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<br />6 <br /> <br /> <br />of natural salt movement from the rocks and soils to the river <br />system. Activities of man which influence the "natural" <br />~ contribution include grazing, logging, mining, o~l exploration, <br />-.J road building, recreation, and others. Natural causes inclUde salt <br />~ contribution from saline springs, ground water discharge into the <br />~ river system, erosion and dissolution of sediments, and the <br />concentrRting effects of evaporation and transpiration. <br /> <br />Of the land within the ColoradO River basin, about 75 percent <br />is owned and administered by the Federal Government or held in <br />trust for Indian tribes.. By far the greatest portion of natural <br />sal t load originates on these federally owned and administered <br />lands. <br /> <br />Human-caused increases in salini ty concentration, as <br />identified by EPA, result from a number of man's activities. EPA <br />estimated that out-of-basin exports account for about 3 percent of <br />increased salinity concentrations, irrigation accounts for about <br />37 percent, reservoir evaporation accounts for about 12 percent, <br />and municipal and industrial uses account for about 1 percent. Of <br />the salinity resulting from human activities, irrigated ag~iculture <br />accounts for the largest share. Much of this contribution is from <br />federally developed irrigation projects. <br /> <br />Evaluations of the salinity of the Colorado River have been <br />made by Reclamation, the U.S. Geological Survey (USGS), and the <br />Bureau of Land Management (BLM). They were published by the <br />agencies during the period of this Review (1987-1989). <br /> <br />In order to evaluate changes in salinity, water quality and <br />streamflow data are obtained on a daily, weekly, monthly, or <br />quarterly basis at various points on streams throughout the basin <br />by the USGS in cooperation with the states and other federal <br />agencies. Average annual salinity concentrations and salt loads <br />are determined on a flow-weighted basis using the most frequent <br />data available. Gaging stations in the basir, which are of <br />significance cO this report, and for which streamflow and water <br />quality records are available, are listed on Table 1. This table <br />shows the mean streamflow and flow-weighted mean dissolved solids <br />concentrations for key stations during the period 1941-1988. Where <br />the water quality information is not complete, the missing data <br />have been estimated by correlation with data from other stations. <br /> <br />Historical Salinity Concentrations <br /> <br />Salinity concentrations of the river have fluctuated <br />signific9ntly over the period of record, (1941-1989; Figure 1). <br />Salini tyconcentrations generally decre<;lse in perie:dS of high flows <br />and increase in periods of low flows. Figure 1 shows the mean <br />annual flow of the Colorado River at Imperial Dam. <br />