LiveOcean

These plots show observed sections of salinity versus depth and along-channel distance for five different estuaries. The ocean is always on the right, and the river generally enters on the left. The numbers on the contours are salinity in "parts per thousand," so 30 means there are 30 kg of salt in 1000 kg of salty water. Salty water is denser than fresh water, and the force of gravity makes the salinity layers want to flatten out, just like the layers of oil and vinegar in salad dressing. If you look at the vertical and horizontal scales for each section you can see that they are very different, yet all of these systems share some similarities: they gradually get saltier from the river to the ocean end, and they have some vertical stratification (a lot for the Chesapeake, and only a little for its neighbor Delaware Bay). The stratification is important because it separates the surface water from the deep water. This allows for example for bottom water to become hypoxic (low in oxygen) even though surface waters might have a lot of oxygen.

These plots show a conceptual sketch of estuarine salinity structure (top) and tidally averaged circulation (bottom). The salinity structure is just an idealized version of the observations from many estuaries, like the five we plotted above. The "circulation" shows that deep water is being pulled in from the ocean, and gradually rises up to join river water and flow back out to sea near the surface. The transports of these three flows are given by Q_in, Q_R and Q_out which are volume transports (cubic meters per second).

The circulation shown above is "tidally averaged" meaning that it is what we find after averaging over several tidal cycles. The tidal currents are often very strong, like 1 m/s sloshing back and forth, whereas the tidally averaged currents may be much smaller, like 0.1 m/s. What makes the tidally averaged currents so important it that they are persistent, always pulling deep water into the estuary, and sending surface water out.

We call the tidally averaged circulation the "Estuarine Exchange Flow" because it efficiently exchanges water with the ocean. Typically Q_in and Q_out are several time larger than Q_R. In Puget Sound the annual average river flow Q_R is about 1000 m3/s, but Q_in = 20,000 m3/s and Q_out = 21,000 m3/s (note that the outflow is the sum of the two inflows).

So, why is there an estuarine exchange flow? Why would deep water flow in persistently when the main forcing from the river is always pushing water out? The physical process is shown separated into three steps in the panels above. The river is an essential ingredient because it introduces stratification to the system. The tides are another essential ingredient because they mix the buoyant water downward, giving it more potential energy that can be turned into the kinetic energy of the exchange flow. The final step is the tendency of the stratified water to want to flatten out its density (salinity) layers. In flattening it has to drag more ocean water in to replace the mixed water going out. The reason the exchange flow can be so much larger than the river flow is that the mixing means that much of the water going out is ocean water, which then must be replaced.