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Surface Water <br /> The San Miguel River has its source in the San Juan Mountains. These mountains are <br /> primarily composed of Tertiary volcanic rocks. Iorns et. al. (1965a) has shown these waters <br /> to be of a calcium bicarbonate-sulfate type during high flow periods. These waters contain <br /> less bicarbonate during low flows while the calcium and sulfate concentrations increase due <br /> to less dilution from snowmelt runoff. The San Miguel River traverses the interior portions <br /> of a basin that is chiefly underlain by Dakota Sandstones, the Morrison Formation and <br /> Mancos Shales. The Morrison Formation and Mancos Shale have the greatest potential for <br /> influencing the river water chemistry. The San Miguel River drains an area of 1,080 square <br /> miles. In addition, approximately 15,500 acres of irrigated land lies between Placerville and <br /> Naturita, Colorado. It is estimated that 2.8 tons of Total Dissolved Solids (TDS) per acre <br /> are contributed to the waters annually from this area. This results in increased levels of <br /> magnesium, potassium, sulfate and chloride. Water quality samples taken from the San <br /> Miguel River at Naturita, Colorado, have a weighted average of 316 milligrams per liter <br /> (mg/1) TDS. Specific conductance varies between 318 and 730 millimhos (mmhos). The <br /> mean sodium adsorption ratio (SAR) is about 0.7 at Naturita. Water curves indicate that, <br /> 90 percent of the time, flow in this river exceeds 60 cubic feet per second (cfs), while flows <br /> exceed 1000 cfs about 10 percent of the time. The mean discharge of the river is 351 cfs. <br /> San Miguel River waters are suitable for domestic usage except at low flow periods when <br /> sulfate levels are high. <br /> The Colorado Cooperative Ditch Company diverts as much as 145 cfs of water from the San <br /> Miguel River approximately ten miles east of the town of Nucla. The main diversion ditch <br /> then flows westerly passing just north of the town of Nucla and then flows northwestward <br /> passing north of the New Horizon Mine. Diversion ditches distribute water to mainly <br /> agricultural users in the area. Additional uses include domestic, municipal and stock pond <br /> consumption. <br /> The New Horizon Mine 2 site is just west of the town of Nucla and is located in the mildly <br /> sloping upland area immediately north of Calamity Draw, approximately in the middle of <br /> First Park. The above-mentioned parks consist of recent undifferentiated aeolian silts and <br /> sands overlaying the Dakota and Burro Canyon Formations. Since the completion of the <br /> irrigation system for this area in about 1910, these areas have been intensively reworked for <br /> irrigated agriculture. Especially in the New Horizon Mine 2 mining area, the surface water <br /> system is characterized by a "disjointed" tributary drainage system almost completely <br /> controlled by return flow ditches of the local irrigation network. The perennial flow stature <br /> of Calamity Draw is largely due to the local irrigation practices. Over the years this more <br /> or less constant source of water in these drainage systems has encouraged the growth of <br /> vegetation. This has resulted in the channels becoming more erosionally stable but more <br /> incised. <br /> Flows in Calamity Draw generally reflect local irrigation practices. Water from the West <br /> Lateral Ditch's return irrigation flow is its primary source. Very little supplemental flow is <br /> derived from precipitation and snowmelt runoff. In 1987, measured flow rates in Calamity <br /> Draw varied from 0.605 cfs in April to 13.4 cfs in July. Flow in Calamity Draw varied from <br /> 1.04 to 2.06 cfs during the non-irrigation period. Shallow ground water alluvial sources <br /> 6 <br />