In 1922 the Colorado River Compact was concluded by the seven states that constitute its drainage area to facilitate federal investment in dams and reclamation. The river was divided at Lees Ferry, Ariz., into the lower compact states—Arizona, Nevada, and California—and the upper compact states—Wyoming, Utah, Colorado, and New Mexico; the total annual flow of the Colorado River was estimated to be 17 million acre-feet (the volume of area that would cover one acre to a depth of one foot) at Lees Ferry, of which 15 million acre-feet were equally, yet somewhat ambiguously, divided between the lower and the upper compact states. A treaty in 1944 allocated 1.5 million acre-feet of water per year to Mexico. It was later discovered that the initial estimate of Colorado River supplies was based upon an abnormally wet period and that substantially less water was available than the amounts specified in the agreements.

The first major development of the Colorado began in 1928, when Congress passed the Boulder Canyon Project Act. The act authorized the construction of Boulder (now Hoover) Dam, a multipurpose water-storage project that was a major engineering feat of its time; since its completion in 1936, the dam and Lake Mead, which it created with its impounded waters, have become major tourist attractions. The Colorado River system thus was the first drainage basin in which the concept of the multipurpose dam was employed—e.g., for hydroelectric-power development, irrigation, recreation, flood control, and navigation.

Many additional projects have since been undertaken. In the mid-1960s Glen Canyon Dam was completed, impounding Lake Powell. The dam was a controversial project: opposition to its construction helped shift policy from building large dams toward concepts of water management, environmental protection, and policy analysis. Three other large multiple-storage projects upstream have been completed on major tributaries. These are Flaming Gorge on the Green River in Wyoming and Utah, Aspinall (Curecanti) on the Gunnison River in Colorado, and the Navajo on the San Juan River in New Mexico and Colorado.

Shortly after the completion of Hoover Dam, planning and construction began downstream on the Parker Dam. From Lake Havasu, the reservoir impounded by the dam, water is transported some 250 miles across California to supply a portion of the water needs for Los Angeles and most of the water supply for San Diego. Davis, Imperial, Laguna, and Morelos dams further regulate flow and diversion in the lower basin.

The facilities described above do not serve all the demands for water from the Colorado. In 1945 the Colorado–Big Thompson Project, the first federal interbasin water-diversion project in the United States, was completed. Water was diverted by tunnel beneath the Continental Divide in Rocky Mountain National Park to help irrigate cropland in northern Colorado. Another large project, the Fryingpan-Arkansas, diverts water from Fryingpan River, a Colorado tributary, under the Sawatch Range of the Colorado Rockies and into the Arkansas River to supply water for the rapidly growing municipal areas of Pueblo and Colorado Springs. On the Blue River, another tributary, the city of Denver has built Dillon Reservoir, the water of which is piped beneath the Continental Divide to the large and growing Denver conurbation. Tunnel diversions also deliver water from the Colorado to the Rio Grande, Great Basin, and North Platte drainages.

In 1963 a decision of the U.S. Supreme Court made explicit the amount of water apportioned among the lower-basin states, as well as the amounts that had been implicitly “reserved” for Indian tribes and federal public lands. This decision paved the way for funding of the Central Arizona Project (completed in the 1980s), which transferred water to the cities of Phoenix and Tucson. The project consists of a mountain tunnel through which water from the southern end of Lake Havasu is pumped up and into an aqueduct that flows southward to the two cities.

Laced with innumerable dams, both large and small, that impound the total flow of the Colorado and by increasingly severe competition for whatever small quantities of water might remain, the basin remains fraught with litigation and controversy. Water projects must now undergo thorough environmental-impact studies in accordance with federal environmental protection legislation. Controversy between the United States and Mexico over the salinity of water delivered to Mexico was addressed in an international agreement in 1972, which led to desalinization experiments in the lower basin and irrigation management and projects for the disposal of saline water in the upper basin.

To see the Colorado River where it empties into the Gulf of California in Mexico is to know that the river is over-apportioned. At this point the river is only a trickle, and, at times, it is dry, its water having been used upstream—in many instances used more than once by ranchers, urban dwellers, and farmers. Since the late 1990s the U.S. Department of the Interior has been experimenting with flow modification at the large Colorado River dams. Large flows are released from the dams during spring, so as to mimic the spring floods that naturally occurred on the river. Ideally, these flows will redistribute sediment, modify the channel bed and sandbars, and flush unwanted detritus and contaminants from the river. More-aggressive attempts to improve the river’s flow are under consideration, including removal of the dams.

Tensions continue to arise among the basin states over water allocation, interstate water marketing, drought management, potential effects of global climatic change, Indian water rights, and reservoir management. These issues have been watched closely not only in the affected region but also in other areas of the world, for which the Colorado River serves as a laboratory of water-resource experimentation and innovation.