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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />I <br />I <br />I <br />. <br />. <br />. <br />I <br />.. <br />. <br />I. <br /> <br />tributary to the San Juan River at pagosa Springs, covers approx- <br />imately 20 square miles. The Rio Blanco watershed, which is also <br />a tributary to the San Juan River near Truillo, Colorado, approx- <br />imates 157 square miles at .the downstream end of the study area. <br />The topography of the study areas in Archuleta County is primarily <br />mountainous regions lying in the southern most parts of the county. <br />The headwaters of the San Juan River start from the southern slopes <br />. of the San Juan Mountains at about 13,000 feet above mean sea level, <br />and fall to about 7,100 feet at pagosa Springs, which results in <br />an average fall of approximately 250 feet per mile. The head- <br />waters of the Rio Blanco Start from approximately 12,000 feet <br />above mean sea level and fall to about 7,000 feet at the downstream <br />end of the detailed study area. Through this reach, the Rio <br />Blanco has an average fall of about 200 feet per mile. <br /> <br />The population in the study areas was estimated to be 1340 for <br />the pagosa Springs area and 40. for the Rio Blanco area in 1977. The <br />economy of these areas depends primarily on iogging.and agricultural <br />activities with the most recent growth corning from tourism and <br />, <br />recreational activities. <br /> <br />CLIMATOLOGY <br /> <br />The climate of the study areas is arid' to semi-arid except in the <br /> <br />high headwaters, where precipitation is moderately heavy. For the <br /> <br />Silverton area, the annual precipitation is approximately 25 inches, <br /> <br />of which 12 inches occurs in the period from May through September. <br /> <br />The Durango area has a mean annual precipitation of 18 inches with <br /> <br />8 inches falling from May to September. The pagosa Springs and <br /> <br />-5- <br />