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<br />o J2 3j 5 <br /> <br />After the low water years from 1989-1993 the reservoir was again drawn down, resulting in a <br /> <br /> <br />peak in the overall salinity trend. Since that time salinity levels have gradually declined and <br /> <br /> <br />reached a level of nearly 400 mgIL TDS in early 1997, the lowest level since 1984. If above <br /> <br /> <br />- average runoff and reservoir releases continue, this level should decline further.__~ <br /> <br />2. Seasonal Patterns <br /> <br />Seasonal patterns in the Glen Canyon Dam releases are a function of meteorological <br /> <br />conditions, internal reservoir mixing processes, inflow hydrology and the operation of the dam. <br /> <br />a. Temperature <br />An asymmetric annual temperature pattern appears in Glen Canyon Dam releases, with <br /> <br />temperatures gradually warming through the year from a low point of 7 to 8 oC in February or <br /> <br /> <br />March to a maximum point in December of 10 to 12 oC. This is followed by a sudden drop to its <br /> <br /> <br />minimum value with a few months (Fig 15). This pattern is most likely due to the gradual <br /> <br /> <br />penetration of surface warming through the summer. Winter convective mixing in the upper <br /> <br /> <br />layers of the reservoir then takes place, drawing relatively warmer surface water to levels at or <br /> <br />near the penstock. The magnitude of the maximum release temperature appears to be a function of <br /> <br /> <br />the thickness of the mixed upper layer. This is the result of the volume of low-density inflow from <br /> <br /> <br />the previous season's runoff, which arrives near the dam and defines initial conditions for the <br /> <br /> <br />winter convection process, and the severity of the winter cooling and its effect on the convective <br /> <br />process. Only in certain years do withdrawals seem to be come from the mixed isothermal <br /> <br />epilimnion. The sudden drop in temperature which follows is most likely due to the continuation <br /> <br />of the convective process, but is also strongly affected by the upwelling of cold saline water from <br /> <br />the hypolimnion caused by the dense winter underflow currents in the reservoir. The period of <br />sudden drop also corresponds with high powerplant releases in the month of January that may <br />augment the amount of hypolirnnetic water discharged while this upwelling occurs. <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page 29 of 62 <br />