About ten million years ago fluvial deposition from the Rocky Mountains established what is now known as the Ogallala Aquifer. Historically found from fifty to three hundred feet below surface, this loosely confined collection of fossil water extends from northwest Texas to South Dakota. Changes in continental geomorphology severed the originating hydrologic linkages. Large scale irrigation beginning in the 1940s caused water levels to drop by more than one hundred feet. The disappearing resource has minimal recharge from surface rainwater and snowmelt.