Pursuant to the United States-Mexico Treaty on the Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande, signed in 1944, Mexico has a Colorado River allocation of 1.5 million acre-feet per year during normal years. The Treaty also established the International Boundary and Water Commission (IBWC), a binational agency that implements the Treaty. Through a binational process, the U.S. and Mexico Sections of the IBWC work with Colorado River stakeholders in both countries, including the United States Bureau of Reclamation and representatives of the seven Colorado River Basin States in the U.S. When the U.S. and Mexico Commissioners reach certain agreements regarding Treaty implementation, these agreements are executed as Minutes to the Treaty. The Director of the Arizona Department of Water Resources serves as the Governor’s representative regarding such negotiations.
The salinity of Colorado River water also has been a sometimes-contentious issue with regard to the water delivered to Mexico pursuant to the Treaty. Salinity impacts to water users in Mexico became an issue after the Wellton-Mohawk Irrigation and Drainage District began discharging saline groundwater and return flows to the Colorado River above Morelos Dam, Mexico’s primary diversion point.
In 1961, Mexico lodged a formal protest with the United States claiming damages to agriculture in Mexico. The United States began a process to address Mexico’s concerns, which culminated in the passage of the Colorado River Basin Salinity Control Act of June 24, 1974, Public Law 93-320 (the Salinity Control Act). The Salinity Control Act authorized a temporary measure to reduce the salinity of the Treaty water delivery by redirecting the brackish Wellton-Mohawk drainage water from the Colorado River to the Cienega de Santa Clara (Cienega) in Mexico. The Cienega now forms an important habitat for migratory birds and other animals. Each year approximately 109,000 acre-feet of water is delivered to the Cienega and is not counted as part of Mexico's Treaty allocation of 1.5 million acre-feet. The Salinity Control Act authorized the construction of the Yuma Desalting Plant, which was intended to capture and treat the drainage water flowing to the Cienega and return most of it (71,000 to 85,000 acre-feet) to the River for delivery as part of the annual Treaty obligation.
The Yuma Desalting Plant was built and operated briefly, at one-third capacity, in 1993. That test run, however, was cut short when flood flows on the Gila River damaged the intake canal. High flows in subsequent years reduced the salinity of the river sufficiently that the plant was not operated for several years. Ultimately, the plant was placed in "ready-reserve" status, when costs to repair the damaged intake canal and to operate the plant were considered too high to warrant its use. The plant was operated again at ten percent capacity for a 90-day test period in 2007. The impact of a prolonged drought in the Colorado River Basin has generated renewed interest in activating the Yuma Desalting Plant. Efforts are currently underway for a pilot run of the plant at one-third capacity to evaluate the potential for future operation.