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<br />area of approximately 2 square miles. Economic activities in the <br />area include timbering, farming, tourism, and small-scale mining. <br /> <br />Flood plain development is scarce, mostly consisting of ranches <br />and farms. Residential development is increasing. Portions of <br />the flood plains also have the potential for park and recreational <br />uses, as well as a visual greenbelt. <br /> <br />The Rio Grande originates in southern Colorado and flows through <br />New Mexico before becoming the boundary between the United States <br />and Mexico. The South Fork Rio Grande is a major tributary to the <br />Rio Grande. It drains the southern portion of the basin from the <br />Wolf Creek Pass area, or approximately 217.5 square miles. Its <br />confluence with the Rio Grande is located just to the north and <br />east of the Town of South Fork. Also feeding the Rio Grande are <br />several other tributaries that drain portions of San Juan, Hinsdale, <br />Mineral, Sagauche, and Rio Grande Counties. <br /> <br />A number of irrigation ditches, such as Treasure Pass, Rober-Lohr, <br />and Fachs ditches, convey water across natural drainage boundaries <br />from the Colorado River basin. Several reservoirs also regulate <br />flow in the basin. The Rio Grande is affected by the Beaver Creek, <br />Santa Maria, Rio Grande, and Continental reservoirs. <br /> <br />The upper Rio Grande and South Fork Rio Grande basins compose the <br />southwestern portion of the Upper Rio Grande Valley in Mineral and <br />Rio Grande Counties. The valley occupies a long, narrow trough <br />that was formed mainly by faulting. The main topographic features <br />of this valley are the two parallel north-south mountain ranges <br />that border the valley, the San Juan and La Garita Mountains in <br />the west and the Sangre De Cristo Range on the east. The Sangre <br />De Cristo Range merges with a low range of hills along the southern <br />portion of the valley. <br /> <br />The San Juan Mountain Range is a well-defined area of rugged moun- <br />tains rising above the San Luis Valley in the Rio Grande basin. <br />The San Juan Mountains consist largely of volcanic rock in rela- <br />tively horizontal layers overlying older sedimentary rock. Soils <br />are moderately deep to shallow with rock outcrops. Permeability <br />and runoff characteristics are highly variable. <br /> <br />Topography has a significant effect on the semiarid climate. The <br />surrounding mountains act as a barrier, preventing the northern <br />cold air or eastern storms from penetrating,the Upper Rio Grande <br />Valley. <br /> <br />Summers are cool in the study <br />reaching into the 900F range. <br />ally drop below 500F. Winter <br /> <br />area, with temperatures occasionally <br />Temperatures for summer nights usu- <br />temperatures, particularly in the <br /> <br />4 <br />