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001983 . Pitt er al, August 2001. Replacing the Bypass Flow On The Colorado River Commission in 1972, and the subsequent passage of the Colorado River Basin Salinity Control Act of 19745 (CRBSCA) were intended to address Mexico's concerns over rising salinity levels in the Colorado. Mexico's complaints about salinity in the Colorado River started when extraordinarily high volumes of salt were introduced to the river by agriculture in theWel1ton~Mohawk hrigation and Drainage District. The Wellton..Mohawk Irrigation and Drainage District (WMIDD) is located in Yuma County, Arizona along the Gila River valley. A division of the Gila. Project, Wellton- Mohawk is located 12 miles east of the city of Yuma and extends 45 miles into the Gila River valley. GroU?dwater irrigation of the area began in 1906 and by 1934 many Wellton-:Mohawk wells were exhibiting excessive levels of salt and the water table had declined dramatically. Many farms were abandoned until 1952 when the Bureau of Reclamation' brought Colorado River water to the area with the Gila project.6 Unfortunately, poor drainage throughout WMIDD resulted in the mixing of irrigation wastewater with highly saline aquifer water. The brackish groundwater eventually rose to the surface of the agricultural fields, killing thousands of acres of crops and flooding basements. In order to mitigate these problems, the" BOR installed an elaborate tile drain system in the early 1960's, at a cost of$14 million.? . The saline drainwater (6300 ppm as compared to 700 ppm before WMIDD irrigated with Colorado River water) was pumped into the Gila River near its confluence with the Colorado River. Delivery of the drainwater to the Colo.rado River resulted in overall increased salinity in the River and extensive damage-to agricultural fields.downstream in the Mexicali Valley, In 1965, the US and Mexico adopted Minute 218 of the International Boundary and Water CommissIon (IBWC) to reroute the saline Wellton-Mohawk drain water away from the Colorado River mainstern, and into Mexico's Gulf of California via a newly constructed canal. 8 However, an extension to this new canal, known as the Main Outlet Drain Extension (MODE),9 was built with its tenirinus in a below sea-level depression in what formerly was the delta of the Colorado River. Now that Wellton-Mohawk drainwater~ known as the bypass flow, has flowed there for several d~cades it has revived some of the delta's former ecosystem, and presently sustains the Cienega de Santa Clara. At present the MODE delivers an annual average of 110,000 acre-feet of water to the Cienega de Santa Clara.lO . The Clenega de Santa Clara The introduction of Wellton-Mohawk's saline drainwater into the Cienega, de Santa Clara reclaimed some of the Colorado River's delta. . Before development upstream diminished the Colorado River's flows in its southernmost reaches, the Colorado River delta was a vast, s 43 U.S.C. ~ 1571 ~ . . 6 U.S. Bureau of Reclamation, Gila Project webpage: < httD://dataweb.usbr.l!ov/html/lrila.html>. Colorado River water was first delivered to Wellton-Mohawk fields in 1952 with completion of the Gila Project in 1957. 7 U.S. Bureau of Reclamation, (1977). Colorado River Basin Salinity Control Project, Title I Division, Desalting Complex Unit, Arizona; Status Report, p. 146. . 8 Full Text of Minute 218 can be found in U.S. Bureau of Reclamation, (1985). Colorado River Basin Salinity Control Act Handbook, A-11-A-13. 9 The MODE is in fact a sequence of canals known (from upstream down) as the Main Outlet Drain, th~ Main Outlet Drain Extension, andthe Bypass Extenstion. 10 International Boundary and Water Commission (1991-1998). Western Water Bulletin: Flow ofthe Colorado and other Western Bounda.Ir StreamS and Related Data. Department of State, USA. [published annually]. 2