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___~_"l;:;G-__ .. ut.:!..'~ Salinity oj the Lower Colorado River/43 YEAR SALINITI UPSTREAM FROM MOREWS DAM IN PPM (U.S. COUN!j 1974 (after June 24) 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 972 964 955 943 928 739 740 924 933 742 676 639 600 The higher salinity levels in the years 1974-1978. 1981. and 1982 reflect the fact that in those years. the annual quan- tity of Colorado River waters reaching Morelos Dam was little more than the treaty-guaranteed quantity. The appreciably lower salinity in the years 1979 and 1980. and 1983-1986 re- flect the dilution effects of the large quantities of water flowing in the river in those years because of high runoff that filled the reservoirs and required larger releases. In 1984 alone, a total of over 13.5 million acre-feet reached Mexico over and above the annually guaranteed quantity. For the past thirteen years, the United States has com- plied with Minute No. 242 under an interim program untu the desalting plant begins operation. No Wellton-Mohawk drainage waters are delivered to Mexico as part of the Mexican treaty entitlement. Instead. all the drainage waters are by- passed directly to the Santa Clara slough. and other waters of the Colorado River - mostly from the storage reservoirs - are substituted for them. The quantities of Wellton-Mohawk drainage waters discharged directly to the Gulf of California ranged from about 215.000 acre-feet in 1975 to 110,000 acre- feet in 1986. Sueh drainage waters have not required replace- ment because the Colorado River Salinity Control Act relieves the obligation when there are surplus waters. as have existed for the past several years. However, in future years, before the desalting complex comes on line and when there are no surplus waters. the Act will require that Wellton-Mohawk drainage waters bypassed to the Santa Clara slough be replaced by the United States.18