Watching San Francisco Bay Around the Clock: USGS Water-Quality and Sediment Monitoring

The San Francisco Bay study area and its monitoring stations. U.S. Geological Survey.

Why watch the Bay

The U.S. Geological Survey (USGS) monitors water quality and suspended sediment in San Francisco Bay as part of a multi-agency effort on estuary management, water supply and ecology. Millions of people live around the Bay, which teems with plant and animal life. Fresh water mixes with salt water here, pushed by rivers — floods, droughts, reservoir releases and diversions — and by the sea — tides, waves and salt. USGS and its partners have monitored the Bay continuously since 1988.

What managers track:

• Salinity, derived from specific conductance, shows how fresh and ocean water mix.
• Water temperature, with salinity, sets the water's density, which drives circulation and layering.
• Turbidity — light scattered by particles — is used to estimate suspended-sediment concentration (SSC).

Why sediment matters: it dims sunlight, affecting phytoplankton; settling on tidal marshes and mudflats, it can help them keep up with rising seas; in ports and channels it can force dredging; and it carries contaminants stuck to its grains, which can build up on the Bay floor and harm the ecosystem.

Tides reverse four times a day, and wind shifts daily, so conditions change constantly from place to place — hence continuous measurement at many sites. Eight stations send data in near real time by cell phone network.

The network

Instruments hang from stainless-steel cables anchored to the Bay floor, with sensors for specific conductance, temperature and turbidity. They record every 15 minutes; data arrive by telemetry within 1 hour (provisional) or are collected on station visits, available within 1 week.

How a monitoring station is installed. U.S. Geological Survey.

The station at Alcatraz Island; the solar panel powers the real-time equipment in the enclosure. U.S. Geological Survey.

At deep stations, sensors at two depths show differences between the top and bottom of the water; shallow stations such as Pier 17 and Alcatraz measure at one. In water years 2018–2019, valid data ranged from 62 to 100 percent, averaging 90 percent, the gaps coming from sensor failure and biological fouling.

Extreme biological fouling on instruments at San Mateo Bridge. USGS / Selina Davila Olivera, June 8, 2020.

Keeping the data honest

• Visits: every 2–5 weeks, crews clean each station, check calibration and download data. Fouling worsens over time, so affected data are corrected or removed.
• Turbidity to sediment: water samples taken at sensor depth link turbidity to SSC statistically.
• Sediment flux: at some stations, crews measure discharge with a boat-mounted acoustic Doppler current profiler while sampling sediment across the channel with a depth-integrating sampler; flux, in mass per unit time, is discharge times average SSC — telling managers how much sediment moves, and which way.
• Review: records are analyzed, approved and audited under USGS guidelines before final approval; methods are at ca.water.usgs.gov/projects/baydelta.

A rating curve at Pier 17, water years 2018–2019, relating SSC in water samples to turbidity. U.S. Geological Survey.

Collecting a suspended-sediment sample in the Sacramento–San Joaquin Delta. USGS / Darin Einhell, March 6, 2018.

Instruments: YSI model 6560 sensors measure conductance and temperature; turbidity comes from Forest Technology Systems DTS-12 and YSI model 6136 sensors (brand names for identification only).

Uses

The data help calibrate numerical models and show how Bay water responds to extremes such as prolonged drought. Records for water years 2018 and 2019 (October 1, 2017, to September 30, 2019) are public in the USGS National Water Information System.

Supported by the U.S. Army Corps of Engineers (San Francisco District), the Interagency Ecological Program, the California State Coastal Conservancy, the Bureau of Reclamation, and USGS programs.

Sources

Based on Continuous Water-Quality and Suspended-Sediment Transport Monitoring in the San Francisco Bay, California, Water Years 2018–19, U.S. Geological Survey Fact Sheet 2021–3043, by Darin C. Einhell, Selina Davila Olivera and Danielle L. Palm, USGS Publications Warehouse; a work of the United States government in the public domain. Figures and the station table recovered from the fact sheet's PDF.