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<br /> <br />dissolved-solids concentrations that were not within the historical range <br />occurred at sites 10 (Yampa River near Maybell, Colo.) and 23 (Colorado River <br />below Parker Dam, Ariz.-Calif.). Streamflow record extension for sites 5 and <br />9 and dissolved-solids record extension for site 23 were made using techniques <br />tv which required data from a base-station site. If the extreme values were <br />~ larger at the base-station site during the extension period than during the <br />t~ concurrent historical period, estimates outside the historical 'range at the <br />to record-extension site are to be expected. Dissolved-solids estimates for site <br />10 were made by regression on streamflow; therefore, estimates that are not <br />within the range of historical values involve extrapolation and may be inac- <br />curate. However, such an estimate occurred only once. This estimate was 8 <br />percent larger than the historical maximum monthly concentration, and was <br />associated with an extremely low monthly streamflow; therefore, the potential <br />error in this estimate has a negligible effect on the computed dissolved- <br />solids discharge at the site. <br /> <br />At four sites (5, 12, 16, and 17) the difference between the last monthly <br />value of dissolved-solids concentration during the record-extension period and <br />the first monthly value during the historical period was greater than 20 <br />percent of the historical value. However, the absolute magnitudes of the <br />differences at each site were within the range of the historical differences <br />between consecutive months. <br /> <br />SUMMARY <br /> <br />Monthly streamflow and dissolved-solids concentrations were estimated for <br />the period from October 1940 through the initiation of data collection at <br />selected sites in the Colorado River basin. The technique used for <br />streamflow-record extension was simple linear regression on streamflow at a <br />base-station site having a period of record that included the entire record- <br />extension period and at least part of the historical period at the record- <br />extension site. Standard errors ranged from 5 to 74 percent of the observed <br />mean monthly streamflow. <br /> <br />The primary technique used to extend dissolved-solids records was <br />weighted regression on streamflow. If a complete record of streamflow was not <br />available at the record-extension site, streamflow at a base-station site was <br />used. Weighted regression was used for all sites upstream from Lake Powell <br />and for a site on the Virgin River in Arizona. Simple regression on <br />dissolved-solids concentration at a base-station site was used for sites on <br />the main stem of the Colorado River downstream from Lake Powell. Standard <br />errors ranged from 3 to 27 percent of observed mean dissolved-solids <br />concentration. <br /> <br />The extended records at all sites were consistent with records from the <br />historical periods. The resultant monthly values provide estimates of stream- <br />flow and dissolved-solids concentration to complete the record back through <br />water year 1941 at 13 sites in the basin. <br /> <br />22 <br />