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Regional studies describing local hydrology and salinity effects have been done for several subbasins including the Green River basin (Bolke and Waddell, 1975; DeLong, 1977; Lowham and others, 1982); Price River basin (Mundorff, 1972; Ponce, 1975; Riley and others, 1982a, 1982b); San Rafael River basin (Mundorff and Thompson, 1982); Duchesne River basin (Mundorff, 1977); Dirty Devil River basin (Mundorff, 1979; Rittmaster and Mueller, 1985); Yampa River basin (Steele and others, 1979; Steele and Hillier, 1981; Turk and Parker, 1982; Adams and others, 1983; Parker and Norris, 1983); White River basin (Weeks and others, 1974; Hackbart and Bauer, 1982; Boyle and others, 1984; Lindskov and Kimball, 1984); Gunnison and Dolores River basins (Kircher and others, 1984); and the western Colorado area (Shen and others, 1981; Larrone and Shen, 1982; Whittig and others, 1982; Evangelou and others, 1984). Acknowledgments We thank the U.S. Bureau of Reclamation, Colorado River Water Quality Office, for their cooperation in pro- viding data used in this report. Background and technical information were provided by D.P. Trueman, D.H. Mer- ritt, and J.B. Miller. We thank B.D. Nordlund and R.B. Bell for compiling much of the data. DESCRIPTION OF STUDY AREA The Upper Colorado River Basin encompasses 113,200 mil in the States of Arizona, Colorado, New Mexico, Utah, and Wyoming (pl. 1). Of this, about 3,960 mil is in the Great Divide basin of Wyoming, which does not contribute to the flow of the Colorado River. The Upper Colorado River Basin is about 550 mi long and 350 mi wide. Stream System The Colorado River begins high in the Rocky Moun- tains of Colorado and winds 640 mi to Lee Ferry, Ariz., before passing through the Grand Canyon, Lake Mead, and into Mexico, where its natural outlet is the Gulf of Califor- nia. The Colorado River, by name, originally extended up- ward only to the mouth of the Green River in Utah. Upstream from that confluence, it was known as the Grand River. The Green River was considered the upper continuation of the Colorado River because it was longer and drained a larger area (La Rue, 1916). In 1921, a joint resolution of Congress changed the name of the Grand River to the Colorado River (Follansbee, 1929). Although the mountainous headwater areas of the basin receive a large quantity of snow, most of the basin is semiarid or and plains that do not contribute substantially to stream- flow. Almost 85 percent of the streamflow originates in only 15 percent of the area (Stockton and Jacoby, 1976). Thus, the average runoff from the basin is very small, compared to the other major rivers that discharge into the ocean. The Colorado River at Lees Ferry, Ariz., has a mean annual streamflow about equal to that of the Delaware River, which drains an area about one-tenth the size of the Upper Colo- rado River Basin. The Columbia River basin (258,000 mil) in the Northwestern United States and Canada is about twice the size of the Upper Colorado River Basin, but the Columbia River has a mean annual streamflow more than 13 times greater than the Colorado River at Lees Ferry, Ariz. (data from Dunne and Leopold, 1978). Annual runoff from the Upper Colorado River Basin varies from 0.5 inches throughout much of the basin to more than 20 inches in the high mountains (fig. 1). Physiography The Upper Colorado River Basin encompasses parts of four major physiographic provinces (fig. 2): the Southern Rocky Mountains, the Wyoming Basin, the Middle Rocky Mountains, and the Colorado Plateau (Hunt, 1974). The Southern Rocky Mountains are a series of mountain ranges and intermontane basins trending north-south and compris- ing the highest part of the Continental Divide with eleva- tions ranging from 5,000 ft to more than 14,000 ft. The Wyoming Basin consists of elevated semiarid basins with isolated mountains and elevations ranging from 5,000 ft to 7,000 ft. The Middle Rocky Mountains are an assortment of mountains and semiarid intermontane basins with features similar to the neighboring provinces. The elevations in the Middle Rocky Mountains range from 5,000 ft to more than 12,000 ft. The Colorado Plateau province has the highest plateaus on the continent with elevations ranging from about 3,100 ft to 11,000 ft. The Colorado Plateau province is semiarid and deeply incised by numerous canyons. Geology The geology of the study area is diverse; it is char- acterized predominantly by igneous and metamorphic rocks in the high mountains and sedimentary rocks elsewhere. The core of the Southern Rocky Mountains consists mainly of Precambrian granite, schist, and gneiss, capped in places by Tertiary volcanics. Sedimentary rocks contain predominantly sandstone, siltstone, and shale, and local occurrences of evaporite. Several widespread formations were deposited in marine or brackish environments resulting in bedded and disseminated sodium chloride (halite) and calcium sulfate (gypsum), as well as clay that has high concentrations of ex- changeable sodium and magnesium (Whittig and others, Description of Study Area 3