Monitoring San Francisco Bay's Water Quality and Sediment Around the Clock, 2020–21

The U.S. Geological Survey has continuously monitored water quality and suspended-sediment movement in San Francisco Bay since 1989, as part of a multi-agency effort to support estuary management, water supply and ecological needs. The Bay region is home to millions of people and a rich variety of marine and land plants and animals. In the Bay, fresh water mixes with salt water, shaped by river influences — floods, droughts, reservoir releases and water diversions — and by the ocean's tides, waves and saltwater.

The San Francisco Bay study area and monitoring stations (USGS).

What's measured, and why

State and federal resource managers rely on four measurements taken at eight key locations:

• Water temperature and salinity, which set the water's density and so control gravity-driven circulation and layering in the water column. Salinity, derived from specific conductance, shows how fresh and ocean water are mixing.
• Turbidity, the scattering of light by particles in the water, used to estimate suspended sediment.
• Suspended-sediment concentration (SSC), also measured directly in water samples.

![6. Electronic cable reel with labeled heavy sampler on an arm extended over the front
of a boat.](https://media.hubnx.com/asset/3197e909504d2babd71a3e4609a4a47abfd8884bde26bf271b360fd86c545f17.png?Policy=eyJTdGF0ZW1lbnQiOlt7IlJlc291cmNlIjoiaHR0cHM6Ly9tZWRpYS5odWJueC5jb20vYXNzZXQvMzE5N2U5MDk1MDRkMmJhYmQ3MWEzZTQ2MDlhNGE0N2FiZmQ4ODg0YmRlMjZiZjI3MWIzNjBmZDg2YzU0NWYxNyoiLCJDb25kaXRpb24iOnsiRGF0ZUxlc3NUaGFuIjp7IkFXUzpFcG9jaFRpbWUiOjE3OTA5Njg3MDV9fX1dfQ__&Key-Pair-Id=K1CWAV4W62RI0D&Signature=gc35btTJ84S7i9WZ9Rdo7w7m3UKIt3igeYt-wywee5Cj6Prn~OWCBxht1zma~cU0ARaeQN2gHKKoSBUI56pVWsesLhZN3FDiNhlCZEEDiWSlbsy69PYxzueTDE87m5kmufJcL7hc6xyjg7-QCSF7v4QMpLg6nxBU~u0cC5JnDJlqb63nBRn~VptmK4dbR-OEv8XilXZ0w~z0FAHtro1hYhIqpNvQZHs8GcTtVBjDxj5P~CAuav7nytOzNajsRAUlQUoIBs84GJlSO7FhTWuovkRoaj5kIJ3jA5Jqp~rukfX2AYIqyYx2lelKtI4GmglfTJhwx3nD2RYI9d51dlmOgg__)

*Figure 6.Depth-integrating sampler used to collect a suspended-sediment concentration sample
in Sacramento–San Joaquin Delta. Photograph by Darin Einhell, U.S. Geological Survey,
March 6, 2018.*

Suspended sediment matters in several ways:

• it dims sunlight in the water, affecting phytoplankton growth;
• deposited on tidal marshes and mudflats, it helps build and sustain those habitats as sea level rises;
• settling in ports and shipping channels, it increases the need for dredging;
• it often carries contaminants stuck to its particles, and too many contaminated sediments on the Bay floor can harm the ecosystem.

Because tidal currents reverse four times a day and winds shift on a daily cycle, conditions vary from place to place and hour to hour — so frequent measurements at many locations are needed.

How the network works

• Instruments hang from stainless-steel cables anchored to the Bay floor and measure specific conductance, temperature and turbidity every 15 minutes.
• Data are sent by cellular telemetry (provisional data within an hour) or collected on station visits, and are posted to the USGS National Water Information System.
• At stations deeper than 50 feet, sensors sit at two depths to capture vertical differences; at shallow stations under 35 feet, such as Pier 17 and Alcatraz Island, at one.
• Crews visit each station every 2 to 5 weeks to inspect, clean and calibrate instruments and collect water samples at sensor depth, which are used to relate turbidity to SSC.
• Occasionally, crews measure flow with a boat-mounted acoustic Doppler current profiler while sampling sediment across a channel, to calculate how much sediment is moving and in which direction.

A monitoring installation at San Francisco Bay near Alcatraz Island (USGS).

Fighting fouling

In water years 2020–21 (October 2019 through September 2021), the share of valid data after review ranged from 55% to 100%, averaging 88% across parameters and stations. The main culprits were sensor failures and biological fouling — growth on the sensors that worsens over time and forces data to be corrected or deleted. Optical turbidity sensors are especially prone to it. Copper tape around the sensors slows the growth.

Clean and fouled instruments at South San Francisco Bay at Dumbarton Bridge (USGS).

The stations

Using the data

Records are carefully reviewed, corrected for fouling, sediment buildup and sensor drift, approved and audited before release. The data help calibrate computer models of the Bay and show how its water quality responds to extreme events such as prolonged drought. All the records, including water years 2020–21, are publicly available through the National Water Information System.

Relating turbidity to suspended-sediment concentration at Suisun Bay at Mallard Island (USGS).

Sources

• U.S. Geological Survey: "Continuous water-quality and suspended-sediment transport monitoring in San Francisco Bay, California, water years 2020–21," USGS Fact Sheet; a work of the United States government in the public domain. Pictures an earlier version of this page left out were restored on 2026-09-26.