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Section 3: Physical and Economic Settings <br />A. Physical Setting <br />The 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 land in the United States and about 2,000 square miles of land in Mexico. <br />The portion of the Basin that resides in the United States encompasses parts of the States of <br />Arizona, California, Colorado, Utah, Nevada, New Mexico, and Wyoming. To facilitate <br />management of water resources, the Basin was divided into Upper and Lower basins (Figure <br />I -3 -1) by agreement between the seven Basin States in the 1922 Colorado River Compact. <br />The Upper Basin begins at the headwaters of the Colorado Basin, and ends just below Lee's <br />Ferry, Arizona (16 miles below Glen Canyon Dam). Major drainages in the Upper Basin <br />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 <br />drainages in the Lower Basin include the Lower Colorado, Little Colorado, Virgin, and Gila <br />rivers. The 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 habitat for invertebrates, amphibians, reptiles, <br />birds, and mammals. The riparian forests and wetland vegetation provided forage and <br />resting areas for migratory waterfowl and songbirds. <br />I -10 <br />