Sediments are an important component of San Francisco Bay because they transport adsorbed toxic substances, provide habitat for benthic organisms, limit light availability and photosynthesis, and deposit in ports and waterways which require dredging.
When most people look at San Francisco Bay, they see a dirty green color that is a strong contrast to the clear blue waters found in parts of the ocean such as the tropical Caribbean Sea. Whereas the visibility in clear tropical waters can be 8 meters (25 feet) or more, the visibility in San Francisco Bay is rarely more than 1 meter (3 feet). This lack of clarity results from lots of stuff in the water, namely plankton that floats and sediments that are suspended.
Most of the sediment in the Bay comes from erosion of rocks in the Sierra Nevada Range. It is transported by streams and rivers into the Delta region and then into the Bay (Figure 2). Some sediments sink to the bottom, where they collect as Bay mud. Because most of Bay sediment is very small, mud-sized particles, it can remain suspended in the water column for many days (Figure 3A). Tidal currents daily stir up Bay water, which helps to keep sediments suspended and also resuspends sediments that were previously deposited (Figure 1).
Figure 1. These data were collected for 8 days, starting October 6, 1994, with an instrument mounted on the Bay bottom near the San Mateo Bridge in the South Bay (for more information about instrumentation, see the USGS web site). The technique is being used in San Francisco Bay to investigate the transport of particulate matter. The resuspension and movement of particles within the Bay's waters largely determine the eventual sites of accumulation of silt and clay within harbors, movement and deposition of contaminants, and water quality.
The top graph shows variations in water depth (in meters), which change daily with the tide.
1. Describe the tidal variation during this 8-day period. Are they neap tides, spring tides, or is there a change from one type to another? What is the tidal range during this time period, in meters? in feet (1 m = 3.3 ft)? (Refer to Part IIIA, if you need a refresher course about tides.)
The second graph from the top shows current vectors measured at 123 cm above the Bay bottom and plotted hourly. Ebb direction is toward the top of the graph. The maximum hourly current is in the ebb direction at about 85 cm/s.
2. Describe the correlation between the currents and the tidal variation in the top graph, in terms of flow direction and flow speed.
The third graph from the top shows near-bottom water temperature, which has a tidal variation during the first part of the record, but shows a general cooling with pulses of colder water at the site during the last 3 days. The bottom graph shows suspended sediment concentration. The range of concentration of 25 to 120 mg/l during the early part of the record (until day 285) are typical for this part of the Bay. The pulses of higher concentrations (up to 300 mg/l) after day 285 do not represent locally resuspended materials because the bottom currents are lower during neap tides.
3. Speculate about what happened in the South Bay during days 285 to 287 to create pulses of colder water and increased amounts of sediment in the water.
Figure 2. The data above were collected near Mallard Island in the Sacramento-San Joaquin River Delta during a winter storm in 1996 (for more information about the project, see the USGS web site). Measurements were made near the water surface (blue line) and near the sediment bottom (pink line). Daily fluctuations in suspended sediment concentrations correspond to the daily tidal cycle. The larger increase, from about January 10-24, reflect a pulse of river water that flowed through the site from the mountains and Central Valley on its way out to the Bay.
Observe the daily fluctuations that result from the back and forth action of the tides.
4. When during the tidal cycle do you think sediments would be most likely to be deposited: during flood and ebb tides or during the slack tide? When would sediments most likely to be lifted into suspension?
5. In the monthly cycle of tides, do you think sediments would be most likely to be deposited during times of neap tides or spring tides? During which time would sediments be most likely to be lifted into suspension?
Figure 3A. This photograph from the Landsat satellite was taken during a spring following a very wet winter (1983), when lots of fresh water was entering into the Bay through the Delta region. The light-colored areas of the Bay and open ocean are places where the water depth is shallow and where sediments are suspended in the water . During times of high fresh water flow through the Bay, sediments can be flushed from the Bay and out into the open ocean. The Delta region is the triangular area east of the Bay proper. Along with fresh water, sediments eroded from the mountains flow downstream in rivers, then into the Delta and Bay.
Figure 3B. This photograph from the Landsat satellite was taken during a spring following a dry winter (1986), when a smaller amount of fresh water was entering into the Bay through the Delta region. The light-colored areas are here primarily places where the water depth is shallow. For example, in the Bay, the deep tidal channels appear as dark blue and the shallow tidal flats appear as light blue. The ebb-tide delta of shallow water just outside the Golden Gate also appears as light blue. Areas that are heavily vegetated appear on this image as red coloration. Urban areas appear whitish.
In Part II we learned that the Bay is a young geologic feature that formed as sea-level rose during the past 18,000 years. As the rate of rise slowed during the past several 1000 years, sediments began to accumulate and broad shallow tidal flats of mud were created.
5. Given the geologic history of the Bay and evidence from the photos above, what is the ultimate fate of estuaries such as San Francisco Bay, if sea level continues rising at a slow rate? If global warming is a reality, however, the rate of sea-level rise may increase and the size of the Bay may begin to expand once again.
Most sediment in the Bay comes from the Sierra Nevada Range, but a small amount comes from the Coastal Ranges that surround the Bay. These ranges were created during the past several million years by folding and faulting on structures such as the San Andreas and Hayward faults. Examine Figure 1B, where these two major faults show up clearly.
6. Draw a sketch showing the Bay shape and the locations of the San Andreas and Hayward faults. Hints: (a) The San Andreas is located between the Bay and the Pacific Ocean and has several lakes located along it. South of the Bay it curves toward the east. (b) The Hayward fault is just east of the Bay and is located near the abrupt color change between whitish urbanized areas along the Bay and the more vegetated (red in the image) slope of the East Bay hills.