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. .... -. - '- ."-" w CD oc ~ The Colorado River is extremely important to Northwestern Mexico. The Mexicali and San Luis Valleys in that part of Mexico occupy the dry delta lands of the Colorado River and are dependent upon the United States, both for the amount of water received and the usefulness of this water. Colorado River water, supplemented by ground water supplies, has irrigated between 425,000 to 500,000 acres in the Mexicali and San Luis Valleys. For many years the predominant crop was cotton. In 1959, 84 percent of the irrigated acreage was planted to cotton. However, in recent years, other crops have replaced cotton and now wheat is the principal crop, with cotton second, and garden crops, alfalfa, and safflower being the other major crops. The river also supplies the water supply for about 500,000 people in Mexico. Prior to the initial control of the river's flows by Hoover Dam in 1935, the supply ranged from damaging flood flows during the snowmelt season to extremely low flows in the late summer and fall. These natural events acted to limit the use which Mexico was able to make of Colorado River water to an estimated annual maximum historical use of about 750,000 acre-feet. When negotiations on a water treaty commenced with Mexico in 1928, the United States offered Mexico 750,000 acre-feet a year delivered to meet demands, which quantity, by being regulated, was more valuable than the unregulated flows being received by Mexico. The Mexican negotiators did not accept the principle of a limitation based on maximum historical usage, and countered