During 1999, a study was conducted by the U.S. Geological Survey (USGS) in cooperation with the Bureau of Reclamation to evaluate the effect of the Paradox Valley Unit on dissolved solids, sodium, and chloride in the Dolores River downstream from the Paradox Valley Unit. This report describes this evaluation and presents the results from this study. Daily mean flow and daily mean specific conductance, measured at gages upstream and downstream from the Paradox Valley Unit, and results from monthly water-quality samples are used to estimate changes in the dissolved-solids load and concentrations of sodium and chloride in the river as it crosses the valley and to correlate these changes with withdrawals of brine by the Paradox Valley Unit. The time period for this evaluation was restricted to October 1987—September 1998 (water years 1988—98) because regular collection of water-quality samples from the Dolores River in the valley gaban in 1987.