Arizona has officially joined a legal challenge against the federal government’s 10-year plan for managing the critically low Colorado River, a move that could significantly reduce the state’s water allocation. The Arizona Department of Water Resources filed a motion to intervene in a lawsuit previously initiated by Nevada, asserting that the federal authorities’ strategy for the dwindling river system would "devastate Arizona’s economy." In a statement released by the agency, its director emphasized that the state’s communities, agricultural sector, industries, and businesses are fundamentally dependent on a sufficient supply of Colorado River water for their survival.

This escalation follows Nevada’s lawsuit, filed in late August, which targeted several federal agencies. Nevada argued that the federal plan disproportionately burdens the Lower Basin states. The Colorado River, a vital water source for an estimated 40 million people across seven U.S. states and parts of Mexico, is facing an unprecedented crisis. Decades of over-allocation, coupled with the escalating impacts of climate change which have reduced snowpack and subsequent runoff feeding crucial reservoirs like Lake Mead and Lake Powell, have led to a severe deficit. With the seven basin states unable to reach a consensus on how to manage these scarce resources, federal authorities stepped in to implement a plan aimed at balancing water availability with consumption.

Both Arizona and Nevada contend that they have already made substantial water conservation efforts and are being unfairly singled out in the federal plan. Tom Buschatzke, Director of the Arizona Department of Water Resources, stated that the Lower Basin states have collectively contributed millions of acre-feet of water to stabilize the river system, with Arizona alone having left over 5.5 million acre-feet of its water in Lake Mead since 2014. He asserted that no other state has made a greater sacrifice, and therefore, Arizona is joining Nevada in challenging a framework that places the entire burden of sacrifice on just three states.

The Colorado River system is divided into two main regions: the Upper Basin, comprising Colorado, New Mexico, Utah, and Wyoming, and the Lower Basin, which includes Arizona, Nevada, and California. The foundational agreement governing water distribution, the 1922 Colorado River Compact, allocates 7.5 million acre-feet of water annually to each basin, with an additional smaller portion designated for Mexico. The individual states within each basin then further subdivide their respective allocations. An acre-foot, a standard unit of water measurement, is equivalent to approximately 326,000 gallons, enough to cover an acre of land one foot deep. In Arizona, a single acre-foot is typically sufficient to supply three single-family homes for an entire year.

California receives the largest share of Colorado River water, with an annual allocation of 4.4 million acre-feet, representing roughly 27% of the total. Arizona’s allocation stands at 2.8 million acre-feet per year (about 17%), while Nevada receives a significantly smaller 300,000 acre-feet annually (approximately 2%). These allocations are based on historical agreements and legal frameworks that have governed the river’s use for over a century.

The U.S. Bureau of Reclamation’s recently released plan mandates that the Lower Basin states bear the brunt of potential water use reductions. For Arizona, this translates to a proposed cut of 760,000 acre-feet per year, which is about a quarter of its total allocation and the most significant reduction among the three Lower Basin states. California would be required to reduce its usage by 440,000 acre-feet annually, and Nevada by 50,000 acre-feet. The cumulative potential cuts for the Lower Basin could reach up to 3 million acre-feet. In contrast, the federal government is not mandating cuts for the Upper Basin states. Instead, it is requesting voluntary contributions of 200,000 acre-feet from each of the upstream states over a two-year period.

Arizona Governor Katie Hobbs issued a statement affirming the state’s willingness to contribute to the river’s protection but underscored that Arizona would not accept a federally imposed plan that places an undue burden on its residents, agricultural producers, and businesses. The state’s legal complaint, filed alongside its motion to join Nevada’s lawsuit, alleges procedural flaws in the federal government’s environmental review process that underpinned the decision. Arizona’s attorneys argue that every federal option considered implicitly assumed that the Upper Basin states would not significantly reduce their own water usage, regardless of how low the river levels became. Consequently, the alternatives effectively shifted the entire burden of any resulting water shortfall onto the Lower Basin states.

The complex system of Western water rights is historically governed by the doctrine of "first in time, first in right." This principle prioritizes water users who claimed or acquired rights to river water earlier over those who did so later. Under this doctrine, Arizona’s Colorado River rights are considered lower priority than California’s, as California’s rights were established earlier. This historical prioritization exacerbates Arizona’s vulnerability to significant cuts, particularly when overall water availability diminishes.

Director Buschatzke further elaborated on the potential ramifications, warning that such reductions could effectively render the Central Arizona Project canal system inoperable, impact numerous on-river communities, and more than double the water cuts already agreed upon in the Lower Basin Agreement. Nevada’s lawsuit, filed just three days prior to the U.S. Department of the Interior’s formal announcement of the Colorado River plan in August, similarly stated that the proposed cuts would inflict "devastating environmental, socioeconomic, and health and human safety impacts to southern Nevada and its citizens." The Nevada complaint also alleges that the federal plan contravenes several provisions within the "Law of the River," a comprehensive body of agreements, treaties, laws, and court decisions that collectively govern the Colorado River’s management.

Nevada Governor Joe Lombardo welcomed Arizona’s decision to join the legal action, describing it as a crucial step in challenging a federal plan that unfairly burdens the Lower Basin and fails to require meaningful reductions from the Upper Basin. Arizona’s rationale for joining Nevada’s lawsuit is to avoid the necessity of filing a separate, albeit similar, legal action and to effectively advocate for its own interests within the existing legal framework. Director Buschatzke concluded that Arizona believes this strategic move provides the most effective path forward for safeguarding the state’s water users.