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An area identified in the permit application which may meet the geomorphic criteria <br /> of alluvial valley floors is located along Sylvester Gulch. The lower refuse pile is <br /> being constructed west of the mouth of Sylvester Gulch, in a portion of the area <br /> identified. The applicant estimated the area to be approximately 20 acres in size. <br /> However, upon examination of the Division's aerial photographs, the area of <br /> possible fill may be considerably smaller. Also, it has been confirmed by geologic <br /> mapping by the Colorado Geological Survey (Junge, 1978) that the deposits are not <br /> of alluvial origin, rather colluvial-wedge deposits formed by gravity-dominated <br /> processes. On the basis of this information, this area is found not to meet the <br /> geomorphic criteria for an alluvial valley floor. <br /> Another possible location for an alluvial deposit is found within the west branch of <br /> Sylvester Gulch. However, alluvial origin of the deposit is unlikely. The soils are <br /> deep clay Arquistolls which are susceptible to mass movement and are typically <br /> found on side hill locations. The lack of base level control that would favor clay <br /> deposits in this reach of Sylvester Gulch suggests that the valley deposits are of <br /> glacial or perhaps colluvial (mass movement) origin. <br /> The flood irrigation potential of the site is also questionable. Stream flow records <br /> near the mouth of Sylvester Gulch indicate that flow during the critical months of <br /> July and August are not common. Agricultural development would be limited to <br /> grasses and pasture because of the elevation and short growing season. Based on the <br /> soils review and hydrologic characteristics of this site, the Division does not find the <br /> west branch area of Sylvester Gulch to be an alluvial valley floor. <br /> Isolated alluvial deposits are found within the narrow valley of the North Fork of the <br /> Gunnison River upstream from its confluence with Gribble Gulch. All of these <br /> isolated deposits are less than five acres in size. Some of the deposits are above the <br /> 100-year floodplain, are not subject to flood irrigation and are not subirrigated. The <br /> remainder of the deposits are located within the 100-year floodplain and, therefore, <br /> may be subjected to flooding. Also, due to their proximity to the stream and the <br /> presence of willows and cottonwoods, some of these deposits are apparently <br /> subirrigated. All of these subirrigated deposits are too small and too irregular in <br /> shape to support agricultural development. Many of these deposits are simply <br /> islands created by meander cutoffs and are not accessible. The river poses numerous <br /> difficulties for farming these locations, including access, flooding, and changes in <br /> channel location. Due to the inability of these small isolated alluvial deposits to <br /> support agricultural development, the Division fords that no alluvial valley floors <br /> exist within the narrow valley of the North Fork of the Gunnison River upstream <br /> from its confluence with Gribble Gulch. <br /> Other areas of consideration include isolated high terraces along the North Fork of <br /> the Gunnison River. However, the high terraces are probably not of alluvial origin, <br /> and they may actually represent remnants of glacial debris, particularly in light of <br /> their relatively thin, immature soils. A reconnaissance of the areas showed no <br /> evidence of historic or current agricultural activity. These and other high terraces <br /> along the North Fork have a mining/residential land use history dating back to at <br /> least 1935. Terrace sites' isolation from modern river terraces and general site and <br /> 47 <br />