Reach 2: Primary Driver—Reaeration
Reaeration in Reach 2 is affected by the three 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. Factors affecting wind are, for the most part, not reach specific and are therefore described only in the general discussion on the Basic Concepts page.
DO concentrations in Reach 2 are generally fully saturated, even at low summer flows, suggesting that reaeration and algal photosynthesis are dominant processes in Reach 2. The proportion of DO concentration that is derived from reaeration has not been determined but is expected to vary with BOD.
Water temperature is a primary factor controlling reaeration rates because the amount of oxygen that can be dissolved in water is proportional to water temperature.
The water temperature conditions in Reach 2 are described below. The saturated DO at Mossdale is shown on the figure below. The saturated DO concentration is higher (about 12 mg/L) in winter and lower (about 8 mg/L) in the summer.
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 second figure (below), water temperatures in Reach 2 (Mossdale) are very similar to temperatures in Reaches 1 (Vernalis) and 3 (Rough and Ready Island).
The available information suggests that flow velocity is not an important factor affecting reaeration in Reach 2 because, although the velocity is reduced in Reach 2, the reach remains fully saturated. Table 1 indicates that velocity in Reach 2 is less than 1 ft/sec between Vernalis and Mossdale and less than 0.5 ft/sec downstream of Mossdale at low flows of 750 cfs. Velocity increases to about 2 ft/sec between Vernalis and Mossdale and about 0.7 ft/sec downstream of Mossdale 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 in Reach 2.
The geometry of the San Joaquin River channel in Reach 2 affects reaeration by affecting surface area and depth. The river has a width ranging from about 150 feet at the Head of Old River to about 250 feet where it transitions to the DWSC. The channel along this reach is relatively shallow and averages about 8 feet in depth (Jones & Stokes 2002c; California Regional Water Quality Control Board, Central Valley Region 2004). Table 1 indicates that the average depth of Reach 2 increases from about 4 feet at a flow of 750 cfs to about 9 feet at a flow of 7,000 cfs for the riverine portion of the reach between Vernalis and Mossdale. The mean depth is about 8 feet downstream of Mossdale.
Reaeration is apparently sufficiently strong over the full range of flows to maintain fully saturated conditions in Reach 2. Table 1 indicates that the surface area of Reach 2, which governs the mass of reaeration oxygen transferred to Reach 1, is about 650 acres at a flow of 750 cfs and increases to about 820 acres at a flow of 7,000 cfs.
Reaeration cannot be measured directly and must be inferred from the fully saturated DO conditions observed in Reach 2 at Mossdale and Channel Point. However, because reaeration in Reach 2 is strong enough to maintain satured DO concentrations all year for all flows, uncertainties in the reaeration rate may not be important.