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• <br />2.0 SUBSIDENCE IMPACTS <br />2.1 LAND USE <br />As shown on Map 12, all of the area above the Bowie #2 Mine has been classified <br />as rangeland/wildlife habitat. It cannot be expected that the predicted subsidence will alter <br />this classification. Subsidence cracks that are expected to occur within the zone of the <br />greatest subsidence impact can comprise little danger the domestic stock or wildlife; <br />observations of the existing cracks above the Bowie #1 Mine indicate that the cracks may <br />close after a period of several years. The cracks will stay open specifically on mountainous <br />ridges while they may heal sooner in depressions where occasional runoff will tend to fill <br />them with soil. <br />Land use could be changed in isolated areas by subsidence within zones of <br />domestic stock and wildlife grazing areas if the surface or ground water regime is altered <br />by the subsidence and springs or ponds are disrupted. Such potential disruptions would <br />be temporary and are discussed in Sections 2.2 and 2.3. As discussed in previous <br />sections of this report, disruption of surface and shallow ground water regimes can be <br />expected only in the area of relatively low overburden and are thus limited to Stephen's <br />• Draw, Freemans Gulch, Dove Gulch colluvium and stock ponds. <br />2.2 SURFACE WATER <br />Subsidence can impact surface streams or impoundments in several ways: <br />o vertical settlement of the surface <br />o local changes in surface slopes <br />o vertical or subvertical subsidence cracks caused by <br />tensile strain. <br />Vertical settlement of identical magnitude over large areas frequently has a <br />negligible effect on surface water bodies. The changes of stream gradient are very minor <br />(if any) and the vertical displacement would have very little effect on the impoundments <br />such as stock ponds in the area. <br />Differential vertical settlement over a short distance will result in local changes in <br />ground surface slopes. If such changes occur along any stream channel, its gradient <br />characteristics (flow and material transport) would change. In an extreme case the <br />- 19 - 02/01 <br />P~-y <br />