The waters of the Colorado River are shared by 7 Western States and the Republic of Mexico. Legislation in the last century divided the Colorado's waters among the states, and the Mexican Water Treaty of 1944 signed between the two countries ensures the United States will annually deliver 1.5 million acre-feet of water to Mexico.

In the late 1950s, saline agricultural return flow water from the developing Wellton-Mohawk Irrigation and Drainage District (IDD) in southwest Arizona began entering the Colorado River via the Gila River. This water entered the Colorado River between Imperial Dam-the point of diversion for United States users-and Morelos Dam-the primary point of diversion for users in Mexico.

This saline drainage water, combined with reduced excess Colorado River flows, lowered the quality of water being delivered to Mexico. In 1961, Mexico claimed agricultural damages and lodged a formal protest with the United States. To address this issue and establish a permanent level of salinity for Colorado River water delivered to Mexico, Minute No. 242 of the Treaty of 1944 was agreed upon by the United States and Mexico in 1973. This Minute specifies that water delivered to Mexico be no greater than 115 parts per million, plus or minus 30 parts per million, over the average annual salinity of the Colorado River water at Imperial Dam.

To ensure this water quality level could be maintained, the Colorado River Basin Salinity Control Act of June 24, 1974 authorized the construction of the Yuma Desalting Plant near Yuma, Arizona. This plant would treat (desalinate) the return flows from the Wellton-Mohawk IDD so water being delivered to Mexico would be within the agreed-upon salinity levels. Completed in 1991, the YDP was operated at one-third capacity by the Bureau of Reclamation for a 9-month period in 1992 and 1993 to determine whether it would perform as designed. The test was cut short due to Gila River flood damage to the intake canal, and the YDP has not been operated since. Instead, the U.S. has used a bypass drain to meet salinity control obligations to Mexico.

This Bypass Drain (see map), also authorized by the Salinity Control Act, can be used to transport untreated return flows from the Wellton-Mohawk IDD or concentrated wastewater from the YDP desalting process to the Cienega de Santa Clara in Mexico. Bypassing Wellton-Mohawk return flows (i.e., not returning the water to the Colorado River, but discharging it to the Cienega) helps the United States meet Minute No. 242 salinity requirements without operating the YDP. However, in accordance with the Minute, water discharged to the Cienega is not counted as part of the annual treaty deliveries to Mexico.

Consequently, system storage from the Colorado River has been used in place of the bypass flow. Increasing water demands and prolonged and severe drought in the Colorado River Basin have heightened concerns about this additional demand on the river system. Thus, Reclamation is engaged in a public consultation & planning process to identify, analyze and evaluate potential methods to recover or replace the bypass flow.