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