Climate Change and its Effects on the Salinity and Fish of the San Francisco Bay Estuary
Background |
Background on the San Francisco Bay Estuary
Once, the pristine San Francisco Bay offered 190,000 acres of highly productive tidal marsh, but currently 90% of that has been destroyed or altered (6). The Estuary’s watershed currently covers roughly 60,000 square miles, which is about 40 percent of California. It is mostly shallow, with a third being exposed during a good low tide and another third less than 6 feet deep at the same low tide. Almost half of the state's water supply enters the aquatic system as either rain or snow in this watershed. Half of that water is then diverted for anthropogenic uses, such as agriculture, factories, and households. The half of the freshwater left over continues to flow into the bay (7).
The vast natural resources abundant in the bay provide essential winter habitat for over a million migratory birds. The estuary also acts as a nursery for juvenile fish and crustaceans. It is helpful to consider the aquatic species in the bay as part of three general salinity regions. The first is the delta, which is the freshwater mouths of the entering rivers. The second region is the upper bay/lower salinity region. This is considered the strict estuary. The saline region is the lower bay, where the species are more marine (7).
The San Francisco area is characterized by a climate of warm, dry summers and cool, wet winters. Because of this, there are two distinct seasons, the rainy and the dry, which create a highly variable salinity regime during the year. The salinity can shift as much as 10 parts per thousand (ppt) seasonally. In addition to seasonal fluctuations, however, the salinity is also annually variable and has been rising. Over time, the spring salinity in the San Francisco Bay has been gradually rising and has increased by 3ppt since 1941. This is a matter of great concern to the scientific community (7, 8).
An annual average of 30-40 km3 of freshwater that flows into the the San Francisco Bay Estuary is runoff from rain and snow in the Sacramento San Joaquin watershed. The mouth of the Sacramento River is in the moderate-altitude Cascades and northern Sierra, while San Joaquin flows begin at the high southern Sierra. The annual supply of incoming freshwater includes at least 40% snowmelt. This is defined as freshwater entering the system after April 1st because the rainy season is over and the temperature is warming enough to melt the snow. This flow is only during the late winter and spring months, but the supply must last the estuary for the entire year. To make this happen, the state of California relies on artificial (reservoirs) and natural storage with periodic flooding (3,4).
To learn more about the San Francisco Bay Estuary, click here (7).
