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Fish Creek <br />Fish Creek flows in a generally northerly direction through the NMD., The overburden depth varies from 1,000 to <br />1,400 feet through the panels. The location of the creek is shown on Ivjap 23B, Structures and Renewable Resources. <br />Projected impacts to the creek are presented in Exhibit 7e-6; Twentymile Coal Company, AVF Subsidence Study, <br />Final Report, prepazed by SubTerra, Inc. As noted in this exhibit, subsidence varies from 0 to 67 inches over the <br />panels. Based on the data,presented in Exhibit 7e-6, the maximum increase in the stream gradient is 1.1 percent. It <br />is not anticipated this will cause significant changes to the creek's profile, and resultant erosion should not be a <br />concern. TCC has monitored similaz changes in gradient in the EMD, and NMD, without resultant erosional <br />problems in Fish, Foidel, or Middle Creeks. The same type of subsidence features will occur'along Fish Creek, i.e. <br />trough-subsidence features, with differential lowering of the ground swface, minor surface cracks, and localized <br />ponding or minor flooding~(during high flow periods) over the panels. Natural ponding will tend to mitigate any <br />localized changes in stream gradient, as flow velocities will be reduced as streamflows pass through the ponded <br />areas. The extent of ponding, i.e. flooding of AVF, is presented in Exhibit 7e-6. As stated in this exhibit, <br />approximately 7.3 acres will be flooded by the mining of Panels 17-Right. and 18-Right. <br />Prior to full extraction under the creek, a channel survey was 'completed to establish the baseline used in the <br />preparation of the Probable Hydrologic Consequences section of this.application. <br />Fish Creek Alluvial Vallev Flooc <br />The Fish Creek Alluvial Valley Floor will be undemuned and subsided during mining of Panels 17-Right and 18- <br />Right. Subsidence for the AVF will mirror that projected for the creek. The primary impact anticipated for the AVF <br />is localized temporary pooling of water in the subsidence troughs over Panels 17-Right and 18-Right <br />No irrigation headgates are located over or in close proximity to the panels. The AVF evaluation for this azea <br />• documented that flood irrigation of the bottom lands is not practical, and that any enhancement of vegetative growth <br />m these aeeas results from natural sub-irrigation. Subsidence of portions of the valley floor will increase the potential <br />for sub-irrigation to occur, since seasonal flooding, ponding, and localized lowering of the Bound surface will result <br />in higher ground water levels }elative to the subsided areas. Resulting saturated soil conditions may promote localized <br />shifts in vegetation, with water tolerant species becoming more prevalent in aeeas of saturated soils. Conversion of <br />transitional sage-gross meadow and pastureland to riparian habitat is a potential positive impact of the anticipated <br />mining-related ground subsidence. ' <br />Stock Ponds <br />There aze a number of small stock ponds located throughout the NMD and WMD, as shown on Map 23B. The <br />ponds are constructed of earthem materials and aze typically fed by spring runoff or rain storms. TCC has <br />undemilned and subsided similaz stock ponds during mining of reserves located in the northern and eastern part of <br />the Permit Area. No material damage to the ponds resulted from the mining activity. It is anticipated that the stock <br />ponds will react in the same manner. If cracks develop in the embankments, they can be repaired and regraded to <br />maintain the integrity of the ponds. Similarly, if differential subsidence results in changes in the elevation of the <br />pond embankments relative to the containment basins, minor repairs/reconstruction may be necessary to maintain <br />water storage capacity and assure the integrity of the embankments. <br />U <br />PR06-07 2.05-181.10 11/07/06 <br />