PART II: The origins and watershed of San Francisco Bay

San Francisco Bay's History

Like most estuaries, San Francisco Bay is a very young feature, geologically speaking. Twenty thousand years ago there was no bay. At that time the world was in the grip of the last ice age, and much of the planet's water was frozen into glaciers that covered a large part of the northern continents. With less water to fill the oceans, sea level was nearly 150 meters (over 400 feet) lower and the Pacific coastline was 30 km (20 miles) west of where it lies today. Imagine having to travel all the way to the Farallon Islands to go walking on the beach or surfing in the ocean! The Bay itself was dry land, with rivers running through the low-land areas on their route to the sea.

As the glaciers melted over centuries, the ocean waters rose and the shoreline crept back eastward, toward land. By 10,000 years ago the ocean had spread inland through a gap in the outer Coast Ranges that we know today as the Golden Gate (Figure 1), and seawater began to fill the Bay. For thousands of years, sea level rose rapidly at nearly 2.5 cm (one inch) per year, advancing the shoreline progressively inland. Several thousand years ago, the rate of rise slowed and sediments began to accumulate in the shallows faster than the sea could cover them. These sediments supported the expansion of tidal mudflats and marshes along the Bay's shores, whose vast extent was recorded in the last century, before modern civilization began to reshape the Estuary. We will look at the effects of human modifications on the Estuary in subsequent parts of this exercise.

Figure 1. These diagrams show how sea-level has risen during the past 15,000 years. The blue color indicates ocean water and the white color indicates land. The present-day shoreline is shown as a black line for reference. The black dots on the shelf west of San Francisco are the Farallon Islands.


1. Predict what will happen to San Francisco Bay in the future, as sediment continues to fill in this lowland area.

2. At this time, sea level is continuing to rise at a slow rate. What would happen to San Francisco Bay if sea level began to fall as the planet went into another ice age?

In fact, sediments beneath San Francisco Bay record successive cycles of rising and falling sea level during the past 2 million years.

Suggested classroom activity:

3. Obtain a topographic map (with elevation contours) of an area along the edge of the Bay. Today sea level is rising at about 2.5 mm/yr (0.1 inch/yr). Assuming a constant rate of sea-level rise, calculate how much higher sea level would be in 1000 years (you will probably want to use the inch/yr rate as topographic maps in the U.S. are usually in feet). On your topographic map, color in the area now above water that would be covered by Bay water in 1000 years.

4. Many scientists think that the planet is warming because of human activities associated with burning fossil fuels such as oil. If the planet warms, more polar ice will melt and ocean water will expand, causing the rate of sea level rise to increase. Briefly discuss the implications of sea-level rise on coastal communities, such as the San Francisco Bay Area.

The Parts of San Francisco Bay and its Watershed



Figure 2. San Francisco Bay is divided into several sub-bays, shown in this map. Water from the Sacramento and San Joaquin Rivers flows into Delta region, then into Suisun Bay, San Pablo Bay, and finally into the Central Bay and out the Golden Gate. When river flow is great, during winter and spring, fresh water can also flow into the South Bay. During winter, the South Bay also receives small amounts of fresh water run off from local streams that flow into the Bay. During other times of the year, the South Bay receives little fresh water. The northern parts of the Bay receive less fresh water in the summer and fall, but some flow does continue.



The watershed of San Francisco Bay covers 40% of California and extends north into Oregon (Figure 3). Nearly half of the surface water in California starts as rain or snow that falls within the watershed and flows downstream toward the Bay. San Francisco Bay is one of the largest estuaries in the country and also one of the most altered by human activities. Nearly half of the water flowing toward the Bay is diverted for use on farms, in homes, and in factories.


Figure 3. Map of the San Francisco Bay watershed (in green). Major rivers are shown as light blue lines. Diagram from the EPA National Estuary Program web site for San Francisco Bay, which contains other details about the Bay.

3. Think about the climate of California---when there are periods of rain and dry weather. How might the climate affect the variation of fresh water into the Bay throughout the year?


Tributaries in this vast watershed feed into the major rivers of the Sacramento and San Joaquin, which flow through the Central Valley and enter into the Bay system through the Delta region (Figure 3). The Delta is a large triangle of interconnected sloughs and agricultural "islands" that forms a key link in California's water delivery system. Some of the fresh water flows through the Delta and into Bay, but much is diverted from the Bay. Enormous pumps at the southern end of the Delta suck water into the canals of the state and federal water projects, to be carried to San Joaquin Valley farmers and the distant cities of southern California. Running east to west across the Delta are the pipelines of local water agencies, which deliver Sierra Nevada water to cities of the Bay Area.


4. What possible impacts might the diversion of fresh water have on San Francisco Bay?

Click here to read an article from the San Francisco Chronicle about a law suit against the U.S. Department of the Interior that relates to the impact of water diversion for the Central Valley Water Project.

5. Summarize the issue discussed in this article.


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