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June 1991 Cyprus 7'CC • Sufsidrrue EJferu on Vegemtion • 1 <br />~` 1.0 INTRODUCTION <br />Twentymile Coal Company (I'CC) is evaluating the impacts of eztending 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 the Fish Creek drainage approximately 3,000 feet long. <br />This area can be described as a low to moderate gradient perennial stream valley and is typical <br />of small drainages in this area. Within this 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 aaoss 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 sedge species. <br />The area immediately to the east of the study area was previously disturbed by surface coal <br />mining activities and has been effectively reclaimed. Both the study area and adjacent <br />undisturbed and reclaimed lands are semi-arid rangeland used for seasonal livestock grating. <br />Fish Creek, within the study area, is a dynamic stream rystem with a constantly changing <br />channel and general stream configuration. The dynamic nature of the stream rystem is evidenced <br />by ongoing channel and bank erosion, a number of abandoned meander segments, and the new <br />upper channel segment established by unusually 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 mazs wasting. The flow regime is typial of small <br />perennial drainages in the area, with high volume flows resulting from snowmelt runoff in late <br />spring to early summer, declining to low volume base flows later in the year, and brief intervals <br />of elevated base flow in response to localized [hunderstorm events. While there appears to be <br />a minor sub-irrigation componen[ in the lower terrace materials, it appears to be closely tied to <br />streamflow and is limited to the extent of occurrence of these materials. <br />2.2 SOILS <br />Area soils are generally classified az anOutlet/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 subsoil (13"), and a loam <br />and sandy loam substratum (35"). The depth to seasonal high water table is estimated to be <br />approximately 25". The soil permeability is moderate and available water capacity is high. <br />Effective rooting depth is 60" 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 seazonal high water table is estimated to be 29". Surface <br />.1 CZ /nc. • P.O. Box 774018 • Stwmlmt Sprang; Colorado 80177 ' (303J879-6260 <br />