Ecology of The San Francisco Estuary - Hydrodynamics

Hydrodynamics

The LSZ centers around 2 psu (practical salinity units, a measurement of salinity) and ranges from about 6 psu down to 0.5 psu. The primary fresh water inputs to the Estuary derive from regional precipitation, the Sacramento River, and the San Joaquin River.

River inflow is largely controlled by upstream reservoir releases. A significant fraction of this inflow is exported out of the Delta by the federal Central Valley Project and the State Water Project to southern California for agricultural and urban use. These alterations have removed much of the variation in through-estuary outflow (i. e., freshwater that makes it out the Golden Gate), creating lower outflow in the winter and higher outflow in the summer than historically found in the Estuary. Phytoplankton, zooplankton, and larval and adult fish can become entrained in the export pumps, causing a potentially significant but unknown impact on the abundance of these organisms. This may be particularly true of the endangered delta smelt, a small endemic fish; unexceptional except that is has been described as being tremendously abundant in historical accounts. The delta smelt is believed to migrate and spawn upstream in the Delta during the early summer, placing its eggs and larvae at high risk for entrainment. Management for the smelt is currently the source of controversy as its ecology brings into collision course the disparate water needs of conservation, development and agriculture in California.

The movement of water out of the estuary is complex and dependent upon a number of factors. Tidal cycles cause water to move toward and away from the Golden Gate four times in a 24 hour period. Using 2 psu as a marker for the Low Salinity Zone, the direction and magnitude of fluctuations can be tracked as the distance in kilometers from the Golden Gate, or X2.

Because the position of X2 relies upon a number of physical parameters including inflow, export, and tides, its position shifts over many kilometers on a daily and seasonal cycle; over the course of a year, it can range from San Pablo Bay during high flow periods, up into the Delta during the summer drought. The position of X2 is carefully monitored and maintained by releasing water from upstream reservoirs in anticipation of export demand. This is mandated by State Water Board Decision 1641 and requires that state and federal pumping be curtailed if X2 is shifted east of Chipps Island (75 river kilometers upstream of the Golden Gate Bridge) during the months of February through May, or east of Collinsville (81 river kilometers upstream of the Golden Gate Bridge) during the months of January, June, July and August. (D-1641 pp 150)

Gravitational circulation causes stratified high salinity water at depth to flow landward while low salinity water on top flows seaward. The effect of gravitational circulation may be most pronounced during periods of high fresh water flow, providing a negative feedback for maintaining the salt field and the distribution of pelagic organisms in the Estuary.

Mixing is important at the landward edge of gravitational circulation, often around X2, where the water column becomes less stratified. A fixed mixing zone occurs at the "Benicia Bump" at the east end of the Carquinez Strait, where the deep channel becomes dramatically shallower as it enters Suisun Bay. Mixing is critical in maintaining salinity such that extremely large inputs of fresh water are required to move X2 a short distance to the west. Mixing also assists pelagic organisms in maintaining position in the Estuary slowing the advection of primary and secondary production out of the system.