Often, when planning activities on or along the Bay, the currents generated by the tide are more important than the water level. For example, when boating it is helpful to travel with the direction of the current. In general, one can predict that when the tidal level is changing from low to high, a flooding current will move water inland from the Golden Gate. When the tidal level is changing from high to low, an ebbing current will move water from inside the Bay toward the Golden Gate.
To help people who navigate on the Bay predict the currents more precisely, the U.S. Geological Survey (USGS) and the National Oceanography and Atmospheric Administration (NOAA) have set up a San Francisco Physical Oceanographic Real-Time System (SFPORTS). This site contains continually updated data that are measured by instruments deployed within the Bay. SFPORTS is designed to provide crucial information in real time to mariners, oil spill response teams, managers of coastal resources, and others about San Francisco Bay's water levels, currents, salinity, and winds. To find out more about SFPORTS, click here.
Measurements are made at 5-10 locations within the Bay. Click here to see the map with measurement locations.
Figure 1. Actual measurements are input into a computer model of the Bay. The model is used to generate maps of current direction and speed throughout the Bay, not just at the measurement locations. This map illustrates the depth variations throughout the Bay that are used in the computer model.
Click here to go to the map above on the SFPORTS web site. You may click on any of the colored rectangles to get the most recent model for tidal current direction and speed in the different parts of San Francisco Bay.
The diagrams below show some data obtained from the SFPORTS site during rising (flooding) and falling (ebbing) parts of the tidal cycle. Shown are the actual tidal level and the model of current direction and speed generated for that time.
Figure 2A. This plot was collected at 9.30 on Aug. 5 during a rising (flooding) tide. The full moon was on Aug. 7, so the tides were were increasing toward spring-type large ranges. Notice that there is a small difference between the tidal level listed in the tidal tables (as in those linked to above) and the actual water level. The differences can be caused by local effects such as landward-blowing wind or temperature changes in the water.
Figure 2B. This model of the Central Bay was generated from measurements collected during the part of the cycle illustrated in Figure 2A. The arrows are vectors; that is, their orientation shows the direction of current flow, and the length/size of the arrow shows the speed of the current. The arrow in the upper right corner has a speed of 4 knots (about 2 meters/sec or 4.5 feet/sec). Smaller arrows have proportionally smaller speeds.
1. Describe the direction of the current during the flooding part of the tidal cycle. How does it change as it flows into the Bay?
2. In which parts of the Bay do the currents move the fastest? In which parts do they move more slowly? If you were in a small boat trying to move against the current from the north part of the Bay toward the central Bay, where would you choose to travel?
3. If you were swimming or surfing at Ocean Beach, would the currents have an impact on the movement of the water there? What happens to the strength of the current as you move south along Ocean Beach?
Figure 3A. This plot was collected 6 hours after Figure 2A, at about 15:30 (3:30 PM) on Aug. 5 during a falling (ebbing) tide. Although the tidal range was large during this time of month (spring tides), the larger fall was in the middle of the night, when the currents would have been stronger.
Figure 3B. This model of Central Bay currents was generated from measurements collected during the part of the cycle illustrated in Figure 3A.
4. Compare the current directions in Figure 3B with the current directions in Figure 2B.
5. Compare the variation in current speed in Figure 3B with that in Figure 2B.
6. What do you think happened to the current direction and speed at the Golden Gate at about noon, between the times of Figure 2A and 3A?
7. If you wanted to sail your boat from St. Francis Yacht Club (along the north side of San Francisco) to the port of Oakland (on the east side of the Bay), what time would you choose to set sail to take best advantage of the tidal currents?
Click here to go to the SFPORTS web site and see what the tides are doing in San Francisco Bay now. Click on Get Data (left side of window) to obtain a plot of the tidal cycle (as in Figures 2A and 3A).
8. What is the date and time of the plot? Which part of the tidal cycle is illustrated?
Click here to get a plot that shows measurement locations of tidal currents around the Bay. The left side of each plot shows the direction and speed of the tidal currents at each location; the right side shows the water level, with arrows pointing up for flooding currents and arrows pointing down for ebbing currents. A horizontal arrow shows a slack current.
9. Explain the variation in current speed and direction in different parts of the Bay. Pay particular attention to the variation between the Golden Gate and the location in Suisun Bay; if there is a difference, why is that so?
Click here to bring up the bathymetric model of the Bay. Click on any of the colored rectangles to view a model of current variations throughout the Bay.
10. Describe any interesting features you observe.
All plots were downloaded from the SFPORTS web site.