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" ,- J <;) ,L-J...v It will be noted that operation under Minute No. 218 required a makeup from storage release in 1971 of approximately 31,- 300 acre-feet. During the last six years of op- eration, this makeup water has cost the United States a total of 232,400 acre-feet, or an average of 38,700 acre-feet of water per year. Mexico also requested the bypass of addi- tional Wellton-Mohawk drainage waters when diversions at Morelos Dam were great- er than 900 cfs. This voluntary bypass of 48,- 100 acre-feet was requested by Mexico to fur- ther improve the quality of water diverted at Morelos Dam, Without this voluntary bypass, the United States operations under Minute No. 218 would have resulted in an ' annual average salinity during the sixth year of 1,245 ppm, a reduction of 33 ppm over the fifth year average. Because of Mexico's voluntary bypass of additional drainage wa- ter, the annual average salinity of actual di- versions by Mexico during the sixth year equalled 1,161 ppm. About 66 percent of the water reaching Mexico during 1971 came from River storage above Imperial Dam and the remaining 34 percent from return flows to the River below Imperial Dam. The largest source of return flows is from the drainage waters of the Well- ton-Mohawk Project, the average salinity of which was about 3,680 ppm in 1971. This represented a reduction of about 110 ppm from the 1970 figure. Plate 9 shows the salin- ity and flow of the Wellton-Mohawk Outfall Drain and the Colorado River at Imperial Dam and at the Northerly International Boundary with Mexico, The International Boundary and Water Commission report summarizes river opera- tion under Minute No, 218 for the last six years as follows: 1. 14There has been a reduction in the average annual salinity of waters made available to Mexico at the northerly boundary under United States opera- tions of Minute No, 218, from 1,375 ppm in 1965 to 1,245 ppm in 1971, an average decrease of about 22 ppm per year," 2, "There has been a reduction in the average annual salinity of the total annual quantity of Well ton- Mohawk drainage water discharged in the convey- ance channel, from 4,680 ppm in 1965 to 3,680 ppm in 1971." 3, "There has been a reduction in the average annual salinity of the total annual quantity of South Gila drainage waters discharged to the river channel, from 2,510 ppm in 1965 to 2,320 ppm in 1971." Mexico has also been working on facilities to reduce the impact of Colorado River salin- ity on its lands. The rehabilitation of the Mexicali Valley irrigation and drainage sys- tem, initiated in June 1969, continues to progress. At the close of 1971, about 107 miles of canals were lined and about 167 miles of drains were constructed or rehabilitated, 63 wells were installed and 119 wells were reconstructed. The United States State Department rep- resentatives were present at the previously described meeting with Department of the Interior officials on July 30, 1971, to indicate the international aspects of the Colorado Riv- er Salinity Control Program. Because of the State Department's interest in providing a . solution to the quality of Mexico's water deliveries, that department expressed strong support for the Colorado River Salinity Con- trol Program. State of Colorado Water Quality Standards During 1971, the Board staff reviewed the newly adopted water quality standards for the State of Colorado, which became effec- tive on September I, 1971. Changes in the standards from those adopted by Colorado on May 15, 1968, are minimal. The new stand- ards include an "Anti-degradation" state- ment which reads as follows: Waters of the state, the quality of which exceeds the limits set in these standards, will be maintained at existing quality unless and until it can be demon- strated to the state that a change in quality is justi- fied to provide necessary economic or social development. In that case, the best practicahle de- gree of waste treatment to protect the current clas- sification of such waters will be required. The appropriate federal authority will be provided with information, from time to time, required to dis- charge his responsibilities under the Federal Water Pollution Control Act, as amended. 31