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The effects of mining and subsequent reconstruction of the postmininq landscape w111~ ~~ <br />result Sn a long-term but minlmal and 1ne1gn1f1cant impact on the geomorphology of both <br />the 005 and DO6 basins. Overall drainage areas and elopes viii remain unaffected, and <br />overall watershed slopes and drainage densities wt 11 change only slightly. Changes in <br />landscapes Itopographles) will be limited to the upper portions of each basin, and <br />proposed permanent impoundments Sn combination vlih properly designed and constructed <br />drainage channels will offset any potential impacts to the overall fluvial system that <br />might occur ae a result of the proposed postmining topography. <br />Effects of Sediment Ponds on Channel Characteristics and Downstream Users. During the <br />permit period of the Seneca II-W Mlne, three sediment ponds x111 be utilized to impound <br />runoff from disturbed areas and provide for sufficient detention time before any rater Ss <br />discharged from the ponds. Ona of these ponds has already been constructed (Pond 006). <br />Ths other two ponds, Ponds 005 and 009, will be constructed near the doxnstream permit <br />boundary on tributaries to Dry Creek and Saga Creek, respectively (see Exhibit 13-2). <br />Impacts to the channels as a result of these ponds are expected ae follows. <br />During mining, the reaches immediately above the sediment ponds will gradually aggrade • <br />headward as more water Ss impounded until a pool level Ss reached that is Sn equilibrium <br />with water gains and losses. Channel reaches immediately below the ponds will become <br />incised by smaller channels whose geometries x111 be dictated by dampened runoff <br />potential, channel gradients, and sediment load particle size ranges. Vegetation will <br />begin encroaching on the edges of the new active channels ae there will be insufficient <br />runoff to remove St. <br />Both Ponds 005 and 006 ate proposed to be left intact ae permanent impoundments after <br />mining and reclamation aetivlties have been terminated. The ponds will be designed and <br />constructed to provide adequate detention time for the 100-year, 2d-hour storm event. The <br />channels immediately below these ponds will ad]uat In geometry to the frequency and <br />magnitude of discharges released from the outfalls of these two ponds. These proximate <br />channel teaches will eventually stabilize ae vegetation encroaches on the banksldea, and <br />gradients and bed materials also ad]ust to discharges from the ponds. The Impacts to the <br />channels Srtaaedia[ely belox the proposed permanent impoundment will be long-term, but of <br />minlmal significance, ae they will atlius[ naturally to discharges from the ponds. <br />Pond 009 ii proposed as a temporary pond, and following Sts removal at bond release, there <br />58 <br />