June 30, 2004 — The U.S. Coast Survey tide station installed in San Francisco Bay, Calif., on June 30, 1854, is now being recognized for having produced a 150-year water level record — making it the longest continuous running series of tidal observations in the Americas. The tide gauge, now operated by the NOAA Center for Operational Oceanographic Products and Services (part of the NOAA Ocean Service), is one of 175 stations in the NOAA National Water Level Observation Network, which spans the entire continental United States, Alaska, Hawaii, Pacific Island territories, Puerto Rico and the Virgin Islands. However, the historical water level record captured by this particular tidal station is unique in that it transcends the maritime history of the San Francisco Bay — from the days when clipper ships relied upon tide predictions provided by the gauge to navigate the dynamic waters of the Golden Gate, to the modern day mariner, who utilizes real-time water level data to ensure that large ships and crane barge operators have enough depth in the channels and clearance under bridges to safely navigate the Bay. (Click
NOAA image upper left for larger view of tide gauge readout from 1854. Please credit “NOAA.”)

Not only has the information derived from this tide gauge successfully assisted mariners in navigating the bay area and planning/constructing of waterfront facilities, but it has also been utilized for a number of non-navigational purposes and contains the “signature” of several significant maritime events.

"The San Francisco tide gauge, and others like it, serves society’s needs for weather and water information and support the nation’s efficient and environmentally sound transportation, which happen to be two of NOAA’s primary mission goals," said retired Navy Vice Adm. Conrad C. Lautenbacher, Ph.D., undersecretary of commerce for oceans and atmosphere and NOAA administrator. "Its 150 years water level record will also serve as a key component in the emerging global observing system." he added. (Click NOAA image upper right for larger view of map of the San Francisco Bay area. Please credit “NOAA.”)

The Importance of Water and Tide Level Information
Tidal datums derived from NOAA tide gauges have traditionally been used for navigation (e.g., establishing chart datum and shoreline datums for all nautical chart products and for the Physical Oceanographic Real-Time System operations), however there are many other beneficial applications for this data. In fact, the NOAA Center for Operational Oceanographic Products and Services created its Coastal Oceanographic Applications and Services of Tides and Lakes (COASTAL) Program in 2003 to focus on these non-navigational applications:

• Real-time applications include coastal forecasts (for commercial and recreational users) and tsunami warnings.
• Near real-time applications include monitoring storm surge and assisting emergency managers plan better evacuation routes.
• Long-term applications include marine boundary determinations, tidal predictions, seasonal and long-term sea level trends monitoring, coastal planning and construction, wetland restoration and oceanographic/climate research.

The applications of tide and water level information to these projects are critical to their success in protecting life, saving property, restoring the environment and maintaining the economic vitality of the nation.

Maritime Event Signatures Found in San Francisco Tide Gauge Record
In addition to its many applications, the 150 years of water level data recorded by the San Francisco tide gauge also contains the signatures of important maritime events that have affected human populations and the California culture over time:

• Traces of Pacific Ocean Tsunamis and Local Earthquakes: On December 23, 1854, — six months after the San Francisco tide gauge was installed — an earthquake off the central coast of Japan spawned a series of tsunami waves, which traveled across the Pacific Ocean and were recorded as attenuated waves on the self-registering tide gauges along the western coast, including San Francisco. These waves were then seen superimposed upon the regular tidal record. From these waves, one of the first accurate estimates of the average depth of the Pacific Ocean was made. Over the next 150 years, the San Francisco tide gauge recorded many of the great tsunamigenic events of the Pacific Ocean. The gauge has also survived many major local earthquakes, including the Hayward earthquake of 1868 (which did major damage to the East Bay and to land fill areas in San Francisco), the great earthquake of 1906 and the 1989 Loma Prieta earthquake.
• Record of Changes in Sea Level: The San Francisco tide gauge has generated the longest continuous record of sea level change in existence in the Western Hemisphere. Tidal records show rise rates of 1.4 mm per year over the last century (and even captured the slight downward trend seen globally between 1875 and 1913). These rates of sea level rise have many ramifications for human occupation of coastal areas. (Click NOAA image upper right for larger view of graph showing variations in annual mean sea levels for San Francisco, Calif., 1856 to 2002. Please credit “NOAA.”)
• High Sea Levels Due to El Niño: Extreme high water events during periods of El Niño are clearly seen in the San Francisco historical tide record. El Niño events generally occur every 3 to 7 years in the Pacific Basin and are caused by the interaction between unusually warm sea surface currents and high sea levels generated in the tropical Pacific drifting eastward and colliding with lower temperatures in the Eastern Pacific. By analyzing the interannual to decadal variations in sea level — especially from a long baseline record like San Francisco — it's now possible to better understand the El Niño Southern Oscillation phenomenon and help predict future events.

It is clear that sea level records from the San Francisco gauge are indispensable for conducting climate change research, investigations of global warming and predicting El Niño events and the impacts of sea level change on coastal communities. Analysis of localized sea level trends, when accompanied by geodetic measurements, also provides insight and better understanding of regional tectonic changes and accompanying seismic activity.

The History of Water and Tide Level Measurements
Since the early 1800s, NOAA (and its predecessor organizations) has been measuring, describing and predicting tides along the coasts of the United States. In the early days, most water level measuring systems used a recorder driven by a float in a “stilling” well. A stilling well calms the waters around the water level sensor. A typical stilling well consisted of a 12-inch wide pipe. Inside the stilling well, an 8-inch diameter float was hung by wire from the recording unit above.

Before computers were used, water level data was recorded on a continuously running pencil strip chart. These records were collected by observers once a month and mailed to NOAA headquarters for manual processing. In the 1960s, data were recorded (every six minutes) onto mechanically punched paper tape that was fed into a computer for processing.

Although these systems worked well, they had their limitations. Stations were subject to recording errors and marine fouling, and were constantly in need of maintenance. In addition, the measurement and data processing equipment could not provide users with information until weeks after the data was collected.

Next Generation Water Level Observation Network
In 1985, the NOAA Ocean Service embarked on a major upgrade to its National Water Level Observation Network. The network of old float/wire systems was replaced by the Next Generation Water Level Measurement System, which consisted of an air acoustic water level sensor coupled with an electronic data acquisition system. The new system had numerous advantages including easy maintenance, direct leveling of the water level sensor to local benchmarks (tide observers and tide staffs are no longer required), electronic data storage, a backup pressure water level sensor with its own data logger and ancillary sensor capability (such as water and air temperature, wind speed and direction and barometric pressure). The acoustic sensor capability allowed much more accurate water level readings, but what really set this system apart from the earlier systems was the ability to transmit data to a central facility via telephone line or via the NOAA Geostationary Operational Environmental Satellite data collection system for near real-time data analysis, processing and distribution.

The San Francisco Tide Gauge Today
Today, the San Francisco tide gauge is an important component of the NOAA San Francisco Physical Oceanographic Real-Time System, which supports safe, cost efficient navigation and provides shipping interests with accurate real-time tide, current and meteorological data for operational decision-making. This information is critically important considering that there is an average of 261 deep-draft vessels entering San Francisco Bay each month and that there are approximately 85,000 registered pleasure boats using approximately 100 yacht clubs in the Bay system.

The tide gauge also provides information critical to maintaining and improving economic prosperity for California (and the nation), contributes to maintaining and monitoring port activities essential to Homeland Security and provides water level and reference datum information needed for the increasing number habitat and marsh restoration programs in the bay region.

NOAA Tidal Products And Services
The water level and tide data from the San Francisco and other tide gauges are made available to the public through the CO-OPS Web site. CO-OPS monitors performance of all real-time observing systems on a 24-hour basis, performs operational data quality control/data analysis and distributes oceanographic data to customers by maintaining a collection of up-to-date Web based tools and printed materials, including NOAA Tides Online and Great Lakes Online and NOAA Tide Predictions.

Although much has changed over the last 150 years, the San Francisco tide gauge has been a constant, serving a valuable function and providing many unforeseen benefits. NOAA is proud of the work it does in San Francisco and is honored that it is so closely tied to such a valuable piece of American History.

Relevant Web Sites
Tides and Current Information

Tides Roadmap to Resources

Water Level Datum and Associated Ancillary Information

Water Level Stations

CO-OPS Products Descriptions and Information

CO-OPS Publications

Sea Levels Online

SUBSIDENCE AND SEA LEVEL RISE IN LOUISIANA: A STUDY IN DISAPPEARING LAND

NOAA's TIDES ONLINE

Media Contact:
Glenda Powell, NOAA Ocean Service, (301) 713-3066 ext. 191 or Ben Sherman, NOAA Ocean Service, (301) 713-3066 ext. 178