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<br /> <br />. <br /> <br />~ <br />~ <br />~ <br /> <br />EVALUATIONS OF EXISTING SALI~ITY CONDITIONS <br /> <br />.", <br />, <br /> <br />of ne~ instrumental methods. ~e~ procedures were adopted only after <br />careful investigation to insure results consistent with those obtained <br />previously. Some of the quality of water records are based on analysis <br />of samples by Bureau of Reclamation laboratories. Bureau of Reclamation <br />results and methods have been checked by the Geological Survey to insure <br />comparable records. It is probable that errors in the load computations <br />due to errors in chemical analysis are less than those due to changes in <br />the samples upon storage. inaccuracies in sampling. or inaccuracies in <br />the determination of stream discharges. <br /> <br />Prior to about 1970 the U.S. Geological Survey analyzed water <br />quality on a composite sample basis and also determined and published <br />the annual total dissolved-solids loads. Since that time the results of <br />the analysis in the Colorado River Basin have been those of individual <br />samples rather than composites and no totals for the year have been <br />computed. At present individual samples are taken and analyzed about <br />once a month together with daily conductivities. The annual total <br />dissolved-solids loads since this change, have been determined from <br />daily conductivities applied to a curve or conversion factors relating <br />conductivities and total dissolved-solids concentrations. <br /> <br />C. Historic Conditions <br /> <br />. <br /> <br />1, <br /> <br />Total Dissolved-Solids Concentrations <br /> <br />Historic streamflow, total dissolved-solids salinity concentra- <br />tions, and salt-load data for the 17 key stations for the 1941-74 <br />period of record are presented in Table 1 to 17 with each table number <br />of record are presented in Tables 1 to 17 with each table number corre- <br />sponding to a station number. The concentrations as shown were determined <br />on a flow weighted basis. <br /> <br />To simplify tabulation, monthly values of flow and total dissolved- <br />solids loads were rounded to the nearest 1,000 acre-feet and tons. This <br />resulted in some differences between the recorded and the computed <br />monthly concentrations when the flows were low, for example, below 1,000 <br />acre-feet in the San Rafael and Duchesne Rivers. Similarly, minor <br />differences from published data in monthly concentrations occur in <br />isolated instances in the flow and quality tables for the other stations. <br /> <br />The water quality at the Lees Ferry and the four other key stations <br />on the Lower Colorado River has been affected by abnormal conditions <br />during the 1959-74 period because of low runoff in 1959, 1960, and 1961 <br />and the filling of Lake Powell in 1963 to 1974. Figure 3 shows the <br />historical flow weighted average salinity concentration for these five <br />stations. <br /> <br />. <br /> <br />During the first year of storage in Lake Powell in 1963, the flow <br />at Lees Ferry was reduced to 1,384,000 acre-feet with a salinity con- <br />centration of 1.27 tons per acre-foot. The average concentration for <br />the 1941-74 period was 0.76 tons per acre-foot. <br /> <br />42 <br />