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Description of Study Unit The UCOL study unit (fig. 1) consists of the Colorado River and its tributaries that drain the mountains of central and western Colorado. The major tributaries to the Colorado River within the study unit are the Blue, Eagle, Roaring Fork, and Gunnison Rivers. The Colorado River, the largest river within the study-unit basin, flows southwest for about 230 mi from its headwaters in the mountains of central Colorado to the Colorado-Utah State line. Land use is the major factor affecting nutrient loading within the study unit. Areas that have a rapidly developing infrastructure to support recreational activities within the Fraser and Eagle River Basins and agricultural areas within the Grand Valley and lower Gunnison River Basin are most associated with elevated nutrient loading (Spahr and Wynn, 1997). The study unit is primarily rural and has a population of 234,000 people (Bureau of Census, 1990). More than a quarter of the population resides in the immediate vicinity of Grand Junction, Colo. Although not accounted for in Bureau of Census figures, tourism attracts a large influx of people to the basin during summer and winter (Driver, 1994). An extensive analysis of the environmental setting of the UCOL study unit, including land use, physiography, climate, ecoregion, and hydrologic characteristics, is presented in Apodaca and others (1996). NITROGEN AND PHOSPHORUS DATA Sources Data used in the surface-water nutrient analyses were obtained from two sources: (1) The USGS National Water Information System (NWIS) (Maddy and others, 1990), and (2) the U.S. Environmental Protection Agency (USEPA) STORET (STOrage and RETrieval) system. The STORET system is used as a repository for water-quality data by many agencies. The following agencies provided data for this report: USGS; Colorado Department of Public Health and Environment; U.S. Department of Agriculture, Forest Service; and Denver Board of Water Commissioners. Retrieval Water-quality data for January 1, 1980, through August 31, 1994, were retrieved for sites within the UCOL study unit. Only records containing data for concentrations of nitrate, ammonia, total nitrogen, total phosphorus, orthophosphate, or a related constituent were retained. Effort was made to exclude sample sites that do not reflect ambient stream conditions; therefore, data collected at point sources are not included in this report. In December 1994, STORET records for sites were retrieved. Updates or changes made to data in the STORET system after December 1994 are not included in this report. Screening Many sites and samples were excluded from the final data set if definitive corrections1for erroneous location information, site type, or nutrient data values were not possible. In some instances, data entry errors were discovered and then corrected after verifying data values with the agency that collected the data. The data set was compiled to determine water-quality patterns on a regional scale; therefore, some sites were retained that have only generalized l~titude and longitude values. These sites are identifiable by latitude and longitude values that ell(~ in "00." Other sites that had comparatively large err~rs in latitude and longitude or did not represent ambient surface-water conditions were excluded. Additional screening of sites based on quantity of data was performed during data compilation. Compilation The NWIS and STORET data were combined into a single data set for analysis after screening for errors. The parameter codes for the STORET data were converted, as necessary, to their NWIS equiva- lents before merging the two data sets into a single data base. All the NWIS parameter codes and their definitions in the order that they appear on the data diskette provided with this report are listed in table 1. Additional parameters created during data compilation or analysis are included in table I and on the data diskette. 2 Nitrogen and Phosphorus Data for Surface Water In the Upper Colorado River Basin, Colorado, 1980-94