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increase stream specific conductance. Although infor- mation regarding rock-salt application was quantified only for the Black Gore Creek area, Interstate 70 is adjacent to Gore Creek through the Town of Vail and is subject to traction sanding; therefore, some quantity of salt probably enters Gore Creek from the Interstate 70 roadway downstream from the confluence with Black Gore Creek. An additional source of salt that may affect specific-conductance values in Black Gore Creek is liquid magnesium chloride (MgCI), which has been used since 1995 in conjunction with traction sand to keep Interstate 70 open during snowstorms. The annual number of MgCI applications to Interstate 70 is unknown; however, a single application from Vail Pass to Gore Creek requires approximately 1 ton of material. The use of MgCI is eventually expected to reduce the amount of traction sand required on Interstate 70, but there was no reduction during the winter of 1995-96 (Lorch, 1998). Specific-conductance values in the Gore Creek watershed are relatively low (median value of 145 llS/cm for 32 sites) when compared to other sites sampled in the Southern Rocky Mountains physiographic province (median value of 254 ~/cm for seven sites, Jeffrey R. Deacon, V.S. Geological Survey, written commun., 1998) as part of the VCOL NAWQA Program. Specific conductance increased in a downstream direction in the main stem of Gore Creek (fig. 13). The lowest and least variable specific- conductance values (median value of 40 ~/cm) occurred at site 1, which represents background condi- tions with the crystalline bedrock. Specific conduc- tance increased slightly at sites 4 and 10, where lower specific-conductance water from Gore, Bighorn, Pitkin, and Booth Creeks dilutes the higher specific- conductance water from Black Gore Creek. Site 16, which is downstream from site 10, Mill Creek, Middle Creek, and urban and recreational land uses, had higher specific-conductance values than site 10. The highest specific-conductance values were at site 29, at the mouth of Gore Creek. This site integrates all the natural and land-use-related sources of dissolved constituents that can increase specific conductance in the watershed. During the stable low-flow conditions in August 1996, Wynn and Spahr (1998) determined that increasing values of specific conductance and dissolved solids in Gore Creek were partially caused by inflows from tributaries such as Mill Creek and Black Gore Creek that drain areas where sedimentary rock is predominant. The Northwest Colorado Council of Governments (NWCCOG) (1993) reported that specific conductance was significantly higher at site 29 at the mouth of Gore Creek than at upstream site 10 from 1978 to 1992. The NWCCOG study concluded that periods of low flow coincide with the highest specific-conductance values when Gore Creek is more strongly influenced by discharges from the wastewater-treatment plant. These results are consis- tent with the data discussed in this report (figs. 12 and 13). The NWCCOG study also determined that specific-conductance values showed significant upward temporal trends at sites 10 and 29 between 1978 and 1992. These increases were at least partially caused by stormwater runoff from the increasing amounts of urban land-use areas within the watershed (Northwest Colorado Council of Governments, 1995). Values of specific conductance for site 29 were compared statistically by using Tukey's Significant Difference Test on the rank-transformed data (Helsel and Hirsch, 1992). The 1968-97 period of record was divided into three time periods-1968-79, 1980-92, and 1995-97-and further divided into a lower flow season (November-April) and a higher flow season (May-October). The Tukey test indicated no difference in specific-conductance values among the three time periods when data for all months were compared; however, the months of November-April did contain significantly higher specific-conductance values (alpha level = 0.05) during the 1995-97 period when compared to the 1968-79 and 1980-82 time periods. Dissolved oxygen. Surface water in the Gore Creek watershed is well oxygenated and typical of high-gradient streams in the Southern Rocky Mountains physiographic province. Concentrations below the 6.0-mg/L aquatic-life stream standard were measured twice in Gore Creek. A concentration of 5.9 mg/L was measured just downstream from the wastewater-treatment plant in July 1994, and a concentration of 5.6 mg/L was measured at site 29, the mouth of Gore Creek, in December 1978. Seventy-five percent of the 783 dissolved-oxygen measurements taken at 37 sites in the watershed exceeded 8.8 mg/L. The median dissolved-oxygen concentration was 9.5 mg/L. Well-oxygenated stream conditions like those in the Gore Creek watershed support aquatic organisms such as macroinvertebrates and fish. 28 Gore Creek Watershed, Colorado-Assessment of Historical and Current Water Quantity, Water Quality, and Aquatic Ecology, 1968-98