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Salinity Control on the Lower Colorado River The United States and Mexico are cooperating in a "hands-across-the-border" program to solve a water quality problem between the two countries which has commanded international attention. In 1944, the United States and Mexico signed a treaty which provides that Mexico is entitled to receive 1.5 million acre-feet of water from the Colorado River annually. On the basis of that treaty, Mexico built Morales Dam on the Colorado River near Yuma, Ariz., and began diverting water into the Mexicali Valley in 1950. But that treaty made no reference to the quality of water which would be delivered to Mexico, and in 1961 drainage water from the Wellton- Mohawk Division of the Giia Project began to flow into the Colorado upstream from Morelos Dam. Also in 1961, riverflows were reduced. As a result, the salinity of Colorado River water going into Mexico increased from about 850 parts per million (pfm) to nearly 1,500 p/m. The Mexican Government asked the United States to reduce the salinity of the water being delivered to Morelos Dam. After several years of negotiations and interim agreements, under which some salinity improvements were made, it became apparent that a permanent and definitive solution to the problem was needed. An interagency task force was named to devise a lasting solution to the overall salinity problem. In 1973, the two countries entered into another agreement (Minute No. 242 of the International Boundary and Water Commission) which stated that the salinity of Colorado River water arriving at Mexico's Morelos Dam would not exceed specified limits, based on the salinity of water arriving upstream at Imperial Dam. CL6~JO To make possible the implementation of that agreement, the Congress passed and, on June 24,1974, the President signed legislation authorizing construction of the Colorado River Basin Salinity Control Project. That legislation provides for three major features downstream from Imperial Dam: (1) a desalting complex unit which will reduce the volume and salinity of Wellton-Mohawk drainage water to be returned to the Colorado River; (2) replacement of the first 49 miles of the unlined Coachella Canal with a concrete-lined reach which will reduce water lost through seepage; and (3) a protective and regulatory ground-water pumping plan for the south Yuma Mesa and the southwestern Yuma Valley to recover ground water for use in the United States and for meeting treaty obligations. Morelus Dam-Alamo ellll" 110" 10 left Irrigated larmllllld In Welllon-Mohawtl DlstrlcL The Desalting Complex The agreed-upon salinity level in the Colorado River could have been attained either by bypassing drainage from the Wellton-Mohawk Irrigation and Drainage District around Morelos Dam and wastinc it into the Gulf of California or by desalt: ing and returning it to the Colorado Rivel Since water is a precious resource in the Southwest, the decision was made to reclaim the Wellton-Mohawk drainage. Consequently, the heart of the project wi be a 104-million-gallon-per-day membranE process desalting plant-the world's largest. It is scheduled for construction on a 60- acre site 4 miles west of Yuma and just south of the Colorado River's South Lev, Operations are scheduled to begin in 191 A bypass drain carrying the plant's rejecl water will extend from the desalting plan to the southerly international boundary a then southward into Mexico, emptying in the Santa Clara Slough near the Gulf of California. Most of Well ton-Mohawk drainage flow. pass through the plant. The desalted wat wili be blended with the balance of the drainage flow and returned to the Colora- River for use in meeting the commitmen1 of the 1944 Treaty and Minute No. 242. Using electricity supplied initially by the Navajo Generating Plant in the Four Corners region, the desalting plant is expected to have an annual energy requirement of 372 million kilowatt-hour~ per year. Brine reject from the desalting plant will transported 51 miles to the Santa Clara Slough in the concrete-lined bypass drall The first 16.mile reach will be constructe by the Bureau of Reclamation, while the United States will provide financing for tl construction of the remaining 35-mile segment in Mexico.