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.. OG2254 Salinity of the Lower Colorado River/ 47 lower reaches. farmers turned to pumplng from the underlying groundwater. This rapidly lowered groundwater levels and in- creased salinity. as the naturally saline groundwater was suc- cessively pumped. reused. and returned to the aquifer. Reports showed concentrations of salt in the aquifer varying from 1.500 to 25.000 ppm. and averaglng about 6.000 ppm after sev- eral years of pumplng. In 1950. Congress authorized the Bureau of Reclamation to assist farmers in the Wellton-Mohawk District by construeting a conveyance canal and irrigation works to sup- ply them with Colorado River water. No drainage works were provided with the irrigation works. however. and by 1958 (six years after deliveries began). groundwater levels had risen so much that the District lands were becoming water-logged. A deep well drainage system was then installed to pump waters from the underlying highly saline aquifer and to discharge these waters to the Colorado River. Pumping began in February. 1961. and lowered groundwater levels. but dis- charges of the waters pumped. having a salinity level averaging 6.000 ppm. caused the annual average salinity of treaty water deliveries to Mexico to rise sharply from 800 ppm in 1960 to nearly 1.500 ppm In 1962. The United States scientific panel In 1962 found that the problem caused by the WeIIton-Mohawk dralnage wells might be avoided if deep wells were replaced by gravity drains. including tile drains. The United States Geological Survey soli and groundwater scientists supported this view. Waters col- lected in gravity drains and then delivered to Mexico would be much lower in salinity since most of the dralnage would come directly from the applied irrigation water instead of the deeper, highly saline groundwater. The water would have been more like normal irrigation return flow. Deliveries of this water would have been consistent with the 1944 treaty. which states that "return flow means that portion of diverted water that eventually finds its way back to the source from which it was diverted."25 but Mexico did not consider deliveries of water pumped from wells to be return flows as deflned in the treaty. The Department of the Interior. however. was opposed to re- placing the wells with gravity drains. and Its engineers dis- agreed that gravity drains would reduce the salinity of the drainage waters. If the State Department had been consulted as to the ad- verse effects of the saline waters pumped for the treaty deliver- ies to Mexico before the decision was made for deep well drainage. gravity drains may have been installed initially in- stead of deep wells. The salinity problem might well have been avoided or at least largely mitigated. If the National Environ-