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- 5 - DESCRIPTION OF THE WATERSHED Physical Data Location.- The Pine River Watershed is located in La Plata and Archuleta Counties in southwestern Colorado. It contains 90.2 square miles (57,730 acres) consisting mainly of rangeland and irrigated cropland. The watershed is drained by Spring Creek, a tributary to the Los Pinos River, and Sambrito Creek, a tributary to the San Juan River. A small area in the southeast part of the watershed drains directly into the Piedra and San Juan Rivers. These. streams are all tributary to the San Juan River and all ultimately drain into the Navajo Reservoir recently constructed by the Bureau of Reclamation on the San Juan River. The watershed is about 13 miles in length and ranges from 6 to 10 miles in width. La Plata County contains approximately 59.4 square miles (37,990 acres) and Archuleta County approximately 30.8 square miles (19,740 acres). physiography and Geology.- The watershed lies along the north- eastern border of the San Juan basin in the Colorado Plateau physiographic province. Altitudes within the watershed range from about 8,900 feet at the upper end of the Spring Creek drainage in the northeastern part of the area to about 6,000 feet along the San Juan River at the southeastern boundary, The higher headwaters portion of the watershed lies along the west side of a high, narrow north-south ridge forming the divide between the Los Pinos and Piedra drainage basins. Most of this area consists of narrow ridges and narrow steep-sided valleys with slopes of 30 percent or more being common. Badlands topography is present in a few places, but the majority of the area is brush-covered. Most of the watershed area below the main Pine River Canal consists of gently to moderately rolling upland slopes, broad moderately sloping alluvial fans and terraces, and side valley floodplains. Bedrock exposed within the watershed consists of sedimentary beds belonging to the Wasatch Formation of early Tertiary age. The Wasatch is composed mainly of sandy shale, claystone, and siltstone ranging in color from olive-drab through light green to light red- brown. Some light gray to light brown beds of fine-grained massive sandstone occur in the lower part of the formation. Unconsolidated surficial deposits consisting of alluvial, colluvial, and eolian materials of Quaternary age mantle the bedrock throughout much of the lower watershed area. The alluvial deposits consist of sandy and gravelly materials capping low mesas or benches and clayey and silty materials underlying floodplains and alluvial fans. Slope-wash colluvium mantles most of the valley slopes. Thin deposits of eolian silt and fine sand occur in places.