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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. To make possible the implementation of that agreement, the Congress passed and, on June 24. 1974, the President signed legislation authorizing construction 01 the Colorado River Basin Salinity Control Project. 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 Morelos Dam on the Colorado River near Yuma. Ariz., and began diverting water into the Mexicali Valley in 1950. 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. - ~- II 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 Gila 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 (p/m) 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 thai a permanent and definitive solution to the problem was needed. Morelo. Oam--Alamo Canal now. to lell. An interagency task lorce 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. 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 wasting it into the Gulf of California or by desalt- ing and returning it to the Colorado River. 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 will 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 Levee. Operations are scheduled to begin in 1981. A bypass drain carrying the plant's reject water will extend from the desalting plant to the southerly international boundary and then southward into Mexico, emptying into the Santa Clara Slough near the Gulf of California. Most of Wellton.Mohawk drainage flow will pass through the plant. The desalted water will be blended with the balance of the drainage flow and returned to the Colorado River for use in meeting the commitments 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.hours per year. Brine reject from the desalting plant will be transported 51 miles to the Santa Clara Slough in the concrete.lined bypass drain. The first 16-mile reach wilt be constructed by the Bureau of Reclamation, while the United States will provide financing for the construction of the remaining 35-mile segment in Mexico.