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XVII. Operations on Alluvial Valley Floors <br /> The applicant has performed a detailed reconnaissance investigation for alluvial <br /> valley floors. During the investigation, the applicant identified several areas within <br /> or adjacent to the mine plan area which may meet the geomorphic criteria of alluvial <br /> valley floors. Alluvial deposits were identified along Minnesota Creek, the Dry <br /> Fork of Minnesota Creek, Sylvester Gulch, and the North Fork of the Gunnison <br /> River. <br /> Two of these areas, Minnesota Creek and the Dry Fork of Minnesota Creek, <br /> although geographically adjacent to the permit area, are not hydrologically adjacent <br /> to the activities approved in the permit. Both a surface water divide and a ground <br /> water divide separate these areas from mining activities. However, since part of the <br /> Minnesota Creek drainage lies within the 40-year life-of-mine lease boundary and <br /> may be affected by future mining, it will be considered in this determination. <br /> An area identified in the permit document 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 (lunge, 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 cri�eria 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 drier months of July <br /> and August are not common. Agricultural development would be limited to grasses <br /> and pasture because of the elevation and short growing season. Based on the soils <br /> review and hydrologic characteristics of this site, the Division does not find the west <br /> 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 flood1lain, 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 /> 49 <br />