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Volume H <br />Chapter 2: The Study Region: Geography, <br />Biodiversity Values and Overview of the Basin Economy <br />A. Geography of the Colorado River Basin <br />Headwaters of the Basin originate in the Rocky, Wasatch, 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 by agreement between the seven Basin States in the <br />1922 Colorado River Compact. The Upper Basin begins at the headwaters, and ends at <br />Lee's Ferry, Arizona (16 miles below Glen Canyon Dam). Major drainages in the Upper <br />Basin include the Upper Colorado, Green, Gunnison, and San Juan rivers. The Lower Basin <br />begins at Lee's Ferry, and ends at the United States/Mexico border. Major drainages in the <br />Lower Basin include the Lower Colorado, Little Colorado, Virgin, and Gila rivers. The <br />latter also includes the Salt and Verde river drainages. <br />The sheer size of the Basin in conjunction with the variety of habitats, meant that a variety of <br />different fish species could develop and the overall fish fauna would differ depending on the <br />location in the Basin. Changes in drainage structure due to geological factors also <br />contributed to the creation of new, isolated fish species that added to the diversity of the <br />Basin. The riparian and wetland areas along the streams and rivers provided a habitat for <br />invertebrates, amphibians, reptiles, birds and mammals. The riparian forests and wetland <br />vegetation provided forage and resting areas for migratory waterfowl and songbirds. <br />Until recent times, the Basin supported a fish fauna unique in North America. Of the 36 fish <br />species native to the Basin, 64 percent were not found in other river basins. Only eight fish <br />species are common to both the Upper and Lower Basin. Of these, four are listed as <br />II-2-6