In North San Francisco Bay, tidal ebb and flood move a slab of water with a temperature and salinity gradient (low temperature and high salinity toward the ocean) along a deep, long channel back and forth past stationary instruments at the Romberg-Tiburon (RT) dock (an Eulerian system). If velocity as a function of time were known, these measurements could be converted to reveal the gradient in the water (a Lagrangian system). The observed temperature-salinity curves at the dock are more complex than the sinusoidal curves used by NOAA to represent predicted currents. These complex temperature curves must be due to complex currents which either generate a complex temperature-salinity profile, or transport a regular profile past the dock in a complex way. It seems a priori likely that both occur.
In Richardson Bay, a shallow, short estuary, tidal currents transport water from entrance to shallow wetlands each tidal cycle. Thus, temperatures during ebb and flood can reveal characteristics of the wetlands. Diurnal solar heating of wetlands can produce large temperature changes. However, when solar heating is modest and air temperatures are close to average water temperatures, water temperatures reveal less about the wetlands. There is often stratification of temperature in deep channels. This is at least partially due to the temperature gradient itself, but during the rainy season there are large salinity gradients which can produce even greater density gradients.
Tidal currents, particularly at the water surface, can be complex and change over very short distances. Plumes of warm, fresh water, recognized by color, are commonly seen around the entrance to Richardson Bay. At the edges of these plumes, often marked by a line of foam (a tide rip), velocities can change more than 1 m/s in less than one meter (from the wakes of boats crossing the boundary). In the water in front of Richardson Bay radar has found circular currents. Preliminary data from drift buoys has documented circular currents inside Richardson Bay. Thus, it will require a sustained effort to acquire even a rough map of currents in this region.
The study of estuaries has a particular appeal for the amateur or undergraduate scientist: there are a large number of them, each one is different, they are generally accessible, and you should be able to find one that hasn’t be completely described in the scientific literature. Your work can be original and productive, in the same way that the observations of amateur ornithologists and astronomers have and continue to be.
An impressive collection of meteorological and oceanographic data are freely available from NOAA on the Internet. Temperature loggers, and refractometers used in this study are inexpensive. If you can identify an instrument used in large numbers, not just by a few scientists, you can decrease costs by a large amount.
There is a potential to develop inexpensive instruments by engineers (students, amateurs) who are not dependent on sales volume and its associated profit. An obvious example would be a salinity logger. The innovative engineer might graft a robust salinity sensor (better than the ones now available) to an existing commercial data logger.
Finally, the collection and analysis of data about the water in estuaries can be used to teach principles of mathematics, statistics, computer science, physics, chemistry, etc. to students at a variety of levels of sophistication.
Petzrick et al., p105,(1996), in The San Francisco Bay The Ecosystem, AAAS Pacific Division