Reach 1: Primary Driver—Reaeration
Reaeration is affected by the four 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.
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 has not been determined but is expected to vary with BOD.
Water temperature data measured at selected locations in Reach 1 during 2001 are shown in the figure below. Water temperature affects the saturation DO concentration, which is about 12 mg/L in the winter and 8 mg/L in the summer.
Water temperatures in Reach 1 (at Vernalis) ranged from less than 50°F (10°C) to greater than 77°F (25°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 50°F [10°C]) were recorded only in January, early February, and December (Brown 2002). As shown in the second figure (below), water temperatures in Reach 1 (at Vernalis) were very similar to temperatures in Reaches 2 (Mossdale) and 3 (Rough and Ready Island).
Ambient air temperature, temperature of water entering Reach 1, and residence time are the primary factors affecting water temperature.
The available information suggests that flow velocity is not an important factor affecting reaeration in Reach 1 because Reach 1 is apparently fully saturated at all flows throughout the year. Table 1 indicates that velocity in Reach 1 is about 1.5 ft/sec at low flows of 750 cfs and increases to about 2.5 ft/sec at higher flows of 7,000 cfs. These flow velocities are apparently sufficient to produce reaereation that is greater than the normal BOD decay rates.
Reaeration occurs at the water surface, and the ability of reaeration to change the DO concentration is inversely proportional to the average depth. Table 1 indicates that the average depth of Reach 1 increases from about 3.5 feet at a flow of 750 cfs to about 7.5 feet at a flow of 7,000 cfs. Reaeration is apparently sufficiently strong over the full range of flows to maintain fully saturated conditions in Reach 1. Table 1 indicates that the surface area of Reach 1, which governs the mass of reaeration oxygen transferred into Reach 1, is about 1,000 acres at a flow of 750 cfs and increases to about 2,400 acres at a flow of 7,000 cfs.
Reaeration cannot be measured directly and must be inferred from the fully saturated DO concentrations observed downstream of Reach 1 at Mossdale (midpoint of Reach 2). However, because reaeration in Reach 1 is strong enough to maintain satured DO concentrations year round for all flows, uncertainties in the reaeration rate may not be important.