organisms tolerant of fluctuating salinities, temperature, and 
turbidity. Some of the geophysical facts about San Francisco 
Bay and its comparisons with other estuaries are given in Tables 
1 and 2. 
Although estuaries have many qualities and functions in 
common, the San Francisco Bay estuary has many distinctive 
attributes compared to other estuaries in the United States. 
The estuary is actually a continuum of basins, deltas, and 
rivers -- as my fellow scientists at the Romberg Tiburon Center, 
Dr. Michael Rosengurt, refers to as the River-Delta-Estuary-Sea 
system. Two major rivers flow into the basin: the Sacramento 
and the San Joaquin. Together, they drain over 40 percent of 
the State of California (approximately 153,000 km 2 ). Before 
reaching the estuarine portion of the basin (where salt and 
freshwater mix), the two major rivers mix with other tributaries 
with a vast interconnecting maze of channels called the Sacra- 
mento-San Joaquin Delta (Figure 1). The Delta, once the largest 
(over 1,400 km 2 ), is now largely used for farming and recre¬ 
ation. As our speakers will relate, the Delta also represents 
the single most important element in the vast "pipe-work" of 
water conveyance facilities in the state as well as providing 
important spawning and nursery areas for the state's 
recreational fisheries. 
Although many estuaries have variable inflow rates associat¬ 
ed with storms and snowmelt, the annual inflow pattern to the 
San Francisco Bay estuary fluctuates in response to the Mediter¬ 
ranean-type climate: frequent and heavy winter storms followed 
by dry summers. Winter rains result in both immediate local 
runoff and accumulation of water in the snow pack, which later 
melts and results in heavy discharges in April and May. With an 
approximate annual river discharge of 20.9 x 10 2 m 3 , 80-90 
percent enters the estuary from December to April (Figure 2). 
The climate also affects net water budgets due to the greater 
amounts of evaporation occurring in the region evolution of a 
unique marsh ecosystem, sometimes referred to in creation of a 
major business for salt production in large evaporation ponds. 
For those of you who have flown into San Francisco Bay, these 
large multi-hued basins are often the most distinctive landform 
on the approach path. 
Another unique characteristic of the San Francisco Bay 
estuary is the separation of the basin into two major circula¬ 
tory systems. The "North Bay", consisting of Suisan, San Pablo, 
and the northern portion of San Francisco Bay (sometimes re¬ 
ferred to as the "Central Bay") is dominated by a typical estu¬ 
arine gravitational circulation pattern. Freshwater inflow 
meets the bottom-flowing oceanic water in the region between 
Chipps Island in the western Delta and San Pablo Bay depending 
upon the amount of inflow. During the summer, the usual loca¬ 
tion of this interface region is within Suisun Bay, where it is 
10 
