A study was initiated in 2009 by the U.S. Geological Survey (USGS), in cooperation with Routt County, the Colorado Water Conservation Board, and the City of Steamboat Springs, to compile and analyze historic water-quality data and assess water-quality conditions in the Upper Yampa River watershed (UYRW) in northwestern Colorado. Water-quality data for samples collected by federal, state, and local agencies for various periods from 1975 through 2009 were compiled and assessed for streams, lakes, reservoirs, and groundwater in the UYRW, including the Elkhead Creek subwatershed and the Yampa River watershed that is upstream from Elkhead Creek. For selected physical-property and chemical-constituent data for samples collected from surface-water sites and groundwa- ter wells in the UYRW, this report: (1) characterizes available data through statistical summaries, (2) analyzes the spatial and temporal distribution of water-quality conditions, (3) identifies temporal trends in water quality, where possible, (4) provides comparisons to federal and state water-quality standards and recommendations, and (5) identifies factors affecting the qual- ity of water. In addition, the availability and characteristics of macroinvertebrate data collected in the UYRW are described.