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Central Arizona Project

Courtesy of the Central Arizona Project

The Central Arizona Project (CAP) brings water from the Colorado River to urban areas in central Arizona by way of a massive canal. The 336 mile canal carries 1.5 million acre-feet of water from Lake Havasu City to points east, terminating 14 miles south of Tucson. Water from the canal reaches municipal users (the cities of Mesa, Phoenix, and Scottsdale, for example), agricultural irrigation districts such as the Maricopa-Stanfield Irrigation District, and twelve American Indian communities. Water is also conveyed to groundwater recharge facilities for storage underground.

The mission of the Central Arizona Water Conservation District (CAWCD), administrators of the Central Arizona Project, is “to deliver the full allocation of Colorado River water to central Arizona in a reliable, cost effective and environmentally sound manner.”

This prodigious water delivery system was created in response to two issues at the very heart of Arizona’s water use. One is to address a 2.5 million acre-foot groundwater overdraft. The second is to allow Arizona to draw its full allocation from the Colorado River, a whopping 2.8 million acre-feet annually.

Central Arizona Project History

The Central Arizona Project Association was formed to lobby Congress and educate citizens in 1946, just two years after Arizona signed the Colorado River Compact. In 1944, Arizona’s hard won 4.4 million acre-foot allocation of the Colorado River addressed a large portion of its water needs. However, all that water wasn’t needed in the western districts. That is, Arizona was not using the full river allocation. Meanwhile, Arizona needed water for the middle of the state, where groundwater demands from urban growth and agriculture were dropping the water table at such an alarming rate that the ground was slowly subsiding.

For the next two decades, Arizona tussled with California over Colorado River allotments. In 1966, a decision by the U.S. Supreme Court decreed that despite California’s use of Arizona’s allotment, the doctrine of prior appropriation would not stand and Arizona would receive the allocated 4.4 million acre-feet of river water. Four years later, legislation to build the CAP canal passed through the U.S. Congress as part of the Colorado River Basin Project Act of 1968. The bill provided for the Bureau of Reclamation, part of the Department of the Interior, to fund and construct CAP and for another entity to repay the federal government for certain costs of construction once the system was complete.

The Central Arizona Water Conservation District was formed in 1971. This entity manages and operates CAP and repays the federal government for reimbursable construction costs. In 1973, canal construction began at Lake Havasu. Twenty years later the canal was complete, ending just south of Tucson.

Environmental Considerations

The canal features wildlife bridges that enable migrating animals to cross it with minimum impact. Before the canal was constructed, environmental teams from the University of Arizona and the Arizona Game and Fish Department were contracted by the Bureau of Reclamation to perform wildlife studies and determine migration patterns. These studies were used to place the bridges in strategic locations. Fences and escape ramps were created to assist wildlife near distribution canals, and some watering holes were constructed specifically to give animals access. CAP purchased a 2,157 acre parcel of land near Tucson to fully preserve relevant habitats.


The Central Arizona Groundwater Replenishment District

CAP Recharge Operations explained.Courtesy of the Central Arizona Project.

Artificial groundwater recharge comprises a major portion of Central Arizona Project (CAP) water delivery. Six recharge facilities store almost 400,000 acre-feet in underground reservoirs. This is same amount of water that industry in Arizona uses. Recharge is a long-established and effective water management tool that allows renewable surface water supplies to be stored underground now for recovery later during periods of reduced water supply.

From an environmental perspective, there are many benefits to this practice. Underground storage minimizes evaporation and creates a “reserve” of water that can be used during periods of prolonged drought. Water that seeps into the aquifer undergoes a natural cleansing process, eliminating the need for additional water treatment plants. In fact, the quality of recharged surface water is improved by filtration through underlying sediments in a process known as soil aquifer treatment. Most importantly, recharged water can begin to alleviate a portion of Arizona’s groundwater debt and can actually raise the levels of some area aquifers. Learn more about CAP's Recharge Program.

Consequences of Groundwater Overdraft

Land Subsidence

Severe groundwater overdraft can lead to open crevasses called “earth fissures.”

The state of Arizona is suffering from a 2.5 million acre-foot groundwater overdraft. This means that 2.5 million acre-feet of groundwater are being removed from the ground faster than nature can replace it. The loss of such a volume causes a reduction in the levels of the water table, or aquifer.

When groundwater levels drop, water becomes harder to access. Wells and pumps that initially had to penetrate 50 or 100 feet underground for water must now go deeper. Engineering and construction costs rise, as does the volume of materials and the complexity of the process needed to raise water to the surface.

Drastic drops in groundwater levels cause basin subsidence. Underground water provides structural support for the surface of the earth. If that water is removed rapidly from a vulnerable area, the ground may slowly subside. Land subsidence across Arizona has been occurring since the early 1900s. Areas in Maricopa and Pinal Counties have subsided more than eighteen feet over the last century. These drops in surface level, generally measured both in centimeters and feet, can cause serious structural damage to homes, agricultural lands and industry. The Arizona Department of Water Resources offers information on ground subsidence and an interactive map of active land subsidence areas.

Earth Fissures

In severe cases, a crack in the ground, a phenomenon known as an “earth fissure,” will appear. Earth fissures are associated with basin subsidence that accompanies extensive ground water mining. In Arizona, fissures were first noted near Eloy in 1929; fissures have been identified in Cochise, Maricopa, Pima, and Pinal Counties. Their physical appearance varies greatly, but they may be more than a mile in length, up to 15 feet wide, and hundreds of feet deep. 

During torrential rains they erode rapidly, presenting a substantial hazard to people and infrastructure. Moreover, fissures provide a ready conduit to deliver runoff and contaminated waters to basin aquifers. Rapid population growth in southern Arizona is increasingly juxtaposing population centers and fissures. The Arizona Geological Survey provides information on earth fissures to the State Land Department and the public.