Salinity measures the relative proportion of freshwater and seawater in the Bay. The plot depicts two examples of vertical profiles collected using a CTD. Salinity is measured in practical salinity units, the salinity of the coastal ocean is about 33-34 psu. Salinity data can also be used to analyze long term salinity temporal variability.

Plot Description:
1. This profile was measured near the San Mateo Bridge on April 11, 1995. This profile was made after a period of high river flows that diluted the salt content of Bay waters and established a strong vertical gradient (salinity stratification) between low-salinity surface and higher-salinity bottom waters. Salinity gradients act to slow vertical mixing, and they create distinct layers that have their own water quality characteristics. Salinity gradients are broken down by tidal stirring, so the shape of the salinity profiles changes with the tides.

2. This profile was measured near the San Mateo Bridge on September 21, 1995. This profile was made after months of low river flow when the salt content of Bay waters progressively increased toward ocean salinity. Notice that the salinity was uniform from surface to bottom depths (the salinity gradient disappeared). This condition implies rapid vertical mixing of the water.