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:;\~ , , w Ul c.o 01 I, THE COLORADO RIVER SALINITY PROBLEM WITH MEXICO 1/ A. History The 1~44 United States-Mexico Treaty for Utilization of Waters of the Co10r~do and Tijuana Rivers and of the Rio Grande allots to Mexico a guaranteed annual quantity of 1,500,000 acrefeet "of the waters of the Co 1 orado Ri ver, from any and all sources..." and " whatever thei r origin... " MexicQ and the United States have had differing interpretations of these and related treaty provisions as they may affect water quality, These questions relating to quality did not become a practical issue until 1961, 'partly because until then Mexico had received excess Colorado River ,flows. These excess flows diluted more saline return flows with the result that the quality of the water delivered to Mexico was very nearly the same as that used in the lower Colorado River Basin of the United States. Howev~r, in 1961 two events occurred to make quality a serious issue. Pumping of highly saline drainage from the We11ton-Mohawk District began at abqut the same time that availability of excess waters came virtuaJ1y to an ,end. The effect of these two events was to increase the average annua~ salinity of waters made available to Mexico in the Colorado River aboveiits main diversion structure from an annual average of about 80b parts per mill ion to nearly 1,500 parts per mill ion. In Nov;ember 1961, Mexico formally protested that "the delivery of water that i's harmful for the purposes stated in the Trei)ty constitutes a vio1adion of the Treaty," and that "any contamination of internationaJ wateribyone of the riparian countries that causes damage or loss to the other (riparian party is in itself an act clearly and specifically con- demned by International Law,.." 2/ Concerned over the effects of high salinttyon the farms of the Mexicali Valley, Mexico has continued to press 'its case. In response to Mexican protests, a series of interim measures have been taken is i nee 1961 and the water deli vered to Mexi co has never agai n reache:d the peak salinity of that year, As a result of several studies, the two Governments, in 1965, reached a five-year agreement embodied in Minute No, 218 of the International Boundary and Water Commission. Pursuant to this Minute, the United States engaged in selective pumping of th~ We11ton-Mohawk drainage wells to alleviate salinity at the times 1/ E)(;tracts from the 1972 report of Special Representative Herbert Brownell. ~ Note, Embassy of Mexico, No, 4012, November 9, 1961.