The Ecology of San Francisco Bay Tidal Marshes - USGS National ...

Sacramento-San Joaqu i n river system, the
local drainage system, and water users.
Groundwater sources are mi nor due to the
impermeable bay mud underlying most tidal
marshes.
The drainage basin of the San
Francisco Bay system covers over 40% of
the land area of California, or 163,000
km2. Ninety percent of total surface flow
enters the bay via the Sacramento and San
Joaquin Rivers. Outflow from these rivers
into the bay is termed delta outflow because
of the confluence and mixing of the
two rivers within a low 1y1ng delta area
immediately east of Suisun Bay.
Historical ly, delta outflow has varied
considerably due to yearly variations in
rainfall and snow pack thickness. During
one particularly wet year (1862), fresh
water flowed outward through the Golden
Gate for 10 consecutive days and during
the draught of 1931, salt water extended
almost to Sacramento. Present1 y, delta
outflow is controlled by numerous dams and
pumplng stations operated by state and
federal agencies and can be esti mated with
some precision (Conomos 1979).
Between 1906-1977, total annual outflow
has ranged between 7.7 x lo9 m3 to
5.8 x 10l0 m3 with a median of 2.8 x 101°
m3 (CSWRCB 1978). If flow were continuous,
this would amount to a discharge of
approximatel y 880 m3/sec. The pronounced
seasonal period of rainfall (snowfall at
higher elevations) followed by dry weather
creates a cyclic pattern of de1 ta outflow
(Figure 13). Discharge is highest from
January to April when direct runoff and
snowmelt contribute to outflow and is
lowest during the dry summer months. The
seasonal variation in precipitation can
result in a 10-fold difference in outflow
when flood flows through the Yolo Bypass
are considered (Conomos 1979).
The primary influence of delta outflow
is on water salinities. Water salinities
in Suisun and San Pablo Bays are
directly affected by changes in delta
outflow whereas the south bay is only
influenced during extreme1 y high flows.
South bay salinities usually remain above
25 ppt even during normal winter outflows.
The strong estuarine gradient in the north
bay region influences the distribution of
Figure 13. Monthly averages of delta outflow for the years 1973-1977. Historjca1
outflow includes flood flows through Yolo Bypass, outflow index is standard calcu;atio!~
of deita outflow based on estimated flows in Sacramento and San Joaquin
Rivers only. Years 1973-74 considered "normal" water years; 1975-77 considered
drought years.