In 1908, 24,000 acres were under irrigation in the so-called Magic Valley; by 1930, that figure had climbed to over 400,000. In 1910 Sunset magazine wrote that “primitive wilds…are being rapidly transformed into a domain of farms and gardens.” A developer’s pamphlet advised in 1918 that to bring water to one’s land, all you had to do was “telephone-for-irrigation.” So many did that after a few decades of cabbage and citrus farming, the once-navigable river had dropped to shin-height. Farmers began to fear that their crops would fail in the event of a drought. At the same time, Mexican farmers were expanding the amount of land under irrigation on the Tamaulipas side of the river. In 1938, recognizing that there was not enough water to go around, Colorado, New Mexico, and Texas—the US’s three Rio Grande Basin states—entered into an agreement regarding their shared river.
As the increasing pressure on the river became clear, the International Boundary and Water Commission (IBWC), the joint US–Mexico agency charged with administering the rivers that span the two countries, took note. They sponsored a shared surveying expedition, and in 1944 the countries signed a treaty apportioning the Colorado River, Tijuana River, and the Rio Grande. They also agreed to collaborate on the construction of two dams. The lower of the two would be completed within eight years: the allocation of water outlined in the treaty depended on it.
Falcon Dam was the first major dam to be jointly constructed by two nations on an international border. At the time the border wasn’t marked by anything besides the river. With the 1944 treaty and the creation of the reservoir, the IBWC also oversaw the construction of concrete monuments and massive steel-laminate buoys to mark the line. The governments shared the dam’s eighty-million-dollar cost. Because the treaty only allowed each country to build within its own borders, the project was also shared by two construction companies, Falcon Constructors and Constructora Intercontinental. Laborers were only authorized to work on their side of the river, with exceptions for carpenters who built identical forms together.
The reservoir they built was long, thin, and shallow: sixty by eleven miles when full, storing four million acre-feet. The construction crews also built two identical power plants, one on each side of the border, and six fifty-foot discharge gates to release the river’s flow. But to make this possible, around five thousand residents of the historic towns within the reservoir’s projected boundary—including Zapata, Falcón, Lopeño, Santo Niño, Santa Rosa, Sabinito, and Soledad on the Texas side—learned that their homes would soon be underwater.