Groundwater withdrawal reached its peak during the 1960s with more than being drawn from the aquifer each year – over twice the present flow of the San Joaquin River – which accounted for 69.6% of all the groundwater pumped within the Central Valley, and nearly 14 percent of all the groundwater withdrawn in the United States. In a 1970 study, it was found that more than of the San Joaquin River valley had been affected by land subsidence in excess of . The maximum drop in elevation was near Mendota, the northward bend of the San Joaquin River, at over . In some areas, the water table has declined more than vertically, forcing farmers to sink their wells as deep as to hit more abundant pockets of the underground water.