A primer on estuarine physics

Estuarine scientists use many terms to describe the complicated physical processes in estuaries, where freshwater from the rivers mixes with saltwater from the sea. The salinity of freshwater is 0 practical salinity units (psu) and the salinity of seawater is 35 psu. The gravitational pull of the sun and moon generates tides with flood (landward) and ebb (seaward) currents.

Tidal currents are strongest during full and new moons, called spring tides, and weakest during half moons, called neap tides. This sloshing back and forth is usually much greater than the tidally-averaged (residual) movement of water caused by river inflow or wind. Tidal and residual currents carry and mix (transport) salt, sediment, plankton, and other constituents. Saltwater is heavier than freshwater; therefore, saltier water tends to be near the bottom of estuaries. The difference in the amount of salinity between the top and bottom of the water column (stratification) can be great enough to prevent the top and bottom waters from mixing.

Figure 2: Existing conceptual model of the entrapment and nullzones, Suisun Bay (see next page for further discussion).

Salinity is greatest near the ocean and smallest near the rivers. This difference in longitudinal salinity (gradient) from the river to the ocean can cause the tidally-averaged currents to flow landward along the bottom and seaward along the surface (gravitational circulation) (fig. 2 above). The null zone is the region in the estuary where the residual, near-bottom, landward current reverses and flows in the seaward direction as a result of river inflow. In many estuaries, the null zone contains an estuarine turbidity maximum (ETM) where SSC and turbidity are greatest.