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<br />3 <br /> <br />. <br /> <br />were the 1905 flood, when control of the river was lost and the <br /> <br />Salton Sea was created; and the drought of 1934, which, ironically, <br /> <br /> <br />.occurred just one year before storage was availabie at Hoover Dam. <br /> <br />From 1904 to 1940 the water supply to Imperial and Mexicali <br />Valleys was conveyed by the old Alamo Canal and shared by the <br />United States and Mexico. The qual{ty of the water during this <br /> <br />period was identical for the irrigated areas on both sides of <br /> <br />the border. <br /> <br />" <br /> <br />With closure of Hoover Dam in 1935, the lower river under- <br /> <br />went an abrupt change in regimen. For the next quarter century, <br /> <br />1931-61, it was a controlled river with no damaging flood waters <br /> <br />or excessively low flows. This change in regimen was of great <br /> <br />Imperial Dam and the All American Canal were placed in operation, <br /> <br />, ' <br />; <br />I <br />I ' <br />I <br />I <br /> <br />'benefit to Mexican as well as American water users. In 1940, <br /> <br />enabling diversions to the Imperial Valley, at a point about 20 <br /> <br />miles upstream from the point of diversions to the ~lexicali Valley. <br /> <br />Between the two points of diversion, drainage return flows enter <br /> <br />the river, so that after 1940, the quality of water was not <br /> <br />identical on both sides of the border. This was the physical <br /> <br />situation at the time of the negotiations for an the signing of <br /> <br />the 1944 Water Treaty. <br /> <br />By about 1961, upstream developments had reached the stage <br /> <br />at which much of the dependable water supply was committed and the <br /> <br />flow at the boundary appreciably reduced. <br /> <br />Geoloqical Features <br /> <br />Yuma Valley strdddles the Sonoran Desert and Salton trough <br />