Salinity Space & Time Contours

Salinity measures the relative proportion of freshwater and seawater, and it reflects the changing importance of river flow as a source of dissolved materials carried into the Bay-Delta from runoff. It is an important habitat descriptor, a control of chemical reactions, and the salinity distribution is an important mechanism that drives water circulation and transport in the Bay. Salinity of Bay-Delta waters ranges from about 0.1 to about 33 psu (salinity is measured in practical salinity units, psu, which are nearly identical to parts per thousand, ppt).

Description of Numbered Regions:

1. The thick black line shows the changing position in the Bay where surface salinity was 2 psu (an indicator of the freshwater-seawater boundary). This location was in Carquinez Strait during the period of high Delta outflow in 1993.
2. As Delta outflow recedes during the dry season, the freshwater-seawater boundary moves landward. Surface salinity of 2 psu occurred upstream of Chipps Island during autumn 1993.
3. During dry years, such as 1994, the freshwater-seawater boundary moves further landward. Surface salinity of 2 psu was located in the lower Sacramento river during autumn 1994.
4. During years of exceptionally high river low, such as 1995, the freshwater-seawater boundary moves far seaward. After the large Delta outflows of 1995, surface salinity of 2 psu moved all the way into eastern San Pablo Bay.
5. The color transition here, from yellow to blue, represents the seasonal change of salinity from low values during the wet winter-spring to progressively higher values during the dry summer-autumn. When river flows are small, evaporation and mixing with the coastal ocean cause the Bay's salinity to increase.
6. Salinity in the South Bay also changes in response to riverine inputs of freshwater, both from local streams and from Delta outflows. Notice how quickly the salinity dropped in early 1995, soon after a series of storms brought heavy flows into the Bay.