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<br />n. -f'~, ~") ,,-: <br /> <br />Water-level measurements were made in more than 600 wells in <br /> <br />January 1976. Most of these wells were irrigation wells, but several <br /> <br />stock and government-owned wells were also measured, especially in areas <br /> <br />where there is little irrigation. Measurements for the four preceeding <br /> <br />winters are also included to serve as references illustrating declining <br /> <br />or rising water levels. <br /> <br />Water-level records for the northern High Plains of Colorado are a <br /> <br />part of the National Water-Data System collected and tabulated by the <br /> <br />U.S. Geological Survey, in cooperation with the Colorado Department of <br /> <br />Natural Resources, Division of Water Resources, Office of the State <br /> <br />Engineer, and the Colorado Water Conservation Board. <br /> <br />The northern High Plains of Colorado (fig. 1) is an area of about <br />9,500 mi2 (24,600 km2). The principal aquifer is the Ogallala Forma- <br />tion, an alter~ating sequence of erratically deposited sand, gravel, <br /> <br />clay, and silt. Depths to water range from less than 20 ft (6 m) to <br /> <br />more than 250 ft (76 m). TIle water-saturated part of the formation <br /> <br />ranges from 5 to 50 ft (1.5 to 15 m) thick in a broad band extending 10 <br />to 25 mi (16 to 40 km) eastward from the cap rock that forms the west <br /> <br />edge of the fonnation. TIle saturated thickness ranges from 50 to 350 ft <br /> <br />(15 to 107 m)in the irrigated areas along the Kansas and Nebraska State <br /> <br />lines. The water table slopes generally to the east. Precipitation is <br /> <br />the only source of recharge to the formation. <br /> <br />Water levels fluctuate slightly in areas with little pumping. In <br />areas where pumping for irrigation predominates, water levels have <br /> <br />steadily declined, the rate of decline depending on the volume of water <br /> <br />withdrawn. <br /> <br />2 <br /> <br />