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Floods caused by snowmelt dominate the flood characteristics up to <br />about the 50-year frequency. Beyond that frequency, floods induced by <br />rain have the potential for the greatest discharges, Mountain streams <br />like North Thompson Creek are subject to flashfloods caused by local <br />thunderstorms in the narrow steep-sided valleys. This relationship is <br />similar to that found in a study of the Crystal River hydrology. <br />An analysis of low flow on North Thompson Creek using the USGS data <br />indicates a 10-year daily flow of approximately 0.2 cfs. This flow <br />may occur in the winter months and also in the late summer. Records <br />for the winter period may be affected by ice which causes low relia- <br />bility. <br />Snowmass began monitoring streamflow on North Thompson Creek on June <br />26, 1981, at the upstream and downstream stations located on Figure <br />3.6,1.2-1, Flow measurements are made manually by wading the stream <br />and measuring velocities with a current meter; depths are measured <br />• using a top-setting rod. Flow values for North Thompson creek as <br />measured by Snowmass are listed in Table 3.6.1-12, <br />These values follow the flow patterns observed at the USGS gaging sta- <br />tion. Below normal precipitation during the 1980-81 winter resulted <br />in peak runoff that was less than normal, The downstream site shows <br />an increase in flow over the upstream site during high flow, This can <br />be attributed to the increase in watershed area. During baseflow, no <br />difference was observed. <br />In addition to the flow information developed from the USGS gaging <br />station, crest stage recording devices were installed upstream and <br />downstream of the mine site in 1980. High snowmelt runoff destroyed <br />the downstream device and partially damaged the upstream one during <br />the spring of 1980, It was possible to make an indirect flow <br /> <br />3-68 <br />