Estuaries like San Francisco Bay are constantly changing. They have complex patterns of water quality variability because they are mixing zones between the ocean and the land. fresh water, sediments, nutrients, toxic contaminants, and other materials are carried to the estuary by river flow. River flow changes from season to season and year to year, so the water quality of San Francisco Bay also changes seasonally and from year to year.
The U.S. Geological Survey (USGS) has, since 1968, undertaken a major research effort in San Francisco Bay, including a measurement program to follow changes in some components of water quality. With internationally recognized interdisciplinary expertise and experience in the study of estuarine processes, the USGS has provided much of the fundamental knowledge of and interrelations among the hydrology, geology, chemistry and ecology of this complex estuarine system.
Much of the information and data below were obtained from the USGS Access San Francisco Bay and Delta web site. Other explanatory material was obtained from:
Most of the water quality data in the sections below are from an extensive data set collected by the USGS since 1968. This water quality program is a collaboration between the USGS's San Francisco Bay Ecosystem Program and the Regional Monitoring Program for Trace Substances, administered by the San Francisco Estuary Institute. Click here to go to the home page for the water quality data set. The map below shows the sites in the Bay where water data are collected. Data are collected using a submersible instrument package that includes sensors for measuring: depth, conductivity, temperature, suspended solids, chlorophyll, light penetration, and dissolved oxygen. The package is lowered through the water column at each site to sample from the surface to the Bay bottom. Click here to go to the USGS web site and obtain more information about the sampling instrumentation and R/V Polaris, the ship from which sampling is completed.
Coastal ecosystems are among our most valuable habitats, yet they are also among our most altered ecosystems. The water is a critical element in creating a sustainable environment for life in the Bay and for maintaining recreational, economic, and aesthetic value. Until about 150 years ago, San Francisco Bay was essentially undisturbed by human activities. Since the 1948 Gold Rush, however, extensive development around the Bay has fundamentally changed its character. Land-use changes have increased the sediment supply and toxic input to the Bay, and diversion of water for urban and agricultural uses has increased the overall salinity of the Bay and reduced its ability to flush out toxic substances. The bioto has changed vastly, with many species now depleted and many others introduced from elsewhere, either on purpose or by accident. For example, an Asian clam, unintentionally released from cargo ship ballast water, filters from the water huge volumes of phytoplankton, thus reducing the food supply for other organisms (see Part IV for more information about introduced, so-called alien, species). By measuring characteristics of the water, such as salinity, temperature, chlorophyll, dissolved oxygen, turbidity, and toxicity, scientists can evaluate the scale of environmental change and the impact on organisms that depend on the Bay's nourishing water.
1. What aspects of the Bay's water matter to you?
San Francisco Bay is the final catchment in a vast water network through which about 40% of California's water drains on its way to the sea. Rivers that drain the west side of the Sierra Nevada Range are tributaries to the Sacramento and San Joaquin Rivers, which carry their water through the Delta region and into the Bay.
Figure 1. The area outlined with brown is the Sacramento-San Joaquin Basin (San Francisco Bay watershed). The Tuolumne and Merced Rivers are two tributaries that feed into the Sacramento-San Joaquin Basin, which then feeds into San Francisco Bay (solid blue). Diagram from Peterson and others, Variations in Spring Delta Discharge to San Francisco Bay, USGS web site.
The water that flows through the Delta is fresh; once in the Bay this water mixes with salty sea water. Variations in salinity throughout the Bay are a result of mixing processes (Part I) and of the amount of fresh water that enters through the Delta. Direct rain and runoff from local streams contribute some fresh water to the Bay but most (about 90%) arrives through the Delta.
The amount of Delta outflow to the Bay depends on climate and on human modifications. Precipitation in California is highly variable, both seasonally and yearly. With a Mediterranean climate, the state receives most precipitation between October and May. The largest input of fresh water to the Bay typically occurs in the spring, when warming temperatures cause snow in the Sierra Nevada Range to melt.
2. Based on climatic influences, during which months of the year would you expect the Bay to have more salty water? During which months would you expect less salty water?
The natural patterns have been somewhat disrupted by the storage of water in reservoirs and by diversion to agricultural lands and urban usage. Storage and diversion tends to decrease the volume of fresh water in the winter and spring. Water is sometimes released from reservoirs during summer and fall months, which can increase the volume of fresh water during these months. During the past 100 years, water flow into the Bay has been highly regulated and now over 50% of the water is diverted for other uses. About 85% of the fresh water exported from the Delta goes to agriculture and the remainder is used for municipal and industrial uses in the Bay area, the Central Valley, and southern California.
This diagram shows the increase in water diversion from the Delta and San Francisco Bay during recent decades. Naturally, the export of water is greatest during the winter and spring seasons when fresh water runoff in the watershed is greatest. Delta and Bay managers are striving to learn the effects of this water diversion and to insure that sufficient fresh water inflow remains for the Bay and a healthy ecosystem. Diagram from Smith and others, Seasonal / Yearly Salinity Variations in San Francisco Bay, USGS web site.
3b. Are there ways that you benefit from this water diversion from the Bay (for example, think of your food and water supply).
3b. What are some possible effects that this fresh water diversion could have on the Bay-Delta ecosystem?
We will explore some of these effects in subsequent parts of the exercise.
