Ground-water levels in the United States, 1972-74, north-central states

Publication of ground-water level data for the United States in water-supply papers was begun by the Geological Survey in 1935. From 1935 through 1939, a single water-supply paper for each year covering the entire nation was issued (Water-Supply Papers 777, 817, 840, 845, and 886). Since then water-supply papers have been issued covering 6 separate sections of the United States as shown in figure 1. See table on page 2 for the number of the water-supply paper containing records by year and section.