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selection criteria were then applied throughout the historic range of the razorback sucker. <br />For the bonytail, humpback chub, and Colorado squawfish, the biological determination was <br />based on the primary constituent elements and existing recovery plans for these species. The <br />second step was to determine the potential impacts of the proposed designation. This is <br />addressed in an analysis of economic costs and other relevant impacts. The final step is to <br />decide which areas, if any, should be excluded based upon economic or other relevant <br />impacts (social, cultural, etc.) and to determine costs and benefits associated with the final <br />designation. <br />BACKGROUND <br />THE COLORADO RIVER BASIN <br />Headwaters of the Basin originate in the Rocky, Wasatch, Uinta and San Juan mountains of <br />Colorado, Wyoming, Utah, and New Mexico. The Basin drains approximately 242,000 <br />square miles of the United States and about 2,000 square miles of Mexico. The portion in <br />the United States includes the states of Arizona, California, Colorado, Utah, Nevada, New <br />Mexico, and Wyoming. To facilitate management of the water resources, the Basin was <br />divided into Upper and Lower basins (Figure 1) by agreement between seven Basin States in <br />the 1922 Colorado River Compact (1922 Compact). The Upper Basin begins at the <br />headwaters and ends at Lee's Ferry, Arizona (16 miles below Glen Canyon Dam). Major <br />drainages in the Upper Basin include the Upper Colorado, Green, Gunnison, and San Juan <br />rivers. The Lower Basin begins at Lee's Ferry and ends at the United States/Mexico border. <br />Major drainages in the Lower Basin include the Lower Colorado, Little Colorado, Virgin, <br />and Gila rivers. The latter also includes the Salt and Verde river drainages. <br />The Colorado River Basin includes aquatic and terrestrial ecosystems. The fish fauna is <br />composed of endemic fishes (found only in the Basin), native nonendemic fishes, and <br />nonnative fishes that have been introduced by man. <br />The native fish fauna is characterized by only a few species and most of these are restricted <br />to different subbasins or individual river systems. Carlson and Muth (1989) identified 36 <br />native fishes (included in Table 1). Several of the species in Table 1 include one or more <br />subspecies that are more restricted in distribution. Of the 38 fishes listed in Table 1, eight <br />(21 percent) were found throughout the Basin, six (15 percent) in the Upper Basin, and 24 <br />(63 percent) in the Lower Basin. The large number of species in the Lower Basin is likely <br />reflective of climate, geologic change, and a wider variety of aquatic habitats. Because of <br />geologic isolation, the Colorado River contains far fewer native species than other large river <br />systems (e.g., Missouri River Basin has 150 native fish species; Hesse and Sheets 1993). <br />3