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Hydrological system and available data DRAFT 8 <br />Stream base flow was estimated from gauging stations on Morgan/Straight Gulch, Jubb <br />Creek, Wilson Creek, Taylor Creek and Good Spring Creek. Details of the gauging <br />stations and data are provided in the Pre-feasibility Hydrogeology Report (WMC, 2005). <br />Gage data were evaluated to estimate the base flow component (Table 2.2). All stream <br />base flow was assumed to originate within the portion of each basin, which lies within <br />the model region. <br />Groundwater flow through the alluvial fill material in each basin was determined using <br />the saturated cross-sectional area of the valley fill material and hydrogeologic data as <br />described below: <br />The widths of the saturated valley fill at the model boundary were <br />calculated using topographic contours, a geologic map and ortho-rectified <br />color photographs. The widths of the saturated valley fill ranged from <br />360 ft for Morgan/Straight Gulch and Good Spring Creek, to 120 ft for <br />Taylor Creek. <br />• The depth of the valley fill was estimated based on data from borings and <br />the KEC geological model. The available data on valley fill thickness was <br />correlated to the widths of the stream valley to develop a range of valley <br />fill depth for each drainage where it crosses the Trout Creek sandstone. <br />The depths ranged from 100 to 50 ft for Morgan/Straight Gulch, Collom <br />Gulch and Good Spring Creek, and 60 to 35 ft for Jubb, Wilson and <br />Taylor Creeks. <br />• A range of saturated cross-sectional areas (maximum and minimum) was <br />calculated for each drainage assuming that the width of the base of the <br />valley fill is half as wide as the top of the saturated valley fill. <br />The groundwater gradients in the valley fill at the model boundary were <br />calculated. The groundwater gradient was assumed to be the same as <br />the stream gradient at the model boundary, and was measured in GIS <br />using topographic data. The gradients ranged from 0.013 to 0.038. <br />The hydraulic conductivity of valley fill was estimated. Results of <br />hydraulic tests performed in the alluvial fill material, lithologic descriptions <br />from borings drilled in the alluvial valley fill, and literature values for <br />similar materials were reviewed. The estimated range of hydraulic <br />conductivities was 10 to 100 ft/day, and a value of 50 ft/day was used in <br />the mass balance calculation. <br />The groundwater flux through the valley fill material was estimated using <br />Darcy's equation: <br />Q = AiK, <br />where Q is the flux in ft3/day, A is the average of the cross sectional area <br />of the drainage (in ft), i is the average groundwater gradient, and K is the <br />hydraulic gradient (50 ft/day). <br />2572-R3 Colowyo Coal Company, L.P. <br />Water Management Consultants