Reach 3: Primary Driver—Residence Time
Residence time in Reach 3 (i.e., the DWSC) is affected by the two secondary drivers shown above. Click on a secondary driver to jump down to the discussion of that driver. See the Basic Concepts page for a general discussion of how the secondary drivers affect the primary driver.
As indicated in Table 1, residence (travel) times in the DWSC range from 256 hours at flows of 750 cfs to 31 hours at flows of 7,000 cfs. Greater residence times provide more time for the biological and chemical processes that consume DO to occur and decrease the DO concentrations in the DWSC.
It is generally accepted that flow in the DWSC is a significant factor that affects the DO concentrations in the DWSC. San Joaquin River flow that enters the DWSC has two opposing effects: increasing oxygen demand loads in the DWSC and reducing the hydraulic residence time in the critical reach (i.e., between Channel Point and Turner Cut) of the DWSC.
Tidal flows are substantial in the DWSC. The average tidal stage range in the DWSC is approximately 3 feet from high tide to low tide each day (Jones & Stokes 2002c). The results of this tidal stage range are shown in the table below.
Thus, water from the DWSC generally moves upstream into the San Joaquin River during flood tides. Tidal flow in the DWSC can affect the location of the DO sag and mixing of the water column. During flood tides when the tidal flow is negative (i.e., upstream), the DO sag in the DWSC typically occurs farther upstream in the DWSC than during ebb tides. Tidal fluctuations and the resultant increased water column mixing can contribute to the elimination of DO sags (Hayes & Lee 2000a). At DWSC flows of less than 500 cfs, downstream tidal exchange with Sacramento River flows may increase DO concentrations in the DWSC upstream of Turner Cut (Jones & Stokes 2002c). A relatively homogenous vertical DO profile is observed during flood or ebb tides, when the flow of water in the channel promotes mixing of the water column (Litton 2001a). However, during the summer, warm air temperatures may cause the surface layer of the DWSC to become isolated from the mixing and allow algal growth in the afternoon that creates high DO concentrations at the water surface. Tidal flow has also been shown to have a strong influence on lateral and longitudinal dispersion (i.e., mixing) in the DWSC (Jones & Stokes 2004b).
There are no measurable inflows or outflows in Reach 3, except for winter stormwater flows.
The geometry of the DWSC channel in Reach 3 affects residence time by affecting flow velocity. The DWSC was artificially created and is maintained through channel dredging. The geometry of the DWSC controls the flow of water through the DWSC and the tidal exchange and mixing in the DWSC ( Jones & Stokes 2002c). The San Joaquin River channel immediately upstream of the DWSC is 10–15 feet deep and the channel in the DWSC is maintained at a depth of 35–40 feet (Lee and Jones-Lee 2003). The geometry of the DWSC is an important factor affecting the DWSC DO concentrations because it affects the residence times of oxygen demanding materials and tidal exchange and mixing (Jones & Stokes 2002c).
The San Joaquin River downstream of Stockton was dredged to become the DWSC for the Port of Stockton. The channel is roughly 250 feet wide at the Port and is maintained to a depth of 35 feet to permit travel by ocean-going ships (Lee and Jones-Lee 2000, Lee and Jones-Lee 2003). The DWSC is typically about 750 feet wide (Jones & Stokes 2002a). Table 1 indicates that the DWSC volume between the Turning Basin and Turner Cut is about 18,000 acre-feet. The surface area is about 850 acres, with an average depth of 22 feet.
The available information suggests that flow is an important factor affecting residence time in Reach 3.
Uncertainties related to understanding how residence time may affect low DO concentrations in the DWSC include: