Arizona and the Colorado River

Linda StitzerAll Colorado River basin states must embrace innovative water conservation and reuse strategies to ensure a secure water future.

– Linda Stitzer, Arizona Senior Water Policy Advisor

The Colorado River Has More of Its Water Allocated Than Exists in the River

The Colorado River system supports almost 40 million people in the West and makes up 40% of Arizona’s water supply. When seven Western states got together in 1922 to determine how to share the Colorado River, they estimated the amount of water in the river based on data from a historically unusual wet period. This miscalculation has resulted in more water being allocated to different states than the river can actually provide annually. Combine this with growing populations wanting more water and climate change increasing the likelihood of drought, and we are facing a major problem.

Lake Mead is the primary reservoir on the Colorado River that stores and supplies water to Arizona, Nevada and California. When the water level in Lake Mead is predicted to reach the 1075 feet on January 1st of a new year, a “Tier 1” shortage will be declared by the Bureau of Reclamation. This shortage declaration will mean reductions in the amount of water that the lower basin states (Arizona, Nevada and California) and the Republic of Mexico can take out of the Colorado River.

Lake Mead’s water level is currently dropping to levels never before recorded. Declining water levels have also reduced the hydropower generation capacity at Hoover Dam on Lake Mead by 25%, requiring the installation of new turbines that can operate at lower lake levels.

The Colorado River Will Never Have Enough Water to Meet Current Allocations

A few good wet years will not make a difference in a water shortage being declared in the near future. The problem is happening now and is here to stay – the Colorado River will never have enough water to meet the current over-allocations to the Colorado River Basin states. The Lake Mead water level reaching 1075 feet is a warning signal that requires a change in water agreements and water management throughout the West.

The Bureau of Reclamation makes the official water shortage declaration for the coming year in the month of August. Their determination of a shortage is determined by their forecast of whether the lake level on January 1st of the coming year will be above or below the 1075 foot level.

Arizona is First in Line for Water Reductions from the Colorado River

A number of agreements collectively referred to as “The Law of the River” govern how the River’s water supply is allocated.  In addition, a 2007 “Shortage Sharing Agreement” determined how the lower basin states would share Colorado River water in the event of a shortage. Because some of Arizona’s Colorado River water users have a low priority in the allocation system, Arizona will face the largest water reduction.

Central Arizona Project Will See a 20% Reduction – Impacting Farms First

Water reductions will have the greatest impact on the Central Arizona Project, which holds the lowest priority water entitlement among the lower basin states. The Central Arizona Project brings Colorado River water to the cities of Phoenix and Tucson. Arizona will see a 20% reduction in its Central Arizona Project supply, equivalent to an 11% reduction in the state’s total Colorado River supply. By comparison, Nevada would experience a 4% reduction in its Colorado River water supply allotment and Mexico a 3% reduction. California would not experience any reduction because it has higher priority status in the water sharing agreement. As water levels in Lake Mead continue to fall, additional reductions will be triggered (Tiers 2 and 3), with Arizona shouldering a 17% reduction when the water level in Lake Mead hits 1025 feet (Tier 3).

The Central Arizona Project supplies water not only to Phoenix and Tucson, but also to central Arizona farmers, as well as industrial and tribal water users. The Tier 1 shortage will reduce water to the Central Arizona Project by 320,000 acre-feet (enough water to supply 1.2 million individuals for a year). These reductions will affect farmers first. A Tier 3 shortage, if triggered, would mean a reduction of 480,000 acre-feet and would also impact Central Arizona Project municipal and industrial water users.

Strides Have Been Made to Conserve Water but More Extensive Conservation and Reuse Needed

A consequence of reduced water to the Central Arizona Project is that farmers in central Arizona will likely return to pumping groundwater to continue watering their crops, leading to groundwater level declines. These declining water levels can lower people’s water wells, cause land to sink, and affect stream and river wildlife areas.

Water managers in Arizona have been planning for shortage for many years. For example, over 9 million acre-feet of Central Arizona Project water has been stored underground and a recent voluntary agreement between Arizona, Nevada and California is designed to keep 745,000 acre-feet of water in Lake Mead to delay a shortage declaration. Investments by cities and farmers in efficiency and reuse have been impressive. Still, the fact remains that the Colorado River is over-allocated, and in the long term more extensive water conservation and reuse measures are needed to support the state’s economy, environmental and recreational resources, and river restoration efforts.

There are many integrated solutions to addressing this water supply and demand imbalance. Concepts like cloud seeding, importing water from distant rivers, and desalination of Sea of Cortez water are impractical due to cost, time to implement, and/or other factors. Instead, there are mechanisms near at hand that are more affordable and immediately implementable: conserving water by residential, commercial and industrial users; expanding water reuse and recycling; increasing agricultural irrigation efficiency; planting less water-intensive crops; and supporting policies and water management changes that encourage compensated sharing of water between farmers to cities and the environment.  With the first shortage declaration likely to occur in the near future, it’s time to find innovative solutions.

Other Resources

The Bureau of Reclamation provides water and power to Western states and manages the operations of the Colorado River. Its Lower Colorado River Region operations include managing reservoir levels, releases from dams and deliveries to users as well as the development of water supply projections, operating plans and water management reports.

The Central Arizona Project is a 336 mile long system of aqueducts and pumping plants designed to deliver 1.5 million acre-feet of Colorado River water to central Arizona. It provides water to cities, industries and farms, recharges water to underground aquifers for later use, provides water information and education and conducts long-term water resource planning.

Healthy Rivers Program

Colorado River Basin


Related Publications




Celebrating Arizona Rivers -The Colorado River


Download Fact Sheet


Project Staff

Drew Beckwith

Water Policy Manager

Bart Miller

Bart Miller

Healthy Rivers Program Director

Linda Stitzer

Linda Stitzer

Arizona Senior Water Policy Advisor