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~' <br />MINNESOTA CREEK DRAINAGE -LIFE OF MINE AVF DETERMINATION <br />One of the locations for alluvial deposits within the life-of-mine <br />permit area is a 90-acre tract of land on the Dry Fork, immediately <br />above Minnesota Reservoir. These alluvial deposits range from 100 to <br />300 feet in width. The valley floor above the reservoir widens and <br />there is a definite break in slope from the steep sides confining the <br />valley floor. Numerous beaver ponds and willow thickets occupy the <br />valley floor in this location, and grazing is limited to isolated <br />locations along the margins of the valley. <br />The soils in this location are primarily deep clays. The Haplaquoll <br />soil predominates. It is associated with beaver activity and commonly <br />exhibits mottled conditions in subsurface horizons. The alluvial origin <br />of this material is plausible because of the beaver activity and <br />downstream channel blockage from a landslide and from Minnesota <br />Reservoir dam which was placed at the blockage point. Normally the soil <br />is saturated with water at or near the surface. Because of absence of <br />the good aeration, organic matter content is high. <br />The applicant states that the valley floor in this reach of the Dry Fork <br />would be incapable of supporting agricultural activities without proper <br />drainage. Approximately 20 percent of the total alluvial fill deposits <br />above Minnesota Reservoir have an artificially high water table as a <br />result of backwater conditions from the reservoir pool. This area could <br />not be drained adequately without removing the reservoir. Even with <br />adequate drainage on the remaining 72 acres, the agricultural <br />development would be restricted to grasses and pasture because of the <br />high elevations and short growing season. Furthermore, the beaver ponds <br />and willow thickets are an important factor in the general ecological <br />picture, since they help regulate stream flow, provide water for <br />domestic animals and wildlife, and provide wildlife cover. A letter <br />from the Delta Soil Conservation District dated February 17, 1981 <br />indicates that agronomic development in this 90-acre tract is doubtful. <br />During the periods 1950-62, 1964-70, 1972-1974, and 1977 water yields <br />from the Deep Creek diversion canal were greater than releases from <br />Minnesota Reservoir. During water years 1978, 1979, and 1980 all water <br />passing through the Deep Creek Ditch into Minnesota Reservoir was <br />released from the reservoir. Assuming no seepage and transmission <br />losses, contributions of Deep Creek Ditch flow releases from Minnesota <br />Reservoir ranged from only 13.5% to 25.4% between 1978 and 1980. It is <br />assumed that seepage and transmission losses in the diversion canal <br />result in a Deep Creek Ditch yield of Tess than 80%. <br />The applicant has stated that the records of water yield from Minnesota <br />Reservoir are not reliable and that there is no way to develop a <br />meaningful comparison between the Deep Creek Ditch "yield" and the <br />Minnesota Reservoir "yield". As a result, it is difficult to assess the <br />quantity and frequency of "natural" water supporting alluvial functions <br />24 <br />