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<br />~ Discharges from Glen Canyon Dam are regulated for power <br />.~ generation and flood control purposes. This has eliminated <br />~ the spring discharge pulse that occurred historically due to <br />CO runoff from the upper Colorado River drainage system (Figure <br />2). Monthly discharges are now subject to much less varia- <br />tion and peak discharges usually occur in summer when power <br />demands are greatest, <br /> <br />Energy Advection <br /> <br />The alterations in temperature and discharge cycles in <br />the Colorado River have had a significant influence on ener- <br />gy advection into Lake Mead, Investigations conducted in <br />1948 by Anderson and Pritchard [6] and in 1952-53 by Harbeck <br />et al. [4] showed that large quantities of energy were ad- <br />vec t',d into Lake Mead during spring and early summer (Figure <br />3). Advection contributed 300-400 cal/cm2.day of heat to the <br />refJ"l'voir during these periods. This was nearly half that <br />derlv'ed from solar radiation. In contrast to pre-Lake Powell <br />periods, advection now contributes minimal heat to Lake Mead <br />(Figure 3). Cold-water discharges from Glen Canyon Dam re- <br />sulted in a net heat gain of only 9.04 cal/cm2'day during <br />1977-1978. This has had a marked influence Gn evaporation <br />rates from the reservoir. <br /> <br /> 500 <br /> 300 <br />,,., <br />0 <br />"0 100 <br />(\j 0 <br />'E <br /><> <br /> -100 1977 -78 <br />0 <br />U <br /> -300 <br /> <br /> <br />-500 <br /> <br />Ocl Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep <br />Month <br /> <br />Figure 3. Net Advective Energy in Lake Mead During <br />1948 [6],1952-53 [4] and in 1977-78 <br />[This Study]. <br /> <br />7 <br />