The site description of the flow monitoring stations are listed in Table 4.2.1. Figure 4.1 shows the relative locations of the primary sites, including flow monitoring sites in north GWD and along the San Joaquin River. Many of the Monitoring Plan flow monitoring stations have been established for more than a decade and provide data that can be compared to the present flow conditions. These established sites were originally chosen for a variety of purposes, including water delivery accounting by GWD, water quality assessment of returns into the San Joaquin River by the CVRWQCB, and Basin water resource inventory by the USGS.

Flow data in the GBP Monitoring Plan is necessary for accurate selenium load computation at Site B to compare to selenium load targets set for the project (Table 1.1). Flow data at sites A, B, D, F and N will be used for load and mass balance calculations, and to perform comparative analyses with historic selenium, boron and TDS discharges to the San Joaquin River. Flow data collected at site E on a bi-monthly basis will be utilized with data from site D collected during the same sampling event to investigate potential selenium losses in-transit within Mud Slough. In addition flow data will be used in determining 1. seasonal flow patterns within Salt and Mud Sloughs; and, 2. influence of discharge from the SLD on the hydrology of Mud Slough.

Site A, located at Check 17 on the SLD, is the upstream flow monitoring site on the Grassland Bypass section of the SLD. It is an existing, inactive station, with a Stevens recorder, stilling well and access bridge. The existing broad-crested weir is fouled and in poor condition. This control structure will be replaced by twin adjustable sharp-crested weirs. Ventilation tubes will be placed below the nape on the downstream side of the weir to minimize flow turbulence at the weir crest.

Site B was chosen for its proximity to the terminus of the SLD and the availability of single phase power for the site. The USFWS wanted to minimize traffic along the SLD service road adjacent to the newly established wetland to the north-east of Kesterson Reservoir, and suggested locating the site as close to Gun Club Road as practicable. The presence of cattails and sediment accumulation immediately downstream from the Gun Club Road culvert had to be taken into consideration. A footbridge and a cantilevered bridge have been constructed at Site B to which the acoustic sensors, the stage and the water quality sensors have been attached. These sensors are connected to a data logger, which in turn is connected to a cellular phone and modem. This will allow the data to be telemetered to each of the participating agencies upon dial-up.

Sites D and F are USGS flow and water quality monitoring sites which have been in place more than 10 years. These sites were chosen because there are road bridges with bank stabilization at each location, facilitating the deployment of sensors and minimizing the potential for channel cross-section changes. Flow gauging is performed directly from the bridge at Mud Slough (Site D) and from a way cable at Salt Slough (Site F).

Site N, at the Crows Landing Bridge on the San Joaquin River, is a former flow gauging station that has been reactivated. The site is not ideal, as it is close to a bend in the river, and access to sensors from the bridge is a narrow walkway unsafe for personnel. The site retrofit, however, has added a gauge house on the north bank of the river, which has improved monitoring conditions, and a nitrogen bubbler sensor, which has significantly improved the accuracy and reliability of the stage measurement. An improved rating curve for the site will be developed over time.

Site E was initially chosen as a downstream Mud Slough biota sampling site, but it was subsequently decided to include downstream flow monitoring to allow for the computation of selenium load losses in Mud Slough between the SLD discharge point and the San Joaquin River. Flow monitoring will be performed bi-monthly to coincide with flow rating quality assurance monitoring at Site D. Since discharge is measured using the current meter technique at both sites during the bi-monthly site visit the data gathered at Site D can be used to check the stage-discharge rating at the same time. Selenium samples taken at both sites during the synoptic study will allow for the estimation of in-transit selenium losses.

Sites J and M (Camp 13 Canal and Santa Fe Canal, respectively) are not flow sites in the Monitoring Plan. However, daily flow data will continue to be collected and integrated when necessary. Flow monitoring will allow for computation of selenium and salt loads at each site, though neither of these load computations is critical to the monitoring program. Flow at these sites will continue to be measured daily by GWD staff using a stage over the weir boards to compute discharge.