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o CD to *'" SUMMARY This report presents the results of a study initiated by the Colorado River Water Quality Office of the USBR (Bureau of Reclamation) to identify possible systematic changes in salt loading tothe Colorado River and its major tributaries. A systematic change may involve long-term alterations of natural sources of salt or the impact of human activities on salt loading or salt transport in the river. Systematic changes have been suggested as a possible cause ofthe recent decreases in the salt concentration at Imperial Dam in the Lower Basin. Reducing the salt load in the river is the goal of the Colorado River Water Quality Improvement Program. If systematic changes in the salt loading character- istics of the basin have occurred, they must be considered in formulating plans to achievesalinity reduction. A systematic change might manifest itself as a long-term shift in the concentration, load (absolute mass), or mass fraction (relative mass) of one or more of the major solutes in the system. Probable shifts can be identified using trend analysis or by comparison of mean values before and after major alterations within the basin.. Both approaches were used in this study. Regression techniques were employed to test for significant trends and Student's t test was used to assess the mean value changes. Six major ions were selected for analysis: calcium, magnesium, sodium, chloride, sulfate, and bicarbonate. Together these account for more than95 percent ofthetotal solute load atthe major gage sites in the basin. Fifteen sites, representing the major divisions of the basin, were studied. Data for these sites were obtained from the USGS (U.S. Geological Survey) computerized water data base, WATSTORE. To isolate potential sources of significant trends or mean value changes, these changes must be traceable from downstream to upstream gage sites. Therefore, consistent data sets were required for all stations. The first step in the study was to create a data file of representative monthly average streamflow and solute concentration values for the selected gage sites. These computer data files are being maintained for future research on the Colorado River system. The data generated was first used to test various methods relating solute concentration to stream- flow. This was necessary so changes in concen- tration caused by natural flow variations could be accounted for in subsequent regression analyses. A theoretical residence time model was compared with the widely used empirical power model. The two models produced equally good predictions for periods prior to flow regulation. These models are available for updating input data used in the USBR Colorado River Simulation System models. Although 15 gage sites were selected, trend analyses were completed for only the 12 sites that had adequate records of water quality prior to major flow regulation. Several different regression models were used, and their results were generally corroborative. Significant negative (decreasing) trends were found for magnesium, sodium, and sulfate at approximately 75 percent ofthe sites. At the Lees Ferry and Grand Canyon gage sites, concentrations of these three ions declined in proportion to their electrical charge and relative mass, resulting in a reduction in TDS (total dis- solved solids) of approximately 2 mg/L per year for the low flow period, December-March, from 1926to 1961. After construction of the Colorado River Storage Project reservoirs in the early 1960's, the variability of both solute concentrations and mass fractions was reduced at downstream gaging stations. This reduction was generally proportional tothe degree of regulation. At some sites, trends toward further reduction in variability developed after initial reser- voir filling. Several other activities of man, including mining, urbanization, and salinity control practices, may have affected water quality in the basin during this period. Impacts decreased with distance downstream, but were often identified at more than one gaging station. However, the major reservoirs, Lakes Powell and Mead, concealed all impacts that had occurred in the Upper Basin. No significant trends in the load for any solute have been identified for the station below Hoover Dam since 1965; however, the concentration of all solutes except chloride decreased there during at least one season. Additional research is recommended to determine the possibility and probable magnitude of con- tinuing impacts of major reservoirs on down- stream salinity. Also, because of the relative chemical stability at the gage site below Hoover Dam, the changes observed downstream at Imperial Dam may have resulted from changing conditions in the Lower Basin area. Therefore, an extension of the present study, with additional Lower Basin gage sites included, is recommended. This extension should also update the monthly data' base and data base generation procedures, and limit statistical analyses to those recommended in the present work.