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<br />0"'148)') <br />U .1...l ~ r.... <br /> <br />Consequently, the mean monthly concentration of the ungaged inputs <br />for the 1941-1962 period was computed by taking the ratio of the <br />total mass of salts to the total quantity of water. This is equiv- <br />alent to forcing a straight line through the data using a slope <br />equal to the mean concentration and a zero intercept. The mean <br />value was 1.512 tons/acre-foot (1,112 ppm). Recomputed salinities <br />for 1963-1970 are included in the revised results of Tables C-5 and <br />C-6 and in Figures C-S and C-6. Due to rounding, concentrations <br />shown in Table C-6 are slightly different. <br /> <br />Lees Ferry to Grand Canyon <br /> <br />This stretch was treated as a river reach. Results are presented <br />by Figures C-8 through C-IO and Tables C-7 through C-9. Results <br />do not exhibit any obvious problems and they were used for sub- <br />sequent runs. <br /> <br />Lake Mead Reach <br /> <br />The Lake Mead reach extends from the Grand Canyon station to Hoover <br />Da~. Outflows are represented by the regulated present modified <br />releases below lioover Dam. The historic end-of-month contents, <br />Table C-lO and Figure C-ll, were used with the area-capacity rela- <br />tions of Table I and evaporation rates of Table II to compute esti- <br />mated historical evaporation quantities. <br /> <br />A bank storage coefficient of 0.06 was used to account for changes <br />in bank storage. Specific accounting of various streams such as <br />Bright Angel, Kanab, Tapeats, and Havasu Creeks, the Muddy and Vir- <br />gin Rivers, and Las Vegas Wash was not made. These inputs, as well <br />as errors in estimating historic evaporation, are all included in <br />the ungaged tributary inputs computed by the mass balance approach. <br /> <br />The initial reservoir salinity was taken as the January 1941 concen- <br />tration of the present modified data at the station below Hoover Dam. <br />Although this is an approximation, it has a relatively minor effect <br />on computed results. In any case, the importance of initial salinity <br />is rapidly diminished with time. <br /> <br />Results are presented in Figures C-12 through C-14 and Tables C-ll <br />through C-13. Inspection of the plots reveals a distinct change <br />in the ungaged inputs commencing in 1963. Again, closure of Glen <br />Canyon Dam and an altered operating policy for Lake Mead are pos- <br />sible explanations. <br /> <br />Although the mean monthly ungaged flow of 35,000 acre-feet and salin- <br />ity of 46,000 tons are reasonable, inspection of individual monthly <br />values reveals impossible losses of 2,000,000 acre-feet in flow and <br />additions of 3,000,000 tons or more of salt. Runs using these values <br /> <br />12 <br />