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<br />~ <br />o <br />0) <br />..... <br /> <br />Boulder Basin (Figure 6). This was especially evident during <br />the spring and early summer, and only on a few occasions did <br />surface temperatures in Virgin Basin exceed those in Boulder <br />Basin. Although these temperature differences could reflect <br />regional variations in climatology over the reservoir, they <br />are most likely due to advection from the Colorado River <br />inflow. The circulation patterns in Lake Mead are such that <br />the Colorado River inflow is confined primarily to the Upper <br />Basin [5J. The river forms a density current that extends to <br />Virgin Basin and into the Overton Arm. The Virgin Basin <br />appears to act like a large "mixing bowl" [6J and only when <br />river discharges are hi~h does the density current extend <br />into the Lower Basin [5J. This usually occurs during late- <br />summer after periods of prolonged, high discharges from Glen <br />Canyon Dam. <br /> <br /> 30 <br />(,) <br />0 26 <br />0) <br />.. 22 <br />::> <br />+- <br />" <br />.. 18 <br />0) <br />0- <br />E 14 ~Virgin <br />0) Ba 5 i n <br />I- --. Boulder Basin <br /> <br /> <br />10, <br />ua n <br /> <br />Feb Ma r <br /> <br />Apr <br /> <br />May <br /> <br />Jun <br /> <br />Jul <br /> <br />Aug <br /> <br />Sep Oct <br /> <br />Nav <br /> <br />Dec <br /> <br />Month <br /> <br />Figure 6. Surface 'rempera tures in Boulder Basin and <br />Virgin Basin During 1980 [Lake Mead <br />Limnological Research Center Data]. <br /> <br />Historically, adjustments to evaporation rates for <br />changes in energy storage in Lake Mead have been based on <br />temperature measurements made at Hoover Dam intake towers <br />[4J. This decision was reached on the basis of data collect- <br />ed in 1952-1953, which showed that temperature differences <br />between the Upper and Lower Basins were minimal. This is no <br />longer the case with cold-water discharges from Glen Canyon <br /> <br />10 <br />