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n <br />• measurement at the upstream site based upon the crest stage recording <br />device and the high water mark. The estimated 1980 peak flow rate was <br />200 cfs based on this measurement. Calculations on which this <br />measurement is based are presented in Appendix 3-0. <br />Middle Thompson Creek <br />Middle Thompson Creek, the adjacent stream south of the mine, drains <br />an area of approximately 21.5 square miles. Upstream and downstream <br />monitoring stations were established by Snowmass on June 11, 1981 (see <br />Figure 3.6,1,2-1). Table 3.6.1-13 lists the flow values measured at <br />both sites. <br />No unusual flow values were observed during the monitoring period. <br />The seasonal patterns are very similar to those observed on North <br />Thompson Creek, This section of stream appears to be a losing reach, <br />that is, a section where the volume of flow decreases downstream, The <br />• loss is greatest during high flows with little or no loss occurring <br />during baseflow. This type of loss is common in this type of geology. <br />The steeply dipping beds intersecting the stream channel and possibly <br />unmapped faults provide potential pathways for water movement. The <br />downstream station is located in the alluvium, some of the water may <br />be recharging the gravels. The water does not appear to be entering <br />Mine No. 1. The reduction in flow on Middle Thompson Creek shows <br />seasonal patterns, whereas, during the life of the mine, there has <br />been a constant inflow of approximately 0.131 cfs (85,000 gpd); no <br />seasonal fluctuations of mine water inflow has been observed. Section <br />4.6.1.3.4 "Water Usage" discusses the volumes of mine water inflow. <br />• <br />3-71 <br />