Salinity measures the relative proportion of freshwater and seawater in the Bay. The USGS has measured salinity in South San Francisco Bay regularly since 1977. This series shows the fluctuating salinity at USGS station 30, just south of the San Mateo Bridge (to see a map of USGS station locations, click here). Salinity is measured in 'practical salinity units', and the salinity of the coastal ocean is about 33-34 psu. This series shows that composition of the South Bay surface waters ranges from about 93% seawater during the dry seasons to about 50% seawater during the wet seasons when river flow dilutes the seawater. However, there are big exceptions to this rule.

Plot Description:
1. During the extreme drought of 1977 (the driest year of the century), salinity in the South Bay was higher than salinity of the coastal ocean (the bay became hypersaline) because evaporation exceeded inputs of freshwater.

2. At the other extreme, river flows were so high in 1983 (the wettest year of the century, through 1996) that the South Bay salinity was only 8.6 psu (Bay water at this location was only 26% seawater).

3. Record rainfall in February 1986 delivered large flows that rapidly diluted South Bay salinity to 8.6 psu (equivalent to only 26% seawater).

4. Northern California was unusually dry from 1987 to 1992, and during these drought years the annual salinity cycle was damped -- salinity ranged from about 25 to 32 psu.

5. The six-year drought was broken in 1993, when high flows diluted South Bay surface salinity to 17 psu (about 50% seawater). In 1995 and 1996, flows were even higher and salinities dropped even further.