Brief context: The southwestern united states is mostly desert with very little in the way of natural water sources. The Colorado river is major river fed by snow melt from the Colorado Rockies that used to flow down into Mexico. I say used to because cites like Los Angeles, San Diego, Las Vegas, Phoenix and others have been using it as their primary water souce for many years. The famous Hoover Dam is one of many, many dams on the Colorado river. By the 1960s the Mexican portion of the river had dried up completely.