Colorado (river, United States/Mexico)

Colorado (river, United States/Mexico), river, south-west United States and north-western Mexico, 2,334 km (1,450 mi) long, the longest river west of the Rocky Mountains. The Colorado rises just west of the Continental Divide, at Grand Lake in northern Colorado, and, for the first 1,600 km (1,000 mi) of its course, passes through a series of deep gorges and canyons that were created by the eroding force of its current. The river flows in a generally south-western direction across Colorado into south-eastern Utah, where it joins its chief tributary, the Green River. After crossing the northern portion of Arizona, the Colorado flows west for 436 km (271 mi) through the majestic Grand Canyon. It then flows in a generally southerly direction and forms the boundary between Arizona and the states of Nevada and California. Near Yuma, Arizona, the river crosses the international border into Mexico, where it becomes the border between the states of Sonora and Baja California, and flows for about 145 km (90 mi) to its mouth on the Gulf of California, an inlet of the Pacific Ocean.

Besides the Green River, the most important tributaries of the Colorado include the Dolores and Gunnison rivers, in Colorado; the San Juan River, in Utah; and the Little Colorado, Salt, and Gila rivers, in Arizona. With its tributaries, the Colorado drains portions of seven states—a total area, in Colorado, Wyoming, Nevada, Utah, Arizona, New Mexico, and California, of about 626,800 sq km (242,000 sq mi); and 5,180 sq km (2,000 sq mi) more in Mexico.

Both to control the tremendous flow of the Colorado, particularly under flood conditions, and to provide drinking and irrigation water and hydroelectric power to the surrounding areas, an extensive series of dams, many of them constructed by the US Bureau of Reclamation, was built along the river and its tributaries during the 20th century. Notable is the Hoover Dam, which impounds the river at Black Canyon to form Lake Mead, one of the largest artificial lakes in the world. The Glen Canyon Dam, which forms Lake Powell in north-central Arizona just south of the Utah border, is the third highest dam in the United States. These two dams together account for about 80 per cent of the basin’s water storage capacity.

Dams have been instrumental in reclaiming the semi-arid and arid regions (see Arid Zone) through which the river flows, although they have drastically reduced the flow of water reaching the river's mouth. Water diverted from the river’s main course is often transported great distances. For example, some is diverted eastwards across the Rocky Mountains to users in Denver and other cities in Colorado, while the Colorado River Aqueduct carries water to metropolitan Los Angeles, and the Central Arizona Project supplies the Phoenix and Tucson areas.

The water concentrated at dam sites throughout the Colorado River system is a valuable source of hydroelectric power for the region. Hydroelectricity output totals about 12 billion kilowatt-hours per year, which is roughly equivalent to one-sixth of the electricity consumed in Arizona each year. This power is shared among several states. Dams have also controlled flooding, permitting development of land along formerly flood-prone sections of the lower river and enabling the development of large recreation areas. The Imperial Valley of southern California is an excellent example of land reclaimed by the waters of the Colorado. Once desert, the region now receives water via the All-American Canal. A number of reservoirs have been incorporated into recreation areas that have become centres of tourism, and the region is also a productive agricultural area.

Many people consider the Colorado River system to be the most important natural resource of the south-west United States. Nearly 17 million people depend on the Colorado’s waters. Without it, the region could support very little agriculture, and major cities such as Phoenix, Arizona, and Las Vegas, Nevada, could not have grown to their present size. The population of the basin has expanded quickly in recent years. The most rapid growth is in urban areas, where about 80 per cent of people live. Phoenix, Tucson, and Las Vegas are the largest cities in the basin, and they use the Colorado and its tributaries as their primary source of water. The Colorado River offers the only renewable water supply in the region.

The Native Americans who originally lived in the region developed a complex urban culture more than 1,000 years ago. These people, some of whom may have belonged to the Anasazi (see Pueblo (people)), constructed elaborate irrigation canal and reservoir systems. Their descendants, the Hopi and Pueblo people, continued to farm and irrigate using the river's waters.

The first European to visit the Colorado was probably Spanish soldier and explorer Francisco de Ulloa, who explored the mouth of the river in 1539. In 1540 and 1541 another Spaniard, Francisco Vásquez de Coronado, travelled through much of the region. The first major exploration of the Colorado was by the Spanish navigator Hernando de Alarcón, who ascended the river for more than 161 km (100 mi) in 1540-1541; he named the river “Colarado” (Spanish, “coloured red”) because of the water’s colour, caused by the red sediments it carries. The Colorado and its chief tributary, the Green, were thoroughly explored for the first time in 1869 by the American geologist John Wesley Powell. On this survey Powell and his party made the first recorded passage through the Grand Canyon.