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charge of the artesian springs at the same time was estimated to be 47,000 acre-feet, making a total supply of about 189,000 acre-feet. There would seem to be no doubt that the "valley fill," which partly fills this vast tift to form the present flom of the San Luis Valley, extends to as yet unknown depths of many thousand feet. Nor is there any doubt that this fill consists of altemating layers of water- bearing sand and gravel in which water is confined under artesian pressure by caps of impervious mate- rial such as clay, adobe, and lava. From a study of the logs of deep wells rather re- cently put down in the San Luis Valley fm oil ex- ploration, some with depths near 8,000 feet, it is interesting to note that bedrock was not encountered but that the penetration was still in the valley fill, having passed thfOugh Some 70 different aquifers or definite water-hearing strata. In the low central basin and in a few places else- where in the valley, some of the upper of many al- ternating layers of pervious and impervious material rise and dip, thick~ and thin, and appear on and disappear beneath the surface. In some cases, es- pecially on low ground, unconfined or It sub" water is trapped in pervious soil above an impervious layer, usually adobe. In other cases, the adobe layer appears on the surface, and in such cases water under artesian pressure is usually found to be confined in the first pervious stratum immediately below the adobe cap which appears on the surface. Such shallow artesian basins are usually more or less local and the water frequently is under less pressure than that which is confined in deeper strata which constitute vastly larger basins in which water is confined under greater pressure. In many locations it has been found that the tem- perature of water increases. with depth until, at depths mnging fwm 1,800 to 2,000 feet, it may ap- proach the boiling point. This increase of temper- ature clearly indicates that water is confined in multiple artesian traps, one above another, and that there is little or no mixing. This is also indicated by small but significant differences in pressure. Fwm the fmegoing it can readily be understood that ground-water conditions in the San Luis Valley, as has been stated, are indeed different from those which prevail elsewhere. Here we have an almost closed basin in which unconfined water is held above local artesian basins, and in which larger artesian basins overlie many other large artesian basins to great but unknown depths. Mention has been made of a strip of sand, gravel, and boulders at the base of mountains which sur- wund the flom of the San Luis Valley. This matetial is so pervious that many sm~ll streams disappear into it entirely and larger streams loose great quan- tities of water into it. Fed by .many mountain streams resulting from the melting of great quanti- ties of snow in the vast mountainous snowshed 10 which surrounds the valley, the water in the margi- nal sttip of very pervious material at the base of the mountains is unconfined. From this common source, gfOund water moves laterally toward lower parts of the valley, part of it passing into multiple thickening beds of sand and gravel beneath impervious caps/to become artesian, and part of it (pwbably only after lower artesian beds are recharged) moving out into the pefmeable materials on top of upper beds of clay and adobe to become unconfined or H sub" water finding its way towaro the low central basin. Many locations in Colorado have underground water under artesian pressure, but by far the most out- standing such supply in the state, or perhaps in the nation, is that found in the San Luis Valley. C. E. Siebenthal of the U. S. Geological Survey in 1906 reported 3,234 flowing wells here at that time; L. G. Carpenter estimated the number to be as many as 2,000 as early as 1891, some then being more than 1,000 feet deep. As has been mentioned, in most artesian basins the intake supply is far removed and the recharge rate slow and limited, while in the San Luis Valley it is constant and liberal. The recharge of the shallow gwund-water basin is from streamflow, irrigation diversions, and pre- cipitation. For the artesian basin, however, the area of recharge is from the extensive marginal strip around the perimeter of the valley, overlying the upturned edges of the artesian aquifers. There is reason to believe that loss to the under- ground water baSin, particularly the artesian res- ervoir, from streams flowing into the valley over the pervious rim, increases somewhat proportion- ately with the increased streamflow; the year 1952 stwngly substantiates this observation. On the basis of the heaviest snow deposits on record on the watershed of the upper Rio Grande, on May I, 1952, forecasts by the various agencies ranged from 1,050,000 to 1,200,000 acre-feet fm the Rio Grande at the Del Nmte Station for the Apfil-September pedod, while actual total delivery fm the full calen- dar year of 1952 was only 826,400 acre-feet. Like many other areas in the west, the San Luis Valley came to life, lived, and had its being only through the development of its water resources, and so only may it exist today, with less than 7 inches mean annual rainfall on the valley floor. This development, as has been mentioned, now covers a little more than 100 years. During that time, mostly prior to 1890, some 1,800 miles of canals and ditches had been constructed with more than 15,000 cubic feet per second of water decreed. With an average annual delivery of about 980,000 acre- feet covering something over 600,000 acres of land, very little development has taken place since 1900. Each biennial report of the Colofado State Engi- neer thwugh the 1890' s mentions the shortage or ~- ,~ .~,' ~: 1'-. _1: ~;;. . . h :'.". ~;::" '..J .;)0. L'1 i~1 :}.l :fl ~,..1 ',' ~ '~, ..... .-, " r- ,. '''''' \J ',,1 ~ ... IJ. U