2012-03-07_PERMIT FILE - C2010088 (22)

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ALLUVIAL VALLEY FLOOR DETERMINATION CAM - COLORADO PROPOSED COAL LOADOUT NEAR FRUITA MESA COUNTY, COLORADO None of the project area is currently cultivated and according to the NRCS, there is no record of past farming in this area. The narrow swales on the west side of Reed Wash contain grasses that could be grazed, but their limited area makes grazing unlikely and impractical. At the locations where the rail spur is proposed to cross the swales, the swales are approximately 10 to 15 feet wide and only 1 foot lower in elevation than the surrounding areas of greasewood (Figure 2). Mesa County's 1937 aerial photographs confirm that prior to the refinery, this area was not cultivated, consistent with NRCS's conclusion (Appendix E). Geology The proposed loadout is within the historic floodplain of the Colorado River. The Colorado River alluvium ranges from 15 to 80 feet thick in the Palisade to Fruita reach of the river, based on alluvial well depth (Topper et al. 2003). The alluvium consists of all clast sizes —from boulders and cobbles to silt and clay. In the Fruita area, the river is incised within the marine shales of the Mancos Formation. Monitoring wells were installed in 1981 in the area of the proposed loadout (Figure 2; Appendix A). The wells encountered between 34 and 48 feet of unconsolidated alluvium and other deposits. The Mancos Shale was encountered in all boreholes, ranging in depth from 34 to 48 feet below ground surface (bgs), generally shallower toward Highways 6 and 50; and deeper toward the river, although the deepest depth to bedrock is adjacent to Reed Wash (RW 1). The uppermost 15 to 23 feet were described in the well logs as silty clay "slopewash" from sheet flow. This deposit is most likely a combination of windblown fine material and slopewash, much of which was derived from the Cretaceous marine shales. Underlying the silty clay is 19 to 25 feet of alluvium that consists of poorly to well sorted sand and sandy gravel. Because of the sediment source (primarily marine shales), the streamlaid deposits along Reed Wash are limited to silts and clays. However, these deposits are of limited extent and subject to erosion during high flow and flash flood events. The wash is incised within the older alluvial deposits of the Colorado River and the overlying silty clay slopewash deposits near the surface. The bottom of the current channel is probably on the Mancos Shale or 3 8/26/2011