These exercises were created by Dr. Karen Grove (© 1998 Grove) for use in the Introduction to Oceanography course offered by the Department of Geosciences at San Francisco State University. Other educators are welcome to use these exercises in their own classes. Please send comments to kgrove@sfsu.edu. Course home page: http://geosci.sfsu.edu/courses/geol102/home.html Commercial use is prohibited.

VOYAGE 7: Fresh meets salt: the water of San Francisco Bay

Instructions: First print copies of Voyage 7, Lag-time questions, and Real-time questions. Read the text and examine the images. Write answers to the questions on your copies. When you are satisfied with your answers, type them into the computer and submit. Keep the copies as study guides.

Don't forget to check out the course web site where you can obtain preparatory information before each class and summaries that are posted after the class meets. Click here to bring up a new browser window with the course site. You can also use this site to display the text and images part of the voyage on a second window so that you don't have to print anything.


Part I: Background on estuaries and San Francisco Bay

Estuaries such as San Francisco Bay are oceanic embayments where fresh water from rivers flows into and mixes with salt water from the ocean. This mixing of fresh and salt water creates a unique environment with an abundance of life-supporting nutrients from the land and from the ocean.

To understand how estuaries work, we must understand how fresh and salt water circulates and mixes. The details of this mixture has a huge impact on which organisms can live in an estuary, and on how toxic substances are retained or flushed out. Because salt water is denser than fresh water, estuaries tend to contain fresher water at their surfaces and saltier water at their bottoms. Because fresh water enters into estuaries from rivers, and salt water from the ocean, estuaries tend to contain fresher water at their landward ends and saltier water at their seaward ends.

Figure 1. San Francisco Bay is divided into sub-bays, as shown on this map. Fresh water from the Sacramento and San Joaquin Rivers flows into the Delta region shown on the east (right) side of the map. Water then flows into Suisun Bay, San Pablo Bay, and finally into the Central Bay and out to the ocean through the Golden Gate. Fresh water enters the South Bay only during very large flow periods and and when rain produces runoff from local streams in winter. The fresh water mixes with salt water that enters into the Bay from the west, through the Golden Gate. The sampling stations on this map (yellow circles) are locations where water samples have been collected by the U.S. Geological Survey (data below).

Figure 2. The watershed of San Francisco Bay (green area on map) includes about 40% of California and even extends northward into Oregon. 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 1).


Part II: The salinity of water in San Francisco Bay

The salinity of Bay water is a measure of the relative proportion of fresh water and sea water. Salinity varies dramatically in space (that is, at different places in the Bay), and in time (that is, in different seasons and in different years).

Figure 3. These diagrams show how salinity varies with time, throughout the year, at the Golden Gate (mouth of the Bay). The horizontal axis shows months of the year from October to October (O). On the upper diagram, the vertical axis shows Delta discharge; the higher the number, the more fresh water is flowing into the Bay from the watershed and Delta region. On the lower diagram, the vertical axis shows the salinity of Bay water at the Golden Gate.

Salinity is measured in practical salinity units, psu, which are about equal to parts per thousand, ppt. Thus 25 psu is about 25 ppt, or 2.5 percent (parts per hundred). Whereas salinity in the Bay varies between 0 and 35 ppt, the salinity of open ocean water (outside the Golden Gate) only varies between about 33 and 35 ppt.

Figure 4. Since 1968, scientists at the U.S. Geological Survey have been measuring water quality several times each month along the entire length of the Bay (stations shown in Figure 1), and from the surface to the bottom of the Bay. This map shows the transect along which the water samples are collected.

Apr. 4, 1995

Sept. 21, 1995

Figure 5. The diagrams above show salinity data collected in the Bay by the USGS in spring (April 1995) and fall (September 1995) of the same year. Colors correspond to salinity values shown in the scale bar (0 ppt-35 ppt). The yellow line on each diagram indicates a salinity value of 2 ppt, considered to be the boundary between fresh and salt water.

The bottom two diagrams in Figure 5 are profiles that show salinity variations along the length of the Bay and from the surface to the Bay bottom. The black area on each diagram is the Bay bottom. The irregularity is variation in depth throughout the Bay, although vertical exaggeration is large, making slopes look much, much steeper than they really are. Data correspond to the transect in Figure 4, extending from the south end of South San Francisco Bay (left side) to the Sacramento River Delta (right side). The deep area near the diagram's center is the Central Bay. In the top diagram the yellow line is located at Carquinez Strait; in the bottom diagram the yellow line is located in Suisun Bay (see Figure 4 for locations). These diagrams show variations at different locations in the Bay and at different times of year.

Apr. 4, 1995

Apr. 19, 1994

Figure 6. The diagrams above show how salinity can vary from one year to the next. The top diagram is the same as in Figure 5 (April 4, 1995). The bottom diagram shows data from April of the preceding year (April 1994).

Optional Image:

Figure 7. This diagram is an excellent summary of salinity variation in both space and time. It shows only the salinity at the Bay's surface (colors differ from those in Figures 5 and 6). The horizontal axis shows salinity variation for three years of time (1993-1995). The vertical axis shows data along the transect of Figure 4 (located on the left axis by distance north or south of Angel Island). The black line is the 2 ppt salinity line (like the yellow line in Figures 5 and 6). Ignore the circled numbers. The curve at the top, called Delta Outflow, shows how much fresh water flowed into the Bay through the Delta during this 3-year period. If you can interpret this diagram, you should pat yourself on the back (and consider becoming an oceanographer!!).

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