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<br />~ <br />Q <br />~ <br />\,..if <br /> <br />calcium carbonate (calcite) that occurs in the impoundments. <br />Large quantities of calcite were precipitated in Lake Mead <br />during early impoundment [10J. The formation of Lake Powell <br />,substantially reduced calcite precipitation in the Upper <br />: Basin of Lake Mead, but rates,are still high in the Lower <br />,Basin [16J. Appreciable quantities of calcite also precipi- <br />itate in Lake Powell [12,17J. The combined impoundment system, <br />,may therefore precipitate more calcite than what historical- ' <br />: ly occurred just in Lake Mead. An increase in overall rates <br />:of calcite precipitation and/or a decrease in the rates of <br />; gypsum dissolution could be possible reasons for recent <br />idecreases in TDS observed in the Lower Colorado River Basin. <br /> <br />F; <br /> <br />'DATA SOURCES AND METHODS <br /> <br />" ,The U.S. Geological Survey has monitored flow rates, <br />, lon and TDS concentrations in the Colorado River and storage, <br />'in the impoundments for several years. Prior to 1970, the ' <br />, flow rates and storage data were compiled in "Surface Waters <br />, of the United States" and ion concentrations in "Quality of <br />: Surface Waters of the United States," both of which were <br />i published annually in the, U.S. Geological Survey Water~ <br />: Supply Papers, Part 2L Colorado River Basin ( 1926-1970). <br />,These records have since been published in the "Water Re- <br />sources Data" series of the U. S. Geological Survey lvater- <br />Data annual reports for individual states in the Colorado <br />'River Basin. In our analyses, we used data from the "Water <br />: Resources Data for Arizona", "Water Resources Data for <br />i Nevada" and "Water Resources Data for Utah" reports for <br />i water years 1970-79. <br />Flow vs. TDS concentration relationships were evaluated' <br />i from data collected for various time periods at Lees Ferry, <br />: ,Grand Canyon and below Hoover Dam (Figure 1). Sufficient <br />i data were available to evaluate these during pre-impoundment <br />'periods of 1926-42 and 1951-60 in Grand Canyon and 1951-60 ' <br />. at Lees Ferry. Data were also available to assess these re- <br />i lationships at Grand Canyon and Lees Ferry during the post- <br />'Lake Powell period of 1970-79 and below Hoover Dam during <br />the post-Lake Mead periods of 1935-43, 1951-60 and 1970-79. <br />Flow-weighted average TDS concentrations were computed for <br />_individual years within each time period. Changes in TDS <br />, concentrations vs. flows were computed between each year in <br />these time periods and used to construct statistical rela- <br />, tionships for each location. <br /> <br />3 <br />