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<br />19 <br />Degradation of 1.5 feet is predicted in CH-15. This is based on average degra- <br />dation in a reach and can be more severe at specific locations. Channel 17 is <br />the very downstream portion of the Magpie Creek diversion channel. The peak <br />suspended sediment concentration in this channel is 35,860 ppm. The high sedi- <br />went concentration in this channel is also due to the steep channel slope and <br />fine sediment sizes. Degradation on the order of 5.4 feet is predicted for <br />this channel. Again, this is based on average degradation through a reach and <br />could be more severe at specific locations. These two channels are not natural, <br />but are a result of mining activity. Corrective measures must be taken in these <br />' channels to acquire a hydrologic balance. Comparing the sediment concentrations <br />( entering and leaving the lease area, the sediment concentration leaving is ap- <br />l: proximately four times as large as the sediment concentration entering the lease <br />area. Through the lease area, the increase in sediment concentration is pri- <br />marily caused by the degradation of CH-15 and CH-17. Remedies for stabilizing <br />• CH-15 and CH-17 are presented in the section of diversion channels (Section 6.4.2). <br />Below the mine site, the sediment concentrations gradually decrease in the <br />remaining channels. The channels become wider with milder slopes causing ag- <br />gradation, and therefore, decreased sediment concentrations. At the watershed <br />outlet, the average and peak sediment concentrations are 20,600 and 28,900 ppm, <br />respectively. The total water runoff volume is 51.1 acre-feet and the peak <br />discharge is 897 cfs for the 10-year storm. <br />Within the Magpie Creek watershed are three existing sediment ponds. In <br />addition, there are four proposed sediment ponds. The adequary of these strur <br />tares is discussed in Section 6.3. Two culverts now exist in this watershed <br />and their adequary is. discussed in Section 6.2. <br />3.2 Newlin Creek Drainage <br />There is no difference in the present-mining condition and post-mining <br />condition for the Newlin Creek watershed. However, since the 10-year design <br />storm was only used to assess the present-mining condition of the other water- <br />sheds, the results of the 10-year design storm for Newlin Creek are presented <br />in this section. <br />. Since the Newlin Creek channel at the "Prospecting Pit" is assumed to be <br />reclaimed, the 10-yeas design storm was used only to analyze the adequacy of <br />the existing culvert under the haul road and estimate the naturally occurring <br />sediment concentrations in this undisturbed watershed. <br />