The Colorado RiverThe Colorado River rises in the Rocky Mountains of Colorado and flows into Utah and Arizona.

Colorado River, a major North American river, draining much of the southwestern United States and a small part of Mexico. It originates in the Rocky Mountains of north-central Colorado and flows southwest for 1,450 miles (2,334 km) to the Gulf of California. The river flows through Colorado, Utah, and Arizona and serves as the boundary between Arizona and Nevada and California. It also separates the Mexican states of Sonora and Baja California.

Tributaries include the Green, Gunnison, Dolores, San Juan, and Little Colorado rivers. The entire drainage basin, including parts of Wyoming and New Mexico, consists of 244,000 square miles (632,000 km 2 )—about 7 per cent of the United States. In its course to the sea, the Colorado drops nearly 2 1/2 miles (4 km) and passes through sparsely populated arid and semiarid land, mainly plateaus and deserts. In much of its middle course it flows through deep canyons with sheer walls, the most notable of which is the Grand Canyon—217 miles (349 km) long and 4 to 18 miles (6,400 m to 29 km) wide, with an average depth of 1 mile (1,600 m).

Numerous dams in the Colorado basin provide hydroelectric power, flood control, and water for irrigation, recreation, and municipal use. Hoover Dam is the largest producer of hydroelectric power and the main flood-control unit. Among canals diverting water from the river are the Colorado River Aqueduct, which extends to the southern California coast; the All American Canal, which carries water to California's Imperial Valley; and the Gila Canal, in Arizona. The Central Arizona Project, a system of dams, canals, and aqueducts, brings large amounts of water to the Phoenix and Tucson areas. Tunnels through the Continental Divide channel water from the Upper Basin to cities and farmlands on the high plains.

The allocation of the Colorado's water has long been a major problem. By treaty, congressional acts, and agreements among the states, the Upper Basin and Lower Basin are each entitled to 7 1/2 million acre-feet (9.25 billion m3 ) of water annually; Mexico, to 1 1/2 million acre-feet (1.85 billion m3 ). These commitments, however, often exceed the river's annual flow, which has caused disputes among the states and with Mexico. The decreased flow has caused the lower course of the river to become salty, endangering fish and damaging irrigated farmland.

In 1539 the mouth of the Colorado River was reached by Francisco de Ulloa, a Spanish soldier and explorer. Spaniards explored most of the rest of the river over the course of the next 200 years.