Tecle, Aregai; School of Forestry, Northern Arizona University, Flagstaff, Arizona 86011 (Arizona-Nevada Academy of Science, 2015-04-18)
      Dams are structures constructed across rivers to control their flows. The main objectives for building dams are to capture and store the surface flow from rivers and runoff from adjacent and upstream watersheds in artificial lakes or reservoirs and eventually release the stored water as needed. The system may be designed for purposes such as flood control, hydroelectric power generation, and providing freshwater for drinking and irrigation. Reservoirs may also serve as sanctuaries for fish and wildlife and for providing recreational activities such as swimming, fishing, and boating (Colorado River Research Group 2014). However, there are also many drawbacks to building dams that need to be considered. Dams displace people from their homes, flood productive areas, destroy ecosystems and /or impair services, inundate precious historical and cultural artifacts and eliminate important wildlife sanctuaries. The subject of this paper is the Colorado River and the effects of its extensive damming projects on downstream ecosystems and the environment. The Colorado River is the major river in the arid and semi-arid southwestern United States and northwestern Mexico. It is a 1,470-mi (2,352-km) river with its main headwaters in the Rocky Mountain National Park in north-central Colorado. It is the international boundary for 17 mi (27 km) between Arizona and Mexico in the southwest (U.S. Bureau of Reclamation, Lower Colorado Region 2015). The Colorado River system, including the Colorado River, its tributaries, and the lands that these waters drain, is called the Colorado River Basin. It drains an area of 246,000 mi2 (637,000 km2) that includes parts of seven western U.S. states (Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming) and two Mexican states (Baja California and Sonora) (Fig. 1). Three-fourths of the Colorado River Basin is in federal lands comprised of national forests, national parks, and Indian reservations. The drainage Basin's total runoff is about 24,700 ft3 (700 m3) per second (Colorado River Commission of Nevada 2006, Colorado River Research Group 2014). The river is the primary source of water, which comes mostly from snowmelt in the Rocky Mountains, for a region that receives little annual precipitation. For more than a thousand years, the Colorado River has been a central feature in the history and development of the southwestern part of the United States. During this period, management efforts in the Colorado River Basin embody society's struggle to overcome conflicts between competing interests over a shared water resource. First, there have been Native Americans who irrigated their crops with water from the river (Glenn et al. 1996). One tribe, the Cocopah Indians who reside in the delta region fished and farmed there for about 2,000 years. Unfortunately, the present Colorado River is often drained dry by upstream demands before reaching this part of Baja, California (Glenn et al. 1992, Zielinski 2010). In spite of this situation, irrigation is still one of the main uses of the Colorado River, especially on its lower portion where it supports one of the most extensive irrigated agriculture in the United States. Other equally important uses are generating hydroelectric power, and supplying drinking water to distant urban areas and other communities. For example, water from the Colorado River is diverted eastward across the Rocky Mountains to Denver and other cities in Colorado. The Colorado River Aqueduct carries water to the metropolitan area of Los Angeles, California, and the Central Arizona Project brings water supply to the Phoenix and Tucson areas in Arizona. In addition, the cities of San Diego and Las Vegas and many smaller cities, towns and rural communities in Arizona, Nevada, and California are dependent on the Colorado River for their water supply. All together about 35 million people in the U.S. Southwest and 3 million others in Mexico depend on the Colorado River for their water supply.