Ground-water levels in the United States, 1967-71, north-central States

Publication of ground-water level data for the United States in watersupply 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.

Beginning in 1956, only water-level data from a basic network of observation wells has been published. This basic network contains observation wells so located that the most significant data are obtained from the fewest wells in the most important aquifers. In some States the basic network forms the skeletal index of a more comprehensive observation-well program. Publications that contain substantial additional water-level data are listed at the end of each State report in this volume.

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