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(reestablishment) of the West Return Ditch was completed by the early 1980's. Table 17-22 <br />presents premining and postmining stream lengths and gradients of the West Return Ditch. <br />The reestablishment of the affected West Return Ditch resulted in a postmining stream <br />course featuring fewer meanders. The reestablished channel reach is approximately 11 <br />percent shorter than before mining began. However, the postmining gradient of the <br />affected reach is .001 ft/ft less steep. Following the removal of the 001 pond (after <br />cessation of mining at the Nucla Mine), the overall gradient of the West Return Ditch will <br />be only slightly higher than the premining gradient. The design of the reclaimed channel <br />reach of the West Return Ditch, in combination with the reclamation (topsoil replacement <br />and revegetation) of the channel banks, will largely offset any potential sediment load <br />increases due to the slight increase in channel slope. Historic monitoring conducted in <br />this channel at Site SW-N6 has shown no trends toward elevated concentrations of total <br />suspended solids (see Appendix 7-7). The monitoring conducted at this site since 1980 <br />indicates that this channel has stabilized with regard to additional contributions of <br />suspended sediment. <br />• Impacts to the drainage basins at both the Nucla Mine and Nucla East mining area include <br />changes in watershed areas, elevations, slopes, slope lengths and temporary loss of <br />topsoil structure. <br />Table 17-23 contains select basin parameters calculated using both the pre- and postmining <br />topography for the watershed above the proposed 007 sediment structure in the Nucla East <br />mining area (see Exhibits 17-1 and 17-2). It should be noted that the small portion of <br />The Nucla East area that will be drained by the diversion was not included in the <br />calculations. Descriptions of the methods used to determine the basin parameters can be <br />found in the subsequent section of this Tab titled "Effects of Runoff from Reclaimed Areas <br />on the Quantity of Streamf low". <br />The data presented in Table 17-23 show that mining and reclamation of the Nucla East area <br />will result in almost a 7 percent increase in area for the watershed above the 007 pond. <br />Also, the average watershed slope length will increase by about 27 percent. Average <br />watershed slope will be decreased by approximately 18 percent. Time of concentration, a <br />measure of the time it takes for runoff to travel from the most distant basin point to the <br />• basin outlet, will be increased by about 32 percent. As a result of mining, the overall <br />elevation of the Nucla East area will decrease slightly. <br />17-73 Revised 04/11/88 <br />