Laserfiche WebLink
5 <br />• River confluence. The San Juan River originates on the south flank of the San <br />Juan Mountains in southern Colorado, as do its major tributaries. Major tributaries <br />include the Dirty Devil and Escalante Rivers in Utah, the Paria River in Arizona, and <br />the La Plata, Animas, Los Pinos, Piedera, and Navajo Rivers in New Mexico. <br />Geoloay <br />Rocks underlying the Colorado River Basin range from 625 million years old <br />to recent alluvial deposits (U.S. Department of the Interior 1987a). Most high <br />mountain areas are composed of resistant, nutrient-poor, metamorphic and <br />igneous rocks (lorns et al. 1965). Intermediate- and low-elevation areas of the <br />Upper Basin are predominantly composed of sedimentary rocks of marine and <br />• brackish origin (U.S. Department of the Interior 1987a). These sedimentary rocks <br />are major non-point salinity sources as they are rich in sodium chloride (halite) and <br />calcium sulphate (gypsum) (U.S. Deparment of the Interior 1987a). <br />Climate <br />Most lower elevation areas of the Upper Basin are and or semi-arid receiving <br />as little as 15 cm precipitation annually. Mean annual precipitation for the Upper <br />Basin is 40 cm (lorns et al. 1965). Runoff per unit area is the lowest of any major <br />drainage in the U.S. (U.S. Department of the Interior 1987a). High mountain areas <br />receive up to about 150 cm annually (lorns et al. 1965). The Upper Basin receives <br />95 million acre feet (MAF) of precipitation annually of which 80 MAF is lost by <br />n <br />U