The spatial and temporal distribution of selected physical and chemical surface-water-quality characteristics throughout southwestern Colorado were analyzed using historical data collected from 1990 through 2005 by various local, State, Tribal, and Federal agencies. This analysis was done to provide an understanding of existing water-quality conditions in the Dolores River and San Juan River basins in southwestern Colorado as part of a larger surface-water and groundwater assessment conducted by the U.S. Geological Survey in cooperation with Bureau of Land Management, Bureau of Reclamation, Southwestern Water Conservation District, San Miguel County, and Telluride Power/Water. Summary statistics were calculated for selected physical and chemical surface-water water-quality characteristics at sites with 10 or more samples. These values were then compared and described spatially. Temporal trends in surface-water waterquality characteristics were calculated at sites with adequate data to determine if changes in values had occurred over time.