Tidal Ties
Rising and falling tides have a big influence on water conditions in an estuary like the San Francisco Bay. By examining water-depth measurements, you can make predictions about water temperature and salinity, and reveal some of the natural cycles in the Bay.
Note: The data used in this Snack was collected at the Exploratorium’s Wired Pier Environmental Field Station, located on Pier 15, at the northeast corner of the city of San Francisco. From there, we measure water depth and track many other parameters of water quality, including salinity and temperature. Familiarity with the Field Station’s location will help you with this activity.
- Map of San Francisco Bay
- Printouts of data from the Exploratorium’s Environmental Field Station:
- Colored markers
None needed.
Take a look at the map of the San Francisco Bay (click to enlarge the image below). Find the bay, the Pacific Ocean, and the Sacramento/San Joaquin River Delta. From where do you think the water in the bay comes?
Examine a copy of the Water Depth graph. What is being measured? Where do you think the measurements were taken? What time period do they cover? What patterns do you see in the data? How would you explain these patterns?
Based on the pattern of the Water Depth graph, what do you think a graph of salinity (saltiness) would look like for the same time period? How might the tide affect salinity? Use a marker to draw a predicted graph of salinity on top of the Water Depth graph. (Units are not important here—we're most interested in how the peaks and valleys on the salinity graph relate to high and low tides.) Why did you draw your graph the way you did?
Look at the printed Water Depth and Salinity graph. Does it look like the graph you predicted? The ocean is the water source for high tides in the Bay, and it’s also the source of its salty water. That’s why salinity tends to be higher when the tide (water depth) is high, and lower when the tide is low.
Based on the pattern in the Water Depth graph, what do you think a water-temperature graph would look like for the same time period? How might the tide affect water temperature? Use a different color marker to draw a predicted graph of water temperature on the second copy of the Water Depth graph. (Again, units are not important here.) Why did you draw your graph the way you did?
Look at the printed Water Depth and Temperature graph. Does it look like the graph you predicted? What do you notice about the relationship between water depth and temperature?
The water in the San Francisco Bay is a mix of fresh water from the Sacramento/San Joaquin Rivers and salt water from the Pacific Ocean. The ocean tides control the water depth at Pier 15. Sometimes, changes in atmospheric pressure or the direction of surface winds can make water levels higher or lower than predicted tide heights. For example, storm surges, which are associated with low atmospheric pressure, can cause tidal heights to be higher than predicted.
The tides in this part of the world have a mixed semidiurnal cycle—two high tides and two low tides each day, with one high/low cycle being more extreme than the other. It’s the motion of these semidiurnal tides that causes the pattern you see on the Water Depth graph.
The tides drive a huge flow of ocean water in and out of the Golden Gate (the opening between San Francisco Bay and the Pacific Ocean). Because Pier 15 is relatively close to the Golden Gate, our data shows a large pulse of cold, salty water with each high tide.
The ocean is the water source for high tides, and ocean water is colder than bay water. That’s why the water is colder when the tide (water depth) is high, and warmer when the tide is low. Since ocean water is also saltier than bay water, the data at Pier 15 shows that when the tide is high, salinity is also high, and when the tide is low, salinity drops.
Asking students for predictions before revealing the data will help them become more invested in the results.