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<br />'" <br />~ <br />IV <br />~ <br /> <br />2 <br /> <br />OVERVIEW OF THE STUD'\: AREA <br /> <br />As defined by the U.S. Water Resources Council (WRC), the Upper <br />Colorado River Region includes the Upper Colorado River Basinl and <br />the Great Divide Basin, which is a closed basin in Wyoming,2 Total <br />drainage area of the Region, which covers about 45 percent of the <br />Colorado River Basin, is 113,496 square miles (72,600,000 acres). The <br />Region encompasses portions of the States of Wyoming, Colorado, Utah, <br />New Mexico, and Arizona. <br /> <br />The Region may be disaggregated into either political or physical <br />subdivisions. With respect to political subdivisions, this report will <br />refer to the multicounty "aggregated subareas" (ASAs) used in the WRC <br />197~ National Water Assessment (see fig. 2.1). The Region has also been <br />divided into four "water accounting units" (WAUs) which are coterminous <br />with major river subbasins (see fig. 2.2, which also shows those points <br />used to represent the outflow points of each WAD). Finally, for the <br />purposes of this 13(a} assessment, WAUs have been further divided into <br />"study subunits," denoted SSUs <.see fig, 2.3). <br /> <br />Characteristics of the Natural Environment <br /> <br />The topography of the Region is characterized by mountains, plateaus, <br />and valley basins, with the mountains being located around the Region's <br />perimeter on the west, north, and east. Uplifting of mountain'ranges and <br />stream erosion have been the basic geomorphic agents, although glaciers <br />and wind have contributed some to land formation. <br /> <br />1. The term Upper Colorado River Basin should not be confused with the <br />definition of "Upper Basin" in the Colorado River Compact. Where the <br />compact definition is intended, the term will be given in quotation <br />marks (see chapter 6). <br /> <br />2. Throughout this report the terms Upper Colorado River Region (Region) <br />and Upper Colorado River Basin (>>asin or Upper Basin) are used inter- <br />changeably. While not identical areas, the difference in data that arises <br />by specifically including or excluding the Great Divide Basin are insig- <br />nif ican t. <br /> <br />2-1 <br />