From purposes and approach of study: The primary purposes of this study were to: (1) Define, on a reconnaissance level, current (1980) water-quality conditions in major aquifers (water-yielding rock formations) in the western Snake River basin, Swan Falls to Glenns Ferry; (2) summarize and interpret available geologic and hydrologic data to assist in understanding the natural and man-cause factors that affect present and future understanding the natural and man-caused factors that affect present and future water-quality conditions; and (3) establish a hydrologic data base on which future data can be compared to evaluate changes. A secondary purpose was to evaluate the possible effects percolation of thermal water applied for irrigation may have on the quality of local, less mineralized, nonthermal ground water.