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rv o c..."J o ESTIMATION OF NATURAL DISSOLVED-SOLIDS DISCHARGE IN THE UPPER COLORADO RIVER BASIN, WESTERN UNITED STATES By David K, Mueller and Lisa L. Osen ABSTRACT A statistical method was developed to estimate monthly natural dissolved- solids discharge at selected sites in the Upper Colorado River Basin. Natural dissolved-solids discharge was defined as the rate of inorganic-solute flow past a specific site that would have occurred if there had been no water- resources development in the basin upstream from the site. The method used weighted least-squares regression to fit a model of dissolved-solids discharge as a function of streamflow and several variables representing development. After the model had been calibrated for an individual site, the development variables were assigned a value of zero to yield a relation between dissolved- solids discharge and streamflow for conditions of no upstream development, Natural dissolved-solids discharge was calculated using this relation and estimates of natural streamflow provided by the U.S. Bureau of Reclamation. Limitations of the method included lack of data to adequately represent all the effects of development and verify the estimates of monthly natural dissolved-solids discharge; however, model statistics indicated a good corre- lation between the estimates and historical data. Also, the estimates of natural dissolved-solids discharge had the expected monthly distribution and were consistent between upstream and downstream sites. INTRODUCTION The Colorado River system is one of the most regulated and most legis- lated in the world, It provides water for more than 12 million people and for approximately 2.5 million acres of agricultural land (U.S. Bureau of Recla- mation, 1983). However, most of the Colorado River basin is arid or semiarid, and the average runoff is much smaller than that for other similarly sized river basins in the United States. The Colorado River basin has a total area of 244,000 square miles, including parts of Mexico and seven States (Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming), It is legally divided into an upper and lower basin at a point on the Colorado River 1 mile downstream from the mouth of the Paria River and 2 miles downstream from the streamflow-gaging station at Lees Ferry, Arizona. The Upper Colorado River Basin (fig. 1) has an area of approximately 113,000 square miles, of which approximately 4,000 square miles is. noncontributing, including the Great Divide Basin and nearby areas in Wyoming. Mean annual streamflow at Lees Ferry was approximately 10 million acre-feet from 1942 through 1983 (U.S, Department of the Interior, 1985) . fj' 1 , d