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XV[[. Operations on Alluvial Valley Floors <br />The applicant has performed a detailed reconnaissance investigation for alluvial valley <br />floors. During the investigation, the applicant identified several areas within or <br />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 Fork <br />of Minnesota Creek, Sylvester Gulch, and the North Fork of the Gunnison River. <br />Two of these areas, Minnesota Creek and the Dry Fork of Minnesota Creek, although <br />geographically adjacent to the permit area, are not hydrologically adjacent to the <br />activities approved in the permit. Both a surface water divide and a groundwater <br />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 may <br />be affected by future mining, i[ will be considered in this determination. <br />An area identified in the permit document which may meet the geomorphic criteria of <br />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 possible <br />fill may be considerably smaller. Also, it has been confirmed by geologic mapping by <br />the Colorado Geological Survey (Junge, 1978) that the deposits are not of alluvial <br />origin, rather colluvial-wedge deposits formed by gravity-dominated processes. On <br />the basis of this information, this area is found not to meet the geomorphic criteria for <br />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 [o mass movement and are typically found <br />on side hill locations. The lack of base level control that would favor clay deposits in <br />this reach of Sylvester Gulch suggests that the valley deposits are of glacial or perhaps <br />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 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 />Wray be subjected to flooding. Also, due to their proximity to the stream and the <br />54 <br />