Reach 2: Primary Driver—Sediment Oxygen Demand
SOD in Reach 2 has not been quantified or studied. Although it is assumed that some quantity of SOD exists in Reach 2, because SOD in this reach has not been quantified in the available literature, SOD values are unknown. By definition, SOD is the oxygen demand from the sediments (i.e., the river bed) (Chen and Tsai 2002); therefore, SOD in Reach 2 is not transportable to Reach 3. SOD in Reach 2 is not expected to be large or to have a significant impact on DO concentrations in Reach 2 because of the rapid river velocities that suspend and resuspend algae and particulate organic matter in the water column and prevent substantial particulate settling.
Particulate BOD concentrations in Reach 2 are primarily the result of algae produced in Reach 2 and algal biomass and other sources of organic matter contributed from Reach 1 or the Stockton RWCF. Reach 2 particulate BOD concentrations and trends have not been measured but may be estimated from observed algal and VSS concentrations.
The figure below compares 2002 algal concentrations in the San Joaquin River at Mossdale and Vernalis and concentrations at Buckley Cove in the DWSC (i.e., Reach 3). Mossdale generally appears to have higher algal concentrations than Vernalis during June–September. The concentrations at Vernalis and Mossdale indicate that the river’s characteristics (i.e., relatively shallow depths, high velocities, and sufficient light) are more suitable for algae growth, particularly during the summer, than the DWSC at Buckley Cove. The greater algal concentrations at Mossdale, compared to Vernalis, may indicate that Reach 2 algal growth is an important contributor of Reach 2 particulate BOD. However, in Reach 2, chlorophyll a (algae and particulate BOD) concentrations were always higher at Mossdale than at Channel Point during the summer and fall 2001 ( Lehman 2003). These higher values indicate that other factors, such as algal settling and decay or tidal flow from the DWSC, are affecting the chlorophyll a concentrations in Reach 2 between Mossdale and Channel Point.
The limiting factor to algal growth in the San Joaquin River is assumed to be light (Foe and McCarthy 2002; Leland et al. 2001) because nutrient concentrations are substantially greater than those concentrations that would limit algal growth (Lee and Jones-Lee 2003). Other factors affecting algal-related particulate BOD in Reach 2 include algae and nutrients contributed from Reach 1, discharge of algae from the Stockton RWCF, and flow diversions that increase the residence time for algae growth. The quantity of particulate BOD that settles in Reach 2 and the importance of particulate BOD on SOD in Reach 2 are unknown.
Iron and sulfate compound concentrations in the sediments of the San Joaquin River in Reach 2 have not been measured. In addition, sources of iron and sulfate compounds to Reach 2 have not been identified. Factors affecting iron and sulfate concentrations in Reach 2 and their importance on SOD in Reach 2 are not identified in the literature.
Because SOD is the result of biochemical processes, SOD rates would be expected to be highest in summer and lowest in winter in Reach 2 (Brown 2002). The literature does not describe the importance of water temperature on the SOD rates in Reach 2 or compare the importance of temperature to other factors affecting SOD rates.
The water temperature conditions in Reach 2 are described below.
Water temperatures in Reach 2 (at Mossdale) ranged from less than 50°F (10°C) to greater than 68°F (20°C) in 2001 (Brown 2002). In portions of June, July, and August, water temperatures in the San Joaquin River were greater than 77°F (25°C) (Brown 2002). The coldest temperatures (less than 10°C) were recorded only in January, early February, and December (Brown 2002). As shown in the figure below, water temperatures in Reach 2 (Mossdale) are very similar to temperatures in Reaches 1 (Vernalis) and 3 (Rough and Ready Island).