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<br />~ <br />Q <br />W <br />w.:.. <br /> <br />,:'; <br />C1 <br /> <br />',.' <br />~~~ <br /> <br />," <br /> <br />,-q <br /> <br />chemical processes operating in the impoundments. <br />,The U.S. Geological Survey (USGS) has monitored ion and <br />'TDS concentrations in the inflows and outflow of Lake Mead <br />,and Lake Powell since early impoundment. The purpose here is <br />to present results of our analysis of the USGS salinity data <br />'and describe how these large impoundments have historically <br />influenced ion and TDS concentrations in the Colorado River. <br />The implications of these findings are discussed relative to <br />,current efforts to control salinity in the Colorado River <br />'Basin. <br /> <br />,SALINITY STUDIES DURING EARLY IMPOUNDMENT <br /> <br />Large impoundments, like 'Lake Mead and Lake Powell, are <br />: generally thought to have a detrimental effect on salinity. <br />,This view stems from the observation that TDS concentrations <br />,ln the impoundment outflow exceed those in the inflows. Con- <br />'centration of salt by evaporation is considered a primary <br />,cause for this increase in TDS [1J. Evaporation in Lake Mead <br />ranged as high as 900,000 acre-feet (1.1 x 109m3)/yr during <br />,early impoundment [8,9J. Howard [10J noted that this caused <br />,a slight increase in TDS below Hoover Dam. Evaporation in <br />; Lake Powell has been estimated at about 500,000 acre-feet <br />: (6.2 x 108m3)/yr [11J and causes a 16 mg/l increase in TDS <br />'below Glen Canyon Dam [12J. <br />Evaporation clearly has an effect on TDS, but it <br />appears to be relatively small by comparison to that caused <br />by salt dissolution processes occurring in the impoundments. <br />'The chemical composition of the Colorado River is strongly <br />: influenced by the regional geology. Calcium, sulfate and <br />'carbonate have historically comprised 60-70% of the TDS. The <br />'impoundment of Lake Mead in 1935 further increased calcium <br />'~nd sulfate concentrations in the river [10,13J. This was <br />derived primarily from dissolution of gypsum deposits which <br />'were prevalent in the Muddy Creek geologic formations in Las <br />,Vegas Ba.z: and Virgin Basin [14J. The U.S. Bureau of Reclama- <br />,tion [15J estimated that there were only 22 acres (8.9 x <br />10' m2) of exposed salt outcroppings in the ,reservoir floor <br />prior to inundation. They predicted that rates of dissolu- <br />tion would be high during early impoundment but then dimin- <br />ish as the outcrops dissolved or became silted over. Sub- <br />sequent studies have not been conducted to evaluate this <br />prediction, or to determine if similar dissolution processes <br />occur in Lake Powell. However, Gloss et al. [12J noted that <br />there was a slight increase in sulfate concentrations at <br />Lees Ferry after the formation of Lake Powell-They suspect- <br />, ed that this was caused by dissolution of gypsum and also <br />predicted that rates would diminish as the impoundment aged. <br />The increases in TDS caused by evaporation and dissolu- <br />tion appear to be offset to some extent by precipitation of <br /> <br />2 <br /> <br />';'..,",> <br />