Eighty million years ago, the Rocky Mountains began to rise. As the land to the east emerged from the sea, rivers and streams began to cut into it. By around 20,000,000 years ago, sediment from the mountains, coupled with climate change, filled many of these river valleys. The infillings became the Ogallala Sandstone, a geological formation hundreds of feet thick that can store vast amounts of groundwater. That's 40 percent of all our wheat, flour and cotton.
Crop yields are three times what they are on adjacent, unirrigated farmlands.
Geologists believe most of the Ogallala water was produced at the end of the last Ice Age as glaciers in the Rockies melted back. The Ogallala water is mostly 15,000 years old, or older. It would take at least that long for the aquifer to be replenished at today's rates of rainfall if it were completely drained. We pull out the equivalent of the entire flow of the Colorado River from the aquifer every year. The level of the underground water in the aquifer is dropping at a rate of about 10 feet a year at present.