National Geographic : 1993 Mar
Aquifer close-up Like a bucket full of wet gravel, an aquifer consists of a porous medium filled with water trapped by a bedrock bottom. Precipi tation and surface streams recharge the supply. Most Ogallala water has been held within the formation for a few millennia. The great underground sponge \VER EONS, rain eroded the Rocky Mountains, washing downstream millions of years' worth of grav el and sand. Those sediments soaked up rain and snowmelt, holding enough water to fill Lake Huron. While the Ogallala is the region's most abundant aquifer,; geologists have mapped others above and below it and term the total supply the High Plains aquifer. The names are used interchangeably by most people. Not until the 1950s was the Ogallala aquifer first massively tapped by advanced irrigation technology, including center pivot sprinkler systems, each designed to water about 130 acres. After 20 years of pump ing, a resource that had once seemed infinite began, in some places, to look like a well run ning dry. Better management practices are easing fears that the aquifer will be depleted within several generations. Still, in most regions pumping continues to remove more water than rain can replace each year.