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• 3. Drainage Basin Characteristics 3.1 Basin Geometry and Soils The drainage basin is divided into three parts, the undisturbed eastside, the undisturbed westside and the yard area. These areas were delineated using the U.S. Geological Survey's (USGS) Kline and Hesperus 7'/z' topographic maps. Figure 3 shows the basins delineated on these maps. The east and west basins are further divided into areas on steep hillslopes and those on the top of the plateau. The mine surface drainage map, C-4, was used in delineating and determining the hillslope areas, the yard and the two small areas adjacent to the yard. The measured areas for each sub-basin and composite basins are shown in the table below. Basin length and average slope were also measured from the topographic maps, see table. There are three soil types represented throughout the basin. These boundaries were delineated on Figure 3 using information obtained from the Soil Survey of La Plata County Area, Colorado, Soil Conservation Service, USDA. The most prominent type is Lazear-Rock outcrop complex (SCS unit #42) which occurs on the steeper hillslopes at the edges of mesas. This unit covers 63% of the basin and is about 50% Lazear very • stony loam and 30% Rock outcrop. It is shallow and well drained and is given a hydrologic group rating of D. Approximately 28% of the basin is covered by Plome fine sandy loam (SCS unit #55). This deep well drained soils is found on the top of the mesa. It is a hydrologic group B. The last soil found in this basin, 9% of area, is Witt loam. This is also a deep, we11 drained soil with a hydrologic group 8 and is founds on the mesa top. The following table summarizes the geometric and hydrologic characteristics associated with each drainage basin. 3.2 SCS Curve Number The SCS curve number (CN) for an area is dependent on the land use, hydrologic soil group, and ground cover type and density. Land use for the different parts of the basin was determined by inspection. Ground cover type and densities were obtained from the Soil Survey and field inspection. Gamble's Oak, sage brush, pinion juniper and grasses are all present on this basin. An average of these vegetation covers was used. Aground cover density of 40% for the south facing slopes, 60% for north facing slopes and 50% for the mesa tops was estimated from the Soil Survey maps, aerial photos and field inspection. The following CN's were used for each class represented by the above descriptors. • 3 January 1998