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<br />~ <br />Q <br />.... <br />~ <br /> <br />Net water losses estimated for Lake Mead during the <br />1970-79 period were higher than the 1951-60 period. This was <br />also unexpected because evaporation rates in Lake Mead have <br />,decreased significantl~ due to cold-water discharges from <br />Glen Canyon Dam [19,20J. However, water levels in Lake Mead <br />have risen steadily since Lake Powell was formed in 1963. <br />Significant quantities of water are retained in bank storage <br />in Lake Mead when levels increase after extended periods of <br />draw down [9]. This appears to be the principal reason for <br />higher net water losses during the 1970-79 period. <br /> <br />, DISCUSSION <br /> <br />'.1: <br /> <br />The large impoundments on the Colorado River clearly <br />have ,a significant effect on salinity. The relationships of <br />flow and TDS differed markedly between pre- and post- <br />~mp~uadment periods. The reason for this is that concentra- <br />tions of sulfate, carbonate and calcium, the principal ions ~ <br />in the river, were altered by dissolution and precipitation <br />, processes occurring in the impoundments (Table II). Disso- <br />, lution of gypsum significantly elevated concentrations of <br />, sulfate during all time periods. Halite dissolution also <br />caused slight increases in concentrations of sodium and <br />chloride. Conversely, calcite precipitation caused marked <br />reductions in concentrations of calcium and carbonate. How- <br />ever, this was not sufficient to offset increases caused by <br />dissolution, and TDS concentrations were elevated above <br />expected levels in all time periods. <br /> <br />: Table II. Estimated Average Annual Rates of Calcite <br />(CaCOs) Precipitation and Gypsum (CaSO,) <br />and Halite (NaCl) Dissolution for Various Time <br />Periods in Lake Mead and Lake Powell. Calcite <br />Precipitation Was Estimated from Molar Changes <br />in Carbonate Ion and Gypsum Dissolution from <br />Molar Changes in Sulfate Ion. Calcium Concen- <br />trations Given in Parentheses. [USGS Data]. <br /> <br /> CaC03 CaSO. NaCl <br />Locations/Time Periods (mg/l) (mg/l) (mg/l) <br />Lake Powen/1970-79 23 ( 9) 16 ( 5) 5 <br />Lake Mead/1970-79 15 ( 6) 37 (11) <br />Lake Mead/1951-60 48 (19) 75 (22) 3 <br />I,ake ~lead/1935-48* 47 123 19** <br />*From Howard [10], Gould [13]. <br />**Includes potassium. <br /> <br />9 <br />