WSP07681 - Laserfiche WebLink

Home Browse Search

129 WATER RESOURCES OF THE SAN LUIS VALLEY, COLORADO* ~ by II .~ PHILIP A. EMERY 1 U.S. Geological Survey Pueblo, Colorado .. , ,':;;""., ", '" INTRODUCTION . ~~vestigation of the water resources of the San Luis Va]j~y, Colorado, was begun in 1966 by the U.S. Geological Survey in cooperation with the Colorado Water Conserva- tion Board and the Colorado Division of Water Resources. Preliminary results of the investigations are being pub- lIshed as U.S. Geological Survey Hydrologic Investigations Atlas HA-381 (Emery and others, 1971). This paper is adapted from the atlas, but includes data collected since the atlas was compiled. The San Luis Valley of Colorado (fig. I) extends about 100 miles from Poncha Pass to the Colorado-New Mexico pgte line-an area of about 3,200 square miles. The valley 'floor; which has an average altitude of about 7,700 feet, is . nearly flat except for the San Luis Hills and a few other small areas. Bounding the valley on the west are the San Juan Mountains, and on the east, the Sangre de Cristo Mountains. Most of the valley floor is bordered by alluvial fans, the most extensive being the Rio Grande fan (fig. 1). The Rio Grande fan extends about 30 miles along the west sIde of the valley and about 20 miles eastward into the valley, and has an average gradient of about 12 feet per mile. Most of the streamflow is derived from snowmelt from about 4~700 square miles of watershed in the surrounding mo~ntallls. The northern part of the valley is internally drallled and IS referred to as a closed basin. The lowest part of this area is known locally as the "sump." The southern part of the valley is drained by the Rio Grande and its tributaries. The climate of the San Luis Valley is arid and is charac- terized by cold winters, moderate summers, and much sun- shine. The average annual precipitation on the valley floor ranges from 7 to 10 inches and as much as 50 inches occur in the neighboring highlands. More than one-half of the precipitation occurs from July to September. Owing to the short growing season (90-120 days), crops are restricted mainly to barley, oats, potatoes, and other vegetables. A succes~ful agricultural economy would be impossible with- out lfngatlOn. HYDROGEOLOGY The San Luis Valley is a large north-trending structural depression that is downfaulted on the eastern border and hinged on the western side. The valley contains as much as 30,000 feet (Gaca and Karig, 1966, p. 1) of alluvium, volcamc debris, and interbedded volcanic flows and tuffs of Oligocene to Holocene age. Although Siebenthal (1910, p. 39) subdivided the deposits into the Santa Fe and >I< Publication authorized by the Director, U.S. Geological Survey. Alamosa Formations, later information indicates that it is impossible to differentiate the formations except locally. In thIS paper, all deposits above the Precambrian crystal- line rocks are referred to as valley fill (table 1). The Sangre de Cristo Mountains are composed of ig- neous, metamorphic, and sedimentary rocks, whereas the San Juan Mountains are composed mainly of volcanic flows, tuffs, and breccias (Larsen and Cross, 1956). Many of the lava flows and tuffs from the San Juans dip generally eastward under the valley floor, and in the southwestern part of the valley they restrict the vertical movement of ground water. Geophysical and drillers' logs indicate that a "clay series," 10 to 80 feet thick, occurs throughout much of the central and northern parts of the valley at depths rangi~g from 20 to 120 feet below land surface. The "clay senes restncts the vertical movement of ground water. The total annual water supply to the San Luis Valley averages about 2,500,000 acre-feet. About 1,500,000 acre- feet is streamflow derived chiefly from snowmelt in the sur;ounding mountains and 1,000,000 acre-feet is from pre- CIpItatIOn on the valley floor. Discharge of water from the valley ~verages about 2,000,000 acre-feet per year by evapo- transpIrabon and about 500,000 acre-feet per year as stream flow and ground-water underflow across the state line. The annual streamflow at the state line averages 445.- 000 acre-feet and ground-water underflow accounts for the remainder, currently estimated at 55;000 acre-feet. About ~me-half of the evapotranspiration is non beneficial; that is, It does not contribute to the growth of plants having eco- nomIC value. Much of the non beneficial consumption is by phreatophytes, mostly greasewood (Sarcobatus), rabbit- brush (Chrysothamnus), and saltgrass (Distich/is), in areas where the depth to water is less than 12 feet. Ground water in the San Luis Valley is obtained from unconfined and confined aquifers. These aquifers contain at least 2 billion acre-feet of water in storage in the upper 6,000 feet. They are separated by a confining "clay series" or by confining layers of volcanic rocks. These confining beds are d,scontllluous and lenticular so it is difficult to differentiate between unconfined and confined aquifers ex- cept locally. This discontinuity in the "clay series" creates varying degrees of hydraulic connection between the aquifers. Shallow unconfined ground water occurs almost every- where in the vallev and extends 50 to 200 feet beneath the land surface. Th;depth to water in about one-half of the valley is less than 12 feet (fig. 1). Recharge to the unconfined aquifer is mainly by infiltra- tion of applied irrigation water and leakage from canals and ditches. Water infiltrating from the many streams en- I ,~. ,"'" ;.."'~ " I"" ;,.. ..,v ~ U