The existence of a major metropolitan center like Phoenix within the arid Sonoran Desert is a testament to extensive engineering and complex resource management. Phoenix sustains its population and economy by utilizing a sophisticated, multi-layered water portfolio. This system relies on surface water from two different river basins and managed underground reserves. This combination creates a resilient water supply engineered to mitigate the effects of drought and population growth.
Primary Sources of Phoenix’s Water Supply
Phoenix draws its raw water from three distinct sources, with surface water dominating the overall supply. The most consistent source comes from the Salt and Verde Rivers, which originate in the mountainous regions north and east of the city. This local river system is the largest component of the city’s water portfolio, often providing over half of the total supply. The second major source is the Colorado River, which typically contributes between 30 to 40 percent of the annual volume.
The remaining small percentage of the supply is sourced from a network of groundwater wells. This underground resource serves primarily as a reserve or supplementary supply during periods of reduced surface water availability. The use of this groundwater is tightly controlled by the Arizona Groundwater Management Act (GMA) of 1980. This legislation established the Phoenix Active Management Area (AMA) to achieve “safe-yield,” balancing the amount of groundwater withdrawn with the amount recharged.
The water derived from the Salt and Verde rivers is highly prized because its supply is managed entirely within the state’s borders. Snowpack and rainfall in the central Arizona mountains feed these rivers, which flow into a series of reservoirs. This local system provides a dependable supply that is less susceptible to the interstate political challenges facing the Colorado River. The Colorado River water is subject to the complex legal framework of the Law of the River, which governs allocations among seven western states.
The Major Water Delivery Infrastructure
The movement of raw water to the Phoenix metropolitan area is accomplished through two massive, separate infrastructure systems. The Salt River Project (SRP) manages the local Salt and Verde river flows through a historic network of dams and canals. The SRP system includes seven major reservoirs, such as Theodore Roosevelt Dam and Bartlett Dam, which collectively have a storage capacity of over 3.3 million acre-feet. These reservoirs capture and store seasonal runoff, ensuring a steady, year-round supply for the Valley.
Water released from the SRP reservoirs flows down to the Granite Reef Diversion Dam, located near the confluence of the Salt and Verde rivers. This structure diverts the river flow into the vast distribution network of canals. The SRP canal system includes historic waterways like the 38-mile Arizona Canal and the 22-mile Grand Canal, which deliver water to municipal treatment plants. This system is the foundation of Phoenix’s water security.
The second major conveyance system is the Central Arizona Project (CAP), built to deliver Colorado River water. The CAP begins at Lake Havasu, where water is pumped into a 336-mile-long open-air aqueduct. This immense canal system transports the water across the desert to central and southern Arizona. The project required the construction of 15 pumping plants along its route.
These powerful pumps are necessary to lift the water nearly 3,000 feet in elevation to overcome the natural topography. The CAP aqueduct delivers Colorado River water to the Phoenix area, where it is either treated directly or stored for future use. The CAP water is essential for recharging groundwater reserves and reducing reliance on local sources.
Water Treatment and Supply Management
Before the raw water reaches consumers, it must pass through one of the city’s five major water treatment plants. The treatment process is tailored to the source water, with separate facilities handling the Salt/Verde river water and the Colorado River water.
The initial steps involve screening to remove large debris. This is followed by chemical addition, where chemicals are introduced to make smaller particles clump together (coagulation and flocculation). These heavier clumps then settle out in sedimentation basins.
The water moves on to the filtration stage, typically using layers of granular media like sand and activated carbon to remove remaining suspended matter. The final stage is disinfection, where a regulated amount of chlorine is added to eliminate microorganisms and maintain water safety as it travels through the distribution pipes.
Beyond daily treatment, Phoenix engages in long-term supply management strategies, most notably through “water banking.” This involves storing excess Central Arizona Project water underground in aquifers during years of good supply. This stored water is converted into Long-Term Storage Credits (LTSCs) through the Arizona Water Banking Authority (AWBA). These credits act as insurance, ensuring the city has a recoverable supply during future droughts or when Colorado River allocations are reduced.
The city also maximizes the utility of water through the extensive use of reclaimed water, or effluent. Phoenix recycles approximately 97 percent of its wastewater for non-potable purposes. This highly treated water is used for agricultural irrigation, watering golf courses, and industrial cooling. Furthermore, the city is developing Advanced Water Purification (AWP) technology, known as the Pure Water Phoenix program, to create a future local, renewable source of drinking water.