The Bay Area Context


A range of environmental factors, such as geology, ocean and tidal currents, wave action, sediment transport, and species interactions shape the San Francisco Bay shoreline. More recently, intense human modifications, including the filling and diking of wetlands and extensive urbanization have transformed the Bay shoreline and its ecology. These factors and some information on how they have changed over time.

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Geology, topography, and tectonics

From a geologic perspective, the Bay is a very young feature. It formed less than 10,000 years ago, when rising seas entered the Golden Gate - a gap in the outer Coast Range - and filled the range’s interior valleys. The Bay’s varied geology has led to a varied shoreline. In some places steep ancient headlands thrust into the Bay and its deeper waters, leaving little room for Bayland habitats. Elsewhere, wide valleys and alluvial fans have filled with more recent alluvium, creating broad, gently sloping plains with wide intertidal zones occupied by mudflats, marshes, and salt pannes. The hills that frame the Bay generally run parallel to major fault lines, most notably the Hayward and San Andreas faults (the latter of which generated the famous 1906 San Francisco earthquake).

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Watershed processes: freshwater delivery and sediment supply

The Bay is part of an estuary, where salt water from the Pacific meets freshwater flowing down from the Central Valley and dozens of other local streams that fringe the Bay. In total, the water from nearly half of California’s land area ultimately drains into the Bay. These freshwater flows drive important gradients in salinity that extend from the Golden Gate, where the salt content of the water is usually equal to that of seawater, upstream to the headwater rivers and creeks, where the water is fresh. Many physical and biological properties of the landscape - such as channel density and plant community composition--vary with salinity, which makes these gradients an important feature of the Bay. Because the South Bay’s tributaries provide less than 1/10th of the freshwater than the tributaries of the northern embayments,  the salinity in the South Bay is generally higher and more uniform. Evaporation can sometimes even make the South Bay saltier in the summer than the coastal ocean.

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Tidal processes

The Bay experiences mixed diurnal tides, meaning there are two unequal high tides and two unequal low tides every 25 hours. Mean tidal range (the average vertical difference between the highest and lowest tides at a given location) at the Golden Gate Bridge is approximately 5.5 ft. As one moves from there to the Delta along the northern axis of the estuary, tidal range generally decreases. By the time one reaches Sacramento, the tidal range has decreased to about 1 ft.  The opposite happens when one moves from the Golden Gate bridge towards the South Bay. Because the South Bay is a closed basin, tidal range is amplified to 8.5 ft at its southern end. Variation in tidal range and tidal prism - a related measurement of the amount of water moving into and out of an area with the tides - impacts the quantity and quality of intertidal habitats. Tides transport nutrients, sediment, salt, and other materials to and from the baylands; create gradients of moisture and energy; and provide the physical means for fish and other aquatic organisms to move across tidal habitats at high tides.

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Water quality

Pollutants in water and sediment pose a threat to the health and survival of species at all levels of the Estuary’s food web. In an effort to protect them, water quality laws and regulations require that the Estuary be clean enough to support abundant, diverse native communities of plants and animals. However, human activities continue to add contaminants to the ecosystem via municipal and industrial discharges, agricultural and urban runoff, and other pathways. Several pollutants--including mercury, pesticides, and trash--are still problems. Thoughtful urban design, particularly the implementation of green infrastructure, can greatly reduce pollutant loads from the watershed to the Bay. There are a variety of tools available to regional planners to help site these types of projects.   

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Bayland habitats and ecosystems

The wetlands at the shore of the San Francisco Bay are an integral part of the region’s iconic beauty, and they provide numerous benefits for our economy and quality of life. These baylands - the lands touched by the tides, plus the areas that would be in the absence of any levees or other unnatural structures - support abundant wildlife, clean water, open space for recreation, and flood protection.  

The baylands were historically dominated by two primary habitat types: tidal flats (including mudflats, sandflats, and shellflats), which covered 50K acres, and tidal marshes (including salt and brackish marshes), which covered 190K acres. Other important historical baylands habitat types included sandy beaches, marsh pannes, tidal channels, and lagoons. The baylands also had strong connections to deeper subtidal habitats (such as eelgrass meadows, shellfish beds, and shoals) and higher upland habitats (such as riparian corridors, willow groves, moist grasslands, and oak savannas), creating transition zones up to several miles wide that provided critical habitat, resources, and high-tide refuge for many species.

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Historically, the Bay Area has attracted a diverse population - from those who came out west in search of opportunities for a better life like the fortune seekers during the goldrush to the large migration of southern African American shipbuilders during World War II. Today, the majority of the Bay Area's population are non-white as defined by the US Census, which influences growth and is a benefit to regional and local economies.

Recently, the Bay Area economy experienced two economic bubbles - the dot-com technology bubble of the early 2000s and the real estate-based financial crisis of 2008. Since the end of the Great Recession, the region has experienced a robust recovery.

The economic recovery has brought industry expansion, employment opportunities, and income consequences throughout the region. The Bay Area population and demographics are shifting as a result of continuing immigration, workers moving-in to seek opportunities in the region’s expanding economy, and baby-boomers moving into retirement years. Recent housing trends and housing policy shows a shift in the balance of growth from single to multi-family homes and from suburban and rural to urban job centers.

The Bay Area’s diverse population and expansive growth has led to both challenges and opportunities. With the regional economy booming, why are there rising inequalities, stagnant wages, and continuation of racial disparities? In 2015 State of the Region, ABAG shares the Bay Area’s key uncertainties, including the interrelationships of employment change, population shifts, and housing supply.

  • A history of job change driven by innovative but volatile industries.

  • Housing and location choices of a changing population: to what degree will an increasingly urban lifestyle continue to be the choice for aging retirees as well as for today’s young adults as they begin to form families?

  • Meeting the housing needs for a wide spread of income groups: the concentration of occupation growth at both the low and high ends of the spectrum means the region will need housing affordable to households at multiple income levels.

  • Whether new business centers and residential development will concentrate where transit services exist or can be provided.

  • The effects of changing public resources and public policy on the ability of the region to meet the housing demands of growing population and labor force.

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Research has shown that sea level rise continues to threaten coastal infrastructure, homes, and habitats. The Pacific Institute found that 55 inches of sea level rise (near the higher end of projections for 2100) will put almost half a million residents at high risk of flooding and threaten critical infrastructure,including airports, power plants, sewage treatment plants, and 3,550 miles of roads. Risk prevention plans that also protect coastal ecosystems are needed.

Critical infrastructure includes telecommunications, energy, education and healthcare facilities, transportation, and water systems. It includes lifelines or large, geographically distributed networks that need to become operational quickly after a disaster. Critical infrastructure fulfills important socio-economic and civic functions. It is essential to national security and to local community functioning.

However, critical infrastructure may be vulnerable to sea level rise, flooding, and storm surges. Climate change has the potential to directly damage infrastructure via sea level rise, intense precipitation, extreme heat and increases in hurricane intensity. The Federal Emergency Management Agency’s HAZUS, available on STICS, provides critical facility data for: transportation, medical, emergency response, energy, water, schools. The Department of Transportation has the National Transportation Atlas and National Transportation Statistics.

Housing structures have differing vulnerabilities to earthquakes, liquefaction, sea level rise, flooding, and other storm effects. This difference may be due to its design (e.g. new high rise condos vs. mobile homes). Housing structural and demographic data thus indicates household and community resilience. STICS provides housing data from the Decennial Census and American Community Survey. American Housing Survey data and CPD maps (showing Community Development Block Grants, HOME, ACS, and other housing data) are available from the Department of Housing and Urban Development.

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Housing and Displacement

Low income communities already spend a tremendous amount of their income on necessities like housing, food, water and electricity, and it’s likely increase as the costs for basic necessities skyrocket due to climate change impacts. As a result, low income communities are also often the least resourced to respond to and bounce back from everyday stresses like lack of transportation and large shocks like a disruptive earthquake. 

The Bay Area needs more housing, and not just low-income housing, but also  affordable housing for the middle class. The high cost of housing is a threat to everything that makes the Bay Area a great place to live. The region is increasingly becoming unaffordable for people with modest resources. As gentrification grows, the displacement of residents from San Francisco to Oakland, and all around the Bay creates far reaching socio-economic insecurities. At the individual family level it creates barriers to build capacity, find good jobs and schools, and splits family apart. At the regional scale, there is not enough housing for all the people who want to live here and competition for housing is driving prices up — for both renters and homeowners. This challenge inspires Design Teams to consider regulatory systems, income disparities, and a lack of affordable housing.

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