin, Napa, and Sonoma counties. Whites also make up the majority in the eastern regions of the East Bay centered around the Lamorinda and Tri-Valley areas. San Francisco's North Beach district is considered the Little Italy of the city and was once home to a significant Italian-American community. San Francisco, Marin County and the Lamorinda area all have substantial Jewish communities.

The Latino population is spread throughout the Bay Area, but among the nine counties, the greatest number live in Santa Clara County, while Contra Costa County sees the highest growth rate. The largest Hispanic or Latino groups were those of Mexican (17.9%), Salvadoran (1.3%), Guatemalan (0.6%), Puerto Rican (0.6%) and Nicaraguans (0.5%) ancestry. Mexican Americans make up the largest share of Hispanic residents in Napa county, while Central Americans make up the largest share in San Francisco, many of whom live in the Mission District which is home to many residents of Salvadoran and Guatemalan descent.

The Asian-American population in the Bay Area is one of the largest in the United States. Asian-Americans make up the plurality in two major counties in the Bay Area: Santa Clara County and Alameda County. The largest Asian-American groups were those of Chinese (7.9%), Filipino (5.1%), Indian (3.3%), Vietnamese (2.5%), and Japanese (0.9%) heritage. Daly City has the highest proportion of Asian-American residents (58.4%) in a large U.S. city outside of Hawaii Asian Americans also constitute a majority in Cupertino, Fremont, Milpitas, Union City and significant populations in Dublin, Foster City, Hercules, Millbrae, San Ramon, Saratoga, Sunnyvale, and Santa Clara.

Pacific Islanders such as Samoans and Tongans have the largest presence in East Palo Alto, where they constitute over 7% of the population.

The African-American population of San Francisco was formerly substantial, had a thriving jazz scene and was known as "Harlem of the West." While black residents formed one-seventh of the city's population in 1970, today they have mostly moved to parts of the East Bay and North Bay, including Antioch, Fairfield and out of the Bay Area entirely. The South Park neighborhood of Santa Rosa was once home to a primarily black community until the 1980s, when many Latino immigrants settled in the area.

The Bay Area is the wealthiest region in the United States, due, primarily, to the economic power engines of San Jose, San Francisco, and Oakland. The Bay Area city of Pleasanton has the second-highest household income in the country after New Canaan, Connecticut. However, discretionary income is very comparable with the rest of the country, primarily because the higher cost of living offsets the increased income.