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<br />l1Ul~J' <br /> <br />. <br /> <br />. <br /> <br />Water-level measurements were made in 122 wells in January 1977. Most <br /> <br />were irrigation wells, but several stock and government-owned wells also <br /> <br />were measured, especially in areas where there are few irrigation wells. <br /> <br />Measurements made during the 4 preceding years are included, where avail- <br /> <br />able, to illustrate recent trends in declining or rising water levels. <br /> <br />Water-level records for the San Luis Valley of Colorado are a part of <br /> <br />the National Water-Data System and are collected and tabulated by the U.S. <br /> <br />Geological Survey in cooperation with the Colorado Department of Natural <br /> <br />Resources, Division of Water Resources, Office of the State Engineer; the <br /> <br />Colorado Water Conservation Board; and the Rio Grande Water Conservation <br /> <br />District. <br />The San Luis Valley extends about 100 mi (160 km) southward from <br />Poncha Pass (near the northeast corner of Saguache County) to about 16 mi <br />(25 km) south of the Colorado-New Mexico State line. The total area <br />(fig. 1) is 3,125 mi2 (8,090 km2), of which about 95 percent is in Colo- <br />rado. The valley is bounded on the west by the San Juan Mountains and on <br />the east by the Sangre de Cristo Mountains. <br /> <br />The aquifers in the San Luis Valley contain at least 2 billion acre-ft <br /> <br />(2.5xI012 m3) of water. Ground water occurs in two major systems referred <br /> <br />to as the unconfined and confined aquifers. These basin-fill aquifers are <br /> <br />separated by confining beds, which are either a "clay series" or a layer of <br /> <br />volcanic rocks. The confining beds are discontinuous and lenticular, so it <br /> <br />is difficult to delineate the boundary between the unconfined ~nd confined <br />. <br />aquifers. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />2 <br />