Upper Gallinas Creek in north-central New Mexico serves as the public water supply for the City of Las Vegas. The majority of this 84-square-mile watershed is within national forest lands managed by the U.S. Forest Service. In 1985, the Forest Service planned to conduct timber harvesting in the headwaters of Gallinas Creek. The City of Las Vegas was concerned about possible effects from logging on water quality and on water-supply treatment costs. The U.S. Geological Survey began a cooperative study in 1987 to (1) assess the baseline water-quality characteristics of Gallinas Creek upstream from the Las Vegas water-supply diversion, (2) relate water quality to State water- quality standards, and (3) determine possible causes for spatial differences in quality. During 1987-90, water-quality constituents and aquatic benthic macroinvertebrates were collected and analyzed at five sampling sites in the watershed. Specific conductance, pH, total hardness, total alkalinity, and calcium concentrations increased in a downstream direction, probably in response to differences in geology in the watershed. The water-quality standard for temperature was exceeded at the two most downstream sites probably due to a lack of riparian vegetation and low streamflow conditions. The standards for pH and turbidity were exceeded at all sites except the most upstream one. Concentrations of nitrogen species and phosphorus generally were small at all sites. The maximum total nitrogen concentration of 2.1 milligrams per liter was at the mouth of Porvenir Canyon; only one sample at this site exceeded the water-quality standard for total inorganic nitrogen. At each of the sites, 10 to 15 percent of the samples exceeded the total phosphorus standard of less than 0.1 milligram per liter. Except for aluminum and iron, almost all samples tested for trace elements contained concentrations less than the laboratory detection limit. No trace-element concentrations exceeded the State standard for domestic water supplies. Suspended-sediment concentrations appeared to increase with distance downstream; suspended sediment increased significantly from the uppermost site to the second site near the national forest boundary, most probably caused by runoff from the unpaved forest road adjacent to Gallinas Creek. The aquatic macroinvertebrate assessment indicated that the three upstream sites had good biological conditions and were nonimpaired, whereas the two downstream sites had lowered biological conditions and were slightly impaired. The water- quality and biological assessments provided similar results.