Reach 1: Primary Driver—Imported Dissolved Oxygen
Research indicates that DO concentrations are at or above saturation levels in the San Joaquin River upstream of Merced and on Reach 1 tributaries, such as Mud Slough near Gustine, Spanish Grant Drain, Harding Drain, Westport Drain, and Dry Creek at Gallo Bridge (Kratzer et al. 2004). The imported DO concentration can therefore be assumed to be near the saturation DO, or about 12 mg/L in the winter and 8 mg/L in the summer. The only continuous DO measurement station along the San Joaquin River is at Mossdale. The Mossdale data (middle of Reach 2) indicate that the DO concentration is at or above saturation (from algae photosynthesis), which suggests that DO in Reach 1 also remains at or above saturation.
DO concentrations in Reach 1 are generally fully saturated, even at low summer flows, suggesting that reaeration and algal photosynthesis are dominant processes in Reach 1. The proportion of DO concentration that is derived from reaeration (see Reach 1: Primary Driver—Reaeration) has not been identified but is expected to vary with BOD concentrations (more reaeration needed to maintain saturation) and algal concentrations (less reaeration needed to maintain saturation).