Bay Area Regional Background





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 are summarized below.

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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.


With pronounced decreases in sediment supply over the last 150 years, the Bay is now generally considered to be sediment starved. This is problematic, because sediment carried into the Bay by rivers and creeks provides substrate for marsh development and is an important component in the transport of nutrients within the Bay ecosystem. Though the majority of freshwater (close to 95%) delivered to the Bay comes from the Central Valley, in recent years the majority of suspended sediment (>60%) has come from the smaller local tributaries. This represents a shift from historical conditions, when the Bay’s suspended sediment supply was dominated by contributions from the Central Valley (a change largely driven by the exhaustion of sediment flowing through the system unearthed during the Gold Rush, dam construction, and river armoring). This shift increases the need to rethink the interfaces between local creeks and Bayland habitats, as well as how we manage dredged sediment, in order to promote the delivery of precious sediment to where it is needed most.

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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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General- Tides: with particular reference to San Francisco Bay, currents, and rowing

Detailed- Open Water Processes of the San Francisco Estuary: From Physical Forcing to Biological Responses


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 50,000 acres, and tidal marshes (including salt and brackish marshes), which covered 190,000 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. Between 1800 and 1998, 79% of tidal marsh and 42% of tidal flats were lost to diking and filling, and the baylands are now otherwise dominated by urban and agricultural areas, diked wetlands, managed ponds (including industrial salt ponds). The remaining tidal marshes have generally become more fragmented and isolated, arranged in smaller patch sizes than were found historically with less “core” habitat, situated farther from other patches, and separated by leveeslevieseed off from upland habitats. These changes in habitat configuration likely reduce the quality of habitat for wildlife, compounding the problem of overall habitat loss.


Habitat loss and degradation is worrisome because the baylands provide some form of food, shelter, or other benefits to approximately 500 species of fish, amphibians, reptiles, birds, and mammals, and at least as many invertebrate and plant species. At least 90 species of plants and animals found in the nine counties that border the Bay are endemic. At least 90 species living in and around the Bay are also listed as threatened or endangered under the Endangered Species Act. Among the most iconic of these species are the Ridgway’s Rail and Salt Marsh Harvest Mouse-- the conservation of these two species motivated much of the initial efforts to preserve and restore bayland habitats. The Bay is also a key location on the Pacific Flyway for migratory birds and a nursery for dungeness crab, halibut, and Pacific salmon fisheries.


Although vast wetland areas have been lost since the 1800s, Bay wetland restoration efforts have significantly progressed since the inception of the 1999 Baylands Ecosystem Habitat Goals Project (Goals Project) and more recent 2015 Baylands Ecosystem Habitat Goals Science Update (Science Update), which recommended reestablishing 100,000 acres of tidal wetlands in the Bay. Since the publication of the Goals Project, approximately 12,000 acres have been restored and an additional 30,000 acres are in the process of being restored. The Science Update identifies the latest scientific findings and recommended actions to support continued restoration and ecological enhancements in the face of increasing challenges from climate change and other urban stressors. Following the key guiding principles of the Goals Project, resilient ecological systems should be self-maintaining and highly functional; support native species over non-natives; and prioritize biological communities over individual species. Measure AA funding will further Bay wetland restoration and work towards improving the ecological integrity of the baylands.

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Economy, Technology and Displacement

The Bay Area region is a cornerstone of the global marketplace.  Silicon Valley has attracted the most intelligent...


It is also host to thousands of small businesses that help communities and residents thrive. Economic data help us understand the regional economy’s reliance on its infrastructure and marine and coastal resources. Achieving resilience means that critical economic assets, business operations, agricultural production, and supply chains are preserved or quickly restored to function.


Resilience increases as the diversity, quantity, and equitable distribution of economic resources increases. Economic data can help us identify who is socioeconomically vulnerable to coastal disasters. STICS compiles individual, household, and regional economic data from the Decennial Census, American Community Survey, Bureau of Economic Analysis and Bureau of Labor Statistics for Coastal Shoreline Counties. ENOW provides economic indicators on business establishments, employment, wages, and GDP for coastal counties. National Ocean Economics program provides data on ocean and coastal markets, tourism and recreation, ports and cargo, natural resources, population and housing, non-market values, and public expenditures. Human Development Index for States provides a composite indicator of states’ economic development and well-being. Small Business Administration links to business, trade, GDP, and other economic data. Innovation in American Regions presents innovation indicators for state, counties, and metro regions.


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Public Health and Food Justice

Where can design of the built environment have the greatest impact on public health along Bay Area shoreline? The places where we live and work, and the choices we make to get around the Bay Area, and access to food have a substantial impact on our health and well-being.


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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, transportation, and water. 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 surge. 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 indicate 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 of color already spend as much as 25% of their income on necessities like housing, food, water and electricity, and this number will likely increase as the costs for basic necessities increase due to climate change impacts. As a result, these communities are also often the least resourced to respond to and prepare for climate change.


The Bay Area needs more housing, and not just low-income housing and affordable housing 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. At the regional scale, there is not enough housing for the people who want to live here and competition for housing is driving prices up — for both renting and owning. This challenge inspires Design Teams to consider regulatory systems, income disparities, and a lack of affordable housing.


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