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Streamwater Quality at Selected Sites in the Fraser River Basin, Grand County, Colorado, Water Years 1991-2000 By Jeffrey B. Bails Abstract To determine the effect of population growth on stream- water quality in the Fraser River Basin, the U.S. Geological Survey did a study in cooperation with the Grand County Commissioners and the East Grand County Water Quality Board. During water years 1991 through 2000, the study determined that concentrations of un-ionized ammo- nia and nitrite plus nitrate in the streamwater of the basin are within Colorado State streamwater-quality standards. The study also found that concentrations of chloride are largest at the headwaters and decrease downstream; however, chloride loading in the stream has the opposite relation. Most nutrient loading to the Fraser River happens January through May. Concentrations of ammonia at Fraser River downstream from Vasquez Creek at Winter Park had a downward trend through the period of the study. Nitrite plus nitrate had upward and downward trends at different sites and over different time spans. Orthophosphorus concentrations had upward trends at two sites. In general, the streamwater quality in the Fraser River Basin is good and is not out of compliance with State standards. Introduction The Fraser River Basin in the southeastern part of Grand County, Colorado, encompasses about 287 square miles (fig. 1). The Fraser River originates at the Continental Divide in the Arapaho National Forest, flows about 29 miles in a northerly direction through the towns of Winter Park, Fraser, Tabernash, and Granby, and joins the Colorado River 2 miles downstream from Granby. A more detailed discussion of the characteristics of the Fraser River Basin is found in Apodaca and Bails (1999). The population of Grand County increased more than 56 percent between 1990 and 2000 (2000 census population of 12,442) and increased more than 300 percent since 1970 (U.S. Census Bureau, 2001). These population statistics do not include the thousands of people that visit the Fraser River Basin each year to enjoy the many year-round recreational activities and place demands on the water resources of the basin. The Fraser River upstream from the town of Tabernash is currently the recipient of wastewater discharges from three wastewater-treatment plants that serve the Winter Park ski area, the town of Winter Park, and the town of Fraser (fig. 1). Local water managers are concerned about the effects of population growth on streamwater quality. To address this need, the U.S. Geological Survey, in coop- eration with the Grand County Commissioners and the East Grand County Water Quality Board, conducted a study of streamwater quality in the Fraser River Basin. Basin Characteristics The land-surface elevation of the Fraser River Basin ranges from about 12,500 feet at Berthoud Pass to about 7,860 feet at the mouth of the Fraser River. The major tributar- ies to the Fraser River are Vasquez, St. Louis, Crooked, Ranch, Strawberry, and Tenmile Creeks. In the upper reaches of the Fraser River, three hydrologic modifications divert water from St. Louis Creek, Vasquez Creek, and the main stem of the Fraser River to the Moffat Water Tunnel system. According to the Colorado Water Quality Control Division (1989), these diversions have the potential to reduce streamflow substan- tially and compound water-quality problems in the Fraser River Basin. Climatic data for Fraser from 1989 through the end of 2000 indicate that the average monthly maximum and mini- mum temperatures are 75.7 degrees Fahrenheit in July and -3.9 degrees Fahrenheit in December. Average annual pre- cipitation for Fraser is 19.2 inches, which includes an average annual snowfall of 154 inches. At Berthoud Pass, the head- waters of the Fraser River, the average annual precipitation from 1950 through 1985 is 37.5 inches, which includes an average annual snowfall of 391 inches (Western Regional Climate Center, 2001).