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Characteristics and Trends of Streamflow and Dissolved Solids in the Upper Colorado River Basin, Arizona, Colorado, New Mexico, Utah, and Wyoming By Timothy D. Liebermann, David K. Mueller, James E. Kircher, and Anne F. Choquette Abstract Annual and monthly concentrations and loads of dissolved solids and major constituents were estimated for 70 streamflow- gaging stations in the Upper Colorado River Basin. Trends in streamflow, dissolved-solids concentrations, and dissolved-solids loads were identified. Nonparametric trend-analysis techniques were used to determine step trends resulting from human ac- tivities upstream and long-term monotonic trends. Results were compared with physical characteristics of the basin and historical water-resource development in the basin to determine source areas of dissolved solids and possible cause of trends. Mean annual dissolved-solids concentration increases from less than 100 milligrams per liter in the headwater streams to more than 500 milligrams per liter in the outflow from the Upper Colorado River Basin. All the major tributaries that have high concentrations of dissolved solids are downstream from exten- sive areas of irrigated agriculture. However, irrigation predated the period of record for most sites and was not a factor in many identified trends. Significant annual trends were identified for 30 sites. Most of these trends were related to transbasin exports, changes in land use, salinity-control practices, or reservoir development. The primary factor affecting streamflow and dissolved-solids concentration and load has been the construc- tion of large reservoirs. Reservoirs have decreased the seasonal and annual variability of streamflow and dissolved solids in streams that drain the Gunnison and San Juan River basins. Fontenelle and Flaming Gorge Reservoirs have increased the dissolved-solids load in the Green River because of dissolution of mineral salts from the bank material. The largest trends oc- curred downstream from Lake Powell. However, the period of record since the completion of filling was too short to estimate the long-term effects of that reservoir. INTRODUCTION The Colorado River and its tributaries comprise one of the primary sources of water in the and American West. The availability and quality of water have been central fac- tors in settlement and development in the Colorado River basin and in neighboring and regions. Much legislation and many legal agreements have been established to govern the distribution of water and to maintain water quality in the basin. The Colorado River basin formally was divided into upper and lower basins by the Colorado River Compact of 1922 (Upper Colorado River Commission, 1950). The divi- sion occurs at the compact point, which was named Lee Ferry, Ariz. (pl. 1), located 1 mi downstream from the mouth of the Paria River and 1.4 mi downstream from the streamflow-gaging station at Lees Ferry, Ariz. The Upper Colorado River Basin comprises parts of the States of Arizona, Colorado, New Mexico, Utah, and Wyoming. The Upper Colorado River Basin annually discharges about 7 million tons of dissolved solids from natural sources and sources related to human activities. This dissolved-solids load is a major concern to agricultural, municipal, and in- dustrial users in the Lower Colorado River Basin. "Dissolved solids" is the sum of the individual dis- solved constituents present in water. Dissolved-solids concentration is the quantity of dissolved solids in a unit volume of water. In other reports on the Colorado River basin, dissolved-solids concentration often is referred to as "salinity." Dissolved-solids load is the product of dissolved- solids concentration and streamflow and represents the quan- tity of dissolved material transported downstream. In this report, the unit of measure for dissolved-solids concentra- tion is milligrams per liter, and the unit of measure for dissolved-solids load is tons. The mass fraction of a dissolved constituent is the proportion of that constituent within the overall dissolved load. In the Upper Colorado River Basin, the major dissolved constituents are the cations calcium, magnesium, sodium, and potassium; the anions sulfate, chloride, and bicarbonate; and electrically neutral silica. Compared to the sum of these major constituents, other dissolved constituents such as car- bonate, nitrate, and organic-carbon compounds ordinarily are not present in substantial quantities. For calculating dissolved-solids concentration as the sum of the constituents, Introduction 1