From the newly released archive of British Movietone newsreels, U.S. Secretary of the Interior Harold Ickes in October 1938 hits the button to open the gates on Imperial Dam, diverting the first Colorado River water into the All-American Canal and on its way to irrigate the farms of the Imperial Valley:
The newsreel makes it sound as though the parched desert had never before seen water, but that is untrue. The dam and canal simplified delivery of water the Imperial Valley, but irrigation had been underway there for decades. In 1934, there were 245,117 acres of farmland in Imperial County, according to the Census of Agriculture. In 1939, that had risen to 274,808. Alfalfa was a dominant crop, with 114,164 acres yielding 2.99 tons per acre. (2012 production was 7.4 tons per acre, according to latest ag census data. Like much of agriculture, productivity has risen.)
Even then, this was vegetable and melon farming country. In 1939, 56,728 acres were reported in various vegetable and related crops, nearly half of that in carrots, cantaloupes, muskmelons, and the like.