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Flows <br />North Thompson Creek <br />North Thompson Creek flow quantity data were obtained from the United <br />States Geological Survey (USGS) Gaging Station Number 9-0828, located <br />on North Thompson Creek near Carbondale, Colorado. This recording <br />discharge gage was located 2 miles above the Snowmass Coal Mine and <br />0.4 miles downstream from Yank Creek. The drainage area at the gage <br />is 26.8 square miles with an additional 3,0 square miles of drainage <br />area between the gage and the mine. The gage was installed October, <br />1963. Date were collected through September, 1978, at which time the <br />gage was removed. <br />The 15 years of data obtained from that gage station provides a <br />reliable estimate of the mean and low flow conditions of North <br />Thompson Creek. Table 3.6.1-10 is a summary of the mean monthly <br />• flows at the USGS gaging site and also the range of monthly maximum <br />and minimum flows. The following correction factors are applied to <br />adjust these values for the drainage area at the mine: mean monthly <br />flow, plus 0.2 cubic feet per second (cfs); minimum flow, no <br />correction; maximum flaw, plus 10 percent. The derivation of these <br />correction factors is shown in Appendix 3-M. <br />The peak flow data provide a reliable estimate of high flow discharges <br />up to about the 25-year flood frequency. Flood flows are extrapolated <br />beyond that recurrence interval; however, the accuracy diminishes. <br />Regional flood methods are used to verify the discharge magnitude of <br />events greater than the 25-year occurrence. Table 3.6.1-11 is a sum- <br />mary of the flood flow characteristics of North Thompson Creek. This <br />information is based upon a statistical analysis of the USGS data pre- <br />viously mentioned. Appendix 3-N lists the flood calculations and the <br />flow sheets from the USGS gaging station. <br />• <br />3-64 <br />