Colorado (rivers, United States and Mexico)
Colorado (1 kŏlərăd´ə, –răd´ō, –rä´dō 2 kŏlərā´də, –rä´də). 1 Great river of the SW United States, 1,450 mi (2,334 km) long, rising in the Rocky Mts. of N Colo., and flowing generally SW through Colo., Utah, Ariz., between Nev. and Ariz., and Ariz. and Calif., then into Mexico, flowing toward the Gulf of California; drains c.244,000 sq mi (631,960 sq km). The Gunnison, Green, San Juan, and Little Colorado are the main tributaries in the upper basin of the Colorado; the Gila is the chief tributary of the lower basin. Silt deposited by the Colorado has formed a great delta across the northern part of the Gulf of California, cutting off the head of the gulf; the Salton Sea is a flooded remnant of the severed part. The intensive use of the river's waters now usually leaves the riverbed largely dry in the delta north of its outlet, but a 2012 agreement between the United States and Mexico called for both nations to work to restore the river's delta.
The mouth of the river was seen by Francisco de Ulloa in 1539; the lower part was explored by Hernando de Alarcón in 1540. The river flows through c.1,000 mi (1,610 km) of canyons, including Arizona's Grand Canyon; many national parks, monuments, and recreational areas lie along its banks. The Colorado's waters are used for power and irrigation, especially by means of the Colorado River storage project, the Colorado–Big Thompson project, Hoover Dam, Davis Dam, Imperial Dam, the All-American Canal, Parker Dam, Glen Canyon Dam, and, in Mexico, the Morelos Dam.
Controversies over water rights on the Colorado have long raged between the United States and Mexico and among the bordering states (it supplies most of S California's water); treaties and compacts regulate the river's use. California and, to a lesser degree, Nevada have in the past drawn more water than they were designated to receive. A new compact in 2003 gave California 14 years to reduce its water usage to its legal limits. A greater problem, however, is that the 1922 Colorado River Compact that established the division of water use between the upper and lower basins was based on an estimate of the average annual flow that is 10% to 25% higher than long-term data suggest, due to the use of river gauge data from what is now known to be a relatively wet period in the river basin's history. A 2007 accord established guidelines for reducing allocations in the lower basin when shortfalls occur.
2 River, 894 mi (1,439 km) long, rising in the Llano Estacado, NW Tex., and flowing SE to Matagorda Bay, an inlet of the Gulf of Mexico; drains c.41,500 sq mi (107,485 sq km). Destructive floods, which prevented private development of the river for power, led the Texas legislature to set up the Lower, Central, and Upper Colorado River authorities to undertake projects for flood control, power plants, and irrigation. The Lower Colorado River Authority, with federal assistance, has been especially active, building five major dams (Buchanan, Roy Inks, Alvin J. Wirtz, Marble Falls, and Mansfield). These projects have benefited a large part of Texas, including the city of Austin. The scenic section of the river above Austin, which includes the lakes formed by the dams, is called Highland Lakes Country. The Central Colorado River Authority has constructed many small irrigation dams and also has jurisdiction over several city reservoirs. The Upper Colorado River Authority regulates the upper Colorado and the several branches of the Concho, a principal tributary.