The U.S. Geological Survey measurement program is designed to describe changes in water quality along the deep channel of the San Francisco Bay-Delta system. We make measurements at a series of fixed stations from Rio Vista (lower Sacramento River) to South Bay mouth of Coyote Creek. Data are collected using a submersible instrument package that concurrently measures multiple water quality parameters (depth, conductivity, temperature, suspended solids, chlorophyll, light penetration, and dissolved oxygen). Vertical profiles are obtained at each sample station resulting in a two-dimensional (longitudinal and vertical) description of water quality for each sampling date (see our recently collected data). Continuous sampling along the 145-km transect takes 12-15 hours with the research vessel Polaris.

Discrete Sampling
In concert with the submersible instrument package sampling, each cruise we collect and process discrete water samples for determination of dissolved oxygen (using Winkler titrations), suspended particulate matter (by measuring the dry weight of filtered particles), and chlorophyll a (by measuring the light absorbed by chlorophyll extracted in acetone solvent). The discrete samples are used to calibrate the following CTD sensors (optical backscatter sensor, fluorometer, and oxygen electrode). We create a regression comparing the discrete values and the sensor volts. From the subsequent regression equation we calculate oxygen, chlorophyll, and total suspended solids at all stations and depths.

We also collect discrete water samples for dissolved inorganic nutrients. Ammonium (NH₄⁺), nitrate + nitrite (NO₃^- + NO₂^-), phosphate (PO₄^-3) and dissolved silica (DSi) are determined using colorimetric reaction methods on an auto analyzer.

Discrete samples are collected at the surface and 1 meter off the bottom using either a pump or a Niskin sampler at the following discrete stations; 36, 32, 30, 27, 24, 21, 18, 15, 13, 9, 6, 3, 649, 657. All discrete samples are available in the database.

Calibration Techniques
The submersible instrument package is calibrated on a regular basis. The conductivity and temperature sensors are calibrated annually by the manufacturer. The optical backscatter sensor, fluorometer, and oxygen electrode are calibrated each sampling date using the discrete water measurements. We present the dissolved oxygen data as relative values, scaled to the oxygen solubility of water as it varies with salinity and temperature.

You can view cruise graphics and data in detail one cruise at a time or the complete data set from 1969 to present is available via the database. Descriptions of the methods and the complete data sets were published in annual reports until 1998, these are listed in our citations page.

See our project in action - watch presentation or field work video clips.