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Streamflow has been measured at 11 gaging stations in the watershed, and in 1998,9 of these stations were active (table 4). The first streamflow- gaging station in the area was established in 1944 on Gore Creek at the mouth; however, the longest records of operation in the watershed are for two upstream stations, Gore Creek at Upper Station near Minturn and Black Gore Creek near Minturn. About 80 percent of the annual streamflow is derived from snowmelt, which occurs in May, June, and July. During most of the year, the daily flow is less than one-third of the mean annual streamflow, which ranges from 5.95 to 140 ft3/s among selected stations in the watershed (table 4). Mean annual runoff ranges from 4,310 to 101,340 acre-ft/yr and reflects the large amounts of precipitation in the watershed. Coef- ficient of variation, which is a measure of the vari- ability of streamflow from year to year, is low for all II gaging stations, ranging from 0.26 to 0.37. Water-quality conditions can be affected by variation in the timing and magnitude of streamflow from year to year. Annual streamflow patterns are similar at five selected stations (fig. 10). Streamflow at Gore Creek at Upper Station near Minturn has no upstream diver- sions, and the variability in the streamflow is low (coefficient of variation, 0.26). Streamflow in Black Gore Creek near Minturn is affected by the operations of the Black Lakes, which are used to mitigate the impacts of diversions of instream flows Table 3. Generalized water budget for the Gore Creek watershed [acre- ft/yr, acre-feet per year; <, less than] for snowmaking during the winter months (Weaver and Jones, 1995). However, the annual streamflow pattern is similar to the other four stations and the vari- ability in the streamflow is low (0.30). The streamflow pattern at Red Sandstone Creek near Minturn, a major tributary to Gore Creek, is similar to the other gaging stations. The two downstream main-stem stations, Gore Creek at Lower Station at Vail and Gore Creek at Mouth near Minturn, have a streamflow pattern similar to the other three gaging stations including low vari- ability in streamflow; however, these two stations lack long-term streamflow data. Monthly streamflows for the five gaging stations (fig. 11) are similar and indicate streamflow in the Gore Creek watershed is dominated by snow- melt during May, June, and July. Although the magni- tude of peak flows in this watershed can be quite large, exceptionally large snowmelt flows that cause severe flooding are uncommon. Because of the annual nature of snowmelt flows, most stream channels are capable of carrying these flows without extensive overbank flooding (Apodaca and others, 1996). Reservoir storage and local diversions also diminish the magnitude of the annual snowmelt peak flows. The lO-year flood (table 4), which indicates that a given peak flow has a 10-percent chance of occurring in any given year, ranges from 106 to 1,715 ft3/s at the streamflow-gaging stations in the Gore Creek watershed. Source Percentage Percentage Source 100 Evapotranspiration from nonirrigated land (residual) <1 Surface-water outflow Consumptive water use Interbasin water transfers Reservoir evaporation Ground-water outflow Change in ground-water storage Precipitation Inputs (acre-ftlyr) 185,000 Surface-water inflow Interbasin transfers (negligible) Ground-water inflow (negligible) a500 o o Total (rounded) 185,500 100 aBased on estimated data 1996-98 (Jim Roberts, Vail Associates, oral commun., 1998). bData from the U.S. Geological Survey National Water Information System. cBased on 1995 data (R.G. Dash, U.S. Geological Survey, written commun., 1998). Outputs (acre-ftlyr) 74,900 40 bIOI ,300 c9,300 o Negligible Negligible Negligible 185,500 55 5 100 SURFACE WATER 21