2012-06-20_PERMIT FILE - C2010089 (89)

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assumed to be the few years*. The site location information contains only general descriptions of the leasehold areas to the nearest full section and includes •reee that do not contain strippable (i.e., surface mineable) coal. Alluvial valley floor determinations in this report therefore relate specifically to the description for each the site as listed in Appendix 1. As a result, Identified alluvial valley floors ssv enct+mpaas areas other than those underlain by strippable coal. This situation lted from • lock of detailed information with respect to the locations of strip.,, coal within the proposed mining areas. The classification of lowland areas within the le valley floors was based principally on the following er (1) underlain by alluvial deposits, viler* known; (2) on floodplaias and low terraces alluvial deposits; (3) is areas exhibiting vigorous vegetative growth in area■ and thus where subirripation is likely; and (4) where valley width is greater than fifteen meters. Lowland drainage areas, floodplaln and low terrace, were assumed to have rela- tively thick (on the order of a few meters) alluvial deposits'. The higher inter- stream areas and minor alluvial washes are assured to have thin veneers of alluvium rather than the thicker deposits of water -borne silt and sand materials which are sore suitable for growth of forage. Or, they may be sufficiently high and above the 'water table such that subirrigation does not occur during the dryer months. As noted in the introductory section, in order to support vegetation typical of an alluvial valley floor, the character of this alluvial material must be such that sufficient eater is retained for continued plant growth during the dry summer and fall months. Thus areas with • thin veneer of eolluvium overlying unweathered strata and gravel - filled areas do not generally qualify as alluvial valley floors since too little soil is present. Also, thick, fine - grained silts or clays (such as laeustrine or plays deposits) say be too impermeable to bold sufficient water for aubirrigation of the ' alluvial valley floor vegetation complex. Lowland areas more than 1.5 meters (on minor streams) and more than 2.5 meters (en practical stream) in elevation above the eater level in the channel, or above the intermittent or ephemeral channel bottom, were generally considered to be out of the alluvial valley floor if infrared aerial photography did not show vigorous vege- tative growth during times of lower rainfall and relative to surrounding areas. Simi- larly, narrow streams incised more than 1.5 meters were generally assumed to indicate a water table too deep to sustain agricultural crops. However, it should be noted that crops grown in some situations could be subirrigated by a ground eater system far outside the channel and low terraces of the valley floors. In these instances, the eater table may be maintained by bedrock aquifers which recharge the eelluvttat. While this study did, mot identify any such cases of significance, the potential for such situations remains. The minimum fifteen-aeter width for an alluvial valley floor designation repre- sents what Is perhaps • lower limit of practical "farmability" and geologic stabili- ty. In fact, the fifteen -meter width represents a practicable limit to identifica- tion of elevations and boundaries on both topographic maps and aerial photographs. It Is recognised that, particularly in the Northern Crest plains, some narrower val- ley floors are stable, subirrigated, and farmed. &waver, it was assumed for this analysis that these narrower valleys are not essential to agricultural operations. It is likely that the narrower drainages located upstream of the alluvial valley floors serve as wildlife habitats to some degree. Further, the water supplies for 1 • alluvial surface features of comparison to adjacent • An original listing of coal mine sites was developed in 1975 as representing western mines to be in production In 1979. That list was corrected by removing some sites that were scheduled to be underground operations and by adding a few existing mines. however, production estimates were not updated. I OCT 3 0 1981