Phoenix expands water storage as Scottsdale considers tighter review for big users

Phoenix and Scottsdale are taking different but related steps to prepare for a shrinking Colorado River supply. Phoenix has approved a roughly $88 million construction contract to add a new 20-million-gallon finished-water reservoir at the Union Hills Water Treatment Plant, while Scottsdale is considering whether developments that would use more than 100,000 gallons of water a day should face additional city review.

The moves affect how two of Arizona’s largest cities plan for future growth and water reliability. Phoenix officials say the new reservoir will add storage and flexibility to the system, while Scottsdale’s proposal would add another layer of scrutiny for large water users even if a project otherwise fits existing zoning rules.

Phoenix Assistant Water Services Director Nazario Prieto said the Union Hills project is part of the city’s effort to strengthen its water system for the long term. City officials have said about 40% of Phoenix’s water has historically come from the Colorado River, and Union Hills is one of two city facilities that treat that water. The city also says it operates five water treatment plants.

Cities plan for less Colorado River water

The timing reflects wider uncertainty across the basin. The new Colorado River water year begins Oct. 1, and Arizona is preparing for a 760,000 acre-foot reduction in its Colorado River supply in 2027. Phoenix officials have described the storage project as one more step toward keeping the system resilient, including during maintenance work at existing facilities.

Scottsdale, meanwhile, is weighing a policy that would require additional city approvals for proposed projects expected to use more than 100,000 gallons per day. The proposal has not been adopted, and the city has not announced a date for when it could take effect. If approved, the rule could give city officials more leverage to review water-intensive development as they look for ways to manage demand.

The planning efforts build on years of concern over the Colorado River, which remains a key source for many Arizona communities. Phoenix and Scottsdale have both been working to diversify supplies, increase storage and prepare for tighter allocations as long-term negotiations over the river continue.