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<br />1. Historical trends <br /> <br />Da23a~ <br /> <br />a. Temperature <br />Fluctuation in temperature patterns became reduced to their current levels by about 1973, at <br /> <br />"which point the reservoir had filled sufficientty to'iSalatethe penstock-from the seasonally <br /> <br />warming surface waters (Fig. 12). Since that time, annual mean temperatures have been fairly <br /> <br />stable between 8 and 10 oC. Increases in instantaneous and mean temperatures were seen in 1978 <br /> <br />when the reservoir was drawn down due to drought conditions and in the mid 80's and a result of <br /> <br />the operation of the spillway release structures. Since the implementation of interim flows at Glen <br /> <br />Canyon Dam annual mean temperatures and variation appear to have been reduced because of <br /> <br />constrained release patterns. <br /> <br />b. Salinity <br /> <br /> <br />Salinity patterns in the Colorado River have been representative of hydrological and climatic <br /> <br /> <br />conditions throughout the Colorado River basin. Although much moderated from pre-dam <br /> <br /> <br />patterns some cyclic patterns appear to be present in the relatively short existence of Glen Canyon <br /> <br /> <br />Dam. Annual salinity variations, like temperature, were brought to stable levels in 1973 after the <br /> <br /> <br />reservoir had reached a surface elevation above 3600 ft. Some notable variations occur (Fig 14). <br /> <br /> <br />After initial filling of the reservoir to the penstock level in 1965, the river outlet works and bypass <br /> <br /> <br />tunnels were operated, releasing a large volume of water to keep the reservoir at the penstock <br /> <br />level. This routed the early summer runoff quickly through the reservoir and replaced the existing, <br /> <br />more saline water near the dam with much more dilute water. This is shown by a marked decrease <br /> <br />in salinity levels of the Glen Canyon Dam releases in 1965. Peaks in salinity occurred later in the <br /> <br />late 1960' s and again in 1980 following drought years with associated reservoir drawdown and <br />corresponding higher salinity levels in the reservoir. Salinity was markedly reduced with the high <br />water years of the mid-80's at which time dilute water replaced most of the reservoir's contents. <br /> <br />03/06/98 <br /> <br />DRAFT <br /> <br />Page 28 of 62 <br />