VARIATION OF SALINITY OF SAN FRANCISCO BAY

Since the energy required is related to the salinity of the water intake, it is important to understand the variability of the salinity of the bay area water at the intake under varioius conditions, including seasonal and wet versus dry years.

The salinity of the bay is governed by several variables. The most important of these are 1) the salinity of the ocean water surging in and out of the bay through the Golden Gate and 2) the dilution of this ocean water with the fresh water flowing in from the Sacramento River through the Carquinez Strait.

Additionally, since fresh water is lighter than the salty sea water it tends to float on top as it works its way towards the Golden Gate. Thus, there is a tongue of sea water that underlies the fresh water that stretches inland all the way to and through the Carquinez Strait, with the specific upper boundary dependent upon the height of the tidal change and the flow rate of the incoming fresh water. Thus, there is a degree of stratification, with a lower salinity at the surface and an increasing salinity with increased depth.

Lastly, there is a degree of mixing of these two layers of water, with the greatest mixing occuring in the North San Pablo Bay where more open stretch of the winds cause more surface waves that extend mixing action below the surface.

The salinity of San Francisco Bay has been extensively monitored for several decades by both portable equipment and by 36 fixed monitoring stations. The following figure shows the locations of these 36 monitoring stations.

Note that station number 15 is the closest station to the site of the proposed MMWD reverse osmosis desalination facility water intake point.

A general feel for the varying salinity of the San Francisco Bay is presented in the following chart, where the reddish color represents the highest salinity and the bluish color represents the lowest salinity.

LSB = Lower South Bay, SB = South Bay, CB = Central Bay, SPB = San Pablo Bay, Carq = Carquinez Stait and SU = Suisun Bay

However, this chart over-simplifies the situation since it is only an average and does not recognize seasonal and wet/dry year circumstances when the fresh water input from the Sacramento River varies.

More detailed data is available from two USGS web sites, The first allows entry into their very large Data Base and the second presents Scatter Plots based on dates and stations that you can select. Ths latter is probably more useful to the novice viewer since by entering the X coordinate as Station Number and the Y coordinate as Salinity it shows a table of salinities for a range of monitoring stations for the month and year that is selected.

Analysis of data from this web site shows that although the salinity of the upper reaches of San Pablo Bay are normally much lower than that shown for station #15, they were nearly as high as ocean water during the extreme drought of 1977.

Thus, locating a desalintion plant - or it's water intake point, far north of the currently proposed site would have a significant energy reduction during wet years but it would not have any energy reduction during extreme drought years.

However, these data also point up the desireablity of obtaining specific salinity data for the final water intake site, especially if the Engineer's recommendation that a revised intake site nearer the Pelican Way plant site are pursued.