Water-quality data were collected and compiled for 59 surface-water stations in the Arkansas River Basin of Colorado. The purpose of the data collection was to describe selected water-quality characteristics of the Arkansas River from the headwaters downstream to the Colorado-Kansas State line. Data are presented for 19 Arkansas River stations, 31 tributary stations, 2 mine-drainage stations, and 7 transmountain diversion stations. Water-quality data presented in this report include instantaneous discharge; onsite measurements of specific conductance, pH, water temperature, and dissolved oxygen; analytical concentrations of bacteria, dissolved solids, major nutrients, trace elements, pesticides, radiochemicals, and suspended sediment; and quality-assurance data for selected water-quality constituents. Sampling began in April 1990 and continued through March 1993 at the 59 surface-water stations. The basinwide water-quality study was initiated in 1988 by the U.S. Geological Survey in cooperation with 14 local agencies and the Bureau of Reclamation.