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Section 3: Physical and Economic Settings <br />A. Physical Setting <br />The headwaters of the Colorado River Basin originate in the Rocky, Wasatch, and San Juan <br />mountains of Colorado, Wyoming, Utah, and New Mexico. The Basin drains approximately <br />242,000 square miles of land in the United States and about 2,000 square miles of land in <br />Mexico. The portion of the Basin that resides in the United States encompasses parts of the <br />States of Arizona, California, Colorado, Utah, Nevada, New Mexico, and Wyoming. To <br />facilitate management of water resources, the Basin was divided into Upper and Lower Basins <br />(Figure I-3-1) by agreement between the seven Basin States in the 1922 Colorado River <br />Compact. The Upper Basin begins at the headwaters of the Colorado Basin, and ends below <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, Arizona, and ends at the United States/Mexico border. Major drainages <br />in the Lower Basin include the Lower Colorado, Little Colorado, Virgin, and Gila rivers. The <br />Gila River drainage also includes the Salt and Verde river drainages. <br />The size and diversity of the Basin encouraged the development of a variety of fishes. <br />Changes in drainage structure due to geological factors also contributed to the creation of <br />new, isolated fish species that added to the diversity of the Basin. The riparian and wetland <br />areas along the streams and rivers provided a habitat for invertebrates, amphibians, reptiles, <br />birds and mammals. The riparian forests and wetland vegetation provided forage and resting <br />areas for migratory waterfowl and songbirds. <br />I-10 <br />