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/unr 1991 Cypnu 7L,C • Subsidtntc E/frcu on Vtgewtion • / <br />• 1.0 INTRODUCTION <br />Twentymile Coal Company (TCC) is cvaluating the impacts of extending longwall panels under <br />the designated Fish Creek AVF. This analysis addresses potential impacts to area vegetation <br />resulting from subsidence of the AVF. It is based on analysis of ezisting vegetation, subsidence <br />predictions, and projected hydrologic impacts. <br />2.0 EXISTING CONDITIONS <br />2.1 GENERAL <br />The study area includes a segment of [he Fish Creek drainage approxima[ely 3,000 feet long. <br />This azea can be described as a low to moderate gradient perennial stream valley and is typical <br />of small drainages in this area. Within [his area, the Fish Creek floodplain is relatively flat with <br />a distinct slope break at the valley margins. There are two general terrace levels within the <br />floodplain. The main terrace, extending across the entire floodplain, supports asagebrush- <br />meadow vegetation community. The lower terrace, which is limited to the margins of the <br />current flow channel and abandoned channel segments, supports a more mesic vegetation <br />community including rushes and xdge species. <br />The area immediately to the eut of the study area wu previously disturbed by surface coal <br />mining activi[ies and hu been effectively reclaimed. Both the study area and adjacent <br />undisturbed and reclaimed lands are xmi-arid rangeland uxd for xuonal livestock graving. <br />• Fish Creek, within the study area, is a dynamic stream system with a constantly changing <br />channel and general stream configuration. The dynamic nature of the stream system is evidenced <br />by ongoing channel and bank erosion, a number of abandoned meander segments, and the new <br />upper channel segment established by unurually high spring runoff in 1984. Bank and channel <br />materials appear to be relatively fine grained alluvial/colluvial materials mixed with weathered <br />sediments derived from slope wuh and mass routing. The flow regime is typical of small <br />perennial drainages in the area, with high volume flows rerulting from snowmelt runoff in la[e <br />spring to early summer, declining co low volume bax flows later in the year, and brief intervals <br />of elevated bax flow in responx to localized thunderstorm events. While there appears to be <br />a minor sub-irrigation component in the lower terrace materials, it appears to be cloxly tied to <br />streamflow and is limited to the eatent of occurrence of thex materials. <br />2.2 SOILS <br />Area soils are generally clusified u an Outlet/Slocum Taxajunct Complex. This soil mapping <br />unit consists of deep, somewhat poorly drained soils forming on floodplains and low terraces, <br />predominantly in medium and moderately fine grained mixed alluvium. <br />The Outlet soils contain a silt loam surface (12"), followed by a loam ubsoil (13"), and a loam <br />and sandy loam substratum (35"). The depth to seuonal high water table is estimated to be <br />approaimacely 25". The soil permeability is moderate and available water capacity is high. <br />Effective rooting depth is b0" or greater. <br />. The Slocum Taxajunct soils have a clay loam surface (12"), a silty clay loam subsoil (17"), and <br />a loam substratum (31"). Depth to seuonal high water table is estimated to be 29'. Surface <br />ACZ Iru. • P.O. Box 7740/8 • Stmmboar Spring; (vlorado 80/77 •(303)8796160 <br />