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pounds active ingredient applied. These pesticides were primarily applied to corn, dry beans, pasture, and alfalfa. Newer pesticides, such as acetochlor, were not included in the 1989 or 1992 pesticide-use surveys. Nationally, the most heavily used pesticides in agricul- ture in 1992 were the herbicides (1) atrazine, (2) meto- lachlor, (3) 2,4-D, (4) cyanazine, and (5) alachlor (U.S. Geological Survey, 1998a). Study Design The surface-water sampling for this study was designed to investigate the occurrence and temporal and spatial distribution of pesticides in the rivers, streams, and drains of the UCOL study unit. To accomplish this, two sampling networks were devel- oped-the fixed-station network and the synoptic network. The fixed-station network was developed out of the larger surface-water-monitoring network of the UCOL study unit (Spahr and others, 1996). Four stations were designated for pesticide collection, one in the Southern Rocky Mountains physiographic prov- ince and three in the Colorado Plateau physiographic province (fig. 1, table 2). Three of the stations -Gore Creek at mouth near Minturn, Dry Creek near Begonia Road, near Delta, and Reed Wash near Mack-were selected as "indicator" sites. These sites represent a particular land use and in this study represented either urban/forest or agriculture. The watershed for the Gore Creek station includes Vail, Colorado, a resort town with a permanent population of 4,353 in 1995 (North- west Colorado Council of Governments, 1997) and substantial increases in summer and winter because of tourism. Most of the Gore Creek watershed is forest land that is included in the White River National Forest. Both Dry Creek near Begonia Road and Reed Wash near Mack are natural watercourses in the agri- cultural areas of the Uncompahgre River Valley and Grand Valley, respectively, and function as drains for irrigation return flows during parts of the year. The final station in the fixed-station network, the Colorado River near Colorado-Utah State Line, was selected as an "integrator" site. Located at the terminus of the study unit, this station represented the integration of water quality from all land uses in the Upper Colorado River Basin in Colorado, including agriculture, forest, mining, rangeland, and urban. Each station was sampled approximately monthly fora 6-month to 1-year time period; sampling occurred from October 1996 through January 1998. Samples were collected more frequently (weekly or biweekly) during the spring and summer growing season. The total number of samples collected at each site varied, depending on expected pesticide use, and was greater for the Dry Creek near Begonia Road and Reed Wash near Mack agriculture sites. Table 2. Description of surface-water-sampling sites for pesticides in the Upper Colorado River Basin fixed-station network [USGS, U.5. Geological Survey] Number of Site samples Drainage number Site name USGS site Sampling (excluding Site type Land use area (fig 1) number period quality (square assur- miles) ance) FS 1 Gore Creek at mouth near Minturn 09066510 10/96-09/97 10 Indicator Urban/Forest FS2 Dry Creek near Begonia 09149480 10/96-09/97 15 Road, near Delta FS3 Reed Wash near Mack 09153290 10/96-09/97 24 FS4 Colorado River near 09163500 04/97-01/98 8 Colorado-Utah State Line tIncludes agriculture, forest, mining, rangeland, and urban Indicator Agriculture 102 175 Indicator Agriculture 16 Integrator Allt 17,843 6 Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996-98