1016 



THE IRRIGATION AGE. 



Geology and Water Resources of the San 

 Luis Valley, Colorado* 



By C. E. Siebenthal 



Springs. 



The valley affords numerous springs, both large and 

 small. These, with a few exceptions, emerge near the junc- 

 tion of the foothills and the valley bottom. They have mostly 

 the normal temperatures of the shallow artesian waters of 

 the valley, but two springs in the north end of the valley 

 and one near the south end have rather warm temperatures. 



The largest group of springs in the valley is that formerly 

 known as Los Ojos, or commonly as Mclntire's springs, 

 on the south side of Conejos river. These springs rise in 

 the bottom just at the foot of one of the San Luis hills, 

 and some of the springs appear to come up through crevices 

 in the lava. The group is limited to an area not more 

 than 300 feet in diameter, and they all merge into one 

 stream. The flow from these springs, which is practically 

 constant and does not vary with rainfall, has been measured 

 many times and is found to be about 21 second-feet. The 

 temperature of the stream comprising the united flow of 

 the springs is '60. Some of the smaller springs have a 

 temperature of 54, but most of the individual springs are 

 either just over or just under 60. As these springs rise 

 against or even through comparatively recent lava rocks, 

 their temperature is not a reliable clue to the depth from 

 which they come. 



The flow of these springs was filed upon at an early date 

 by the inhabitants of the Mexican village of Los Sauces 

 for irrigation purposes, for which its temperature makes it 

 peculiarly valuable in the early season. 



Other Springs Along Conejos River. Other smaller 

 springs emerge near the base of the San Luis hills along 

 Conejos river from a point near its mouth up to the vicinity 

 of Manassa, as was noted previously, where the supposition 

 was advanced that the water of these springs rises from the 

 water-bearing beds of the Alamosa formation, where they 

 abut against the lava of the Santa Fe formation of the San 

 Luis hills. 



Spring CVe^fc.^Spring creek has its head in sec. 12, 

 t. 37 n., r. 7 e., half a mile west of the Gunbarrel road 

 and just under the rise of the steeper alluvial slope. Water 

 rises over an area 15 by 40 feet, flowing more than a cubic 

 foot a second. It is augmented by seepage until at the 

 point where it crosses the Gunbarrel road it has a volume 

 of several second-feet. The flow is affected by melting snows 

 in the mountains. It has a temperature of 57. 



Russell Springs. Russell springs are situated in the ne. 

 '4 sec. 24, t. 43 n., r. 7 e. These springs rise in a grassy 

 area 40 acres or so in extent, underlain by a peaty black 

 mud. In this area about twenty-five springs display tem- 

 peratures ranging from 44 to 56. The temperature where 

 the water crosses the Gunbarrel road is about 52. The 

 water has no taste. The water from these springs drains 

 eastward 2 miles into Russell lakes. 



Hunt Springs. Hunt springs are in the ne. % sec. 3, 

 t. 44 n., r. 8 e., at the foot of the small lava hill 4 miles east 

 of Saguache. 



By far the greater number of wells in the San Luis basin 

 are along its western slope. Various factors have contributed 

 to this segregation. The principal one, perhaps, has been 

 the presence of greater irrigation systems on that side of 

 the valley and consequent greater population ; but another im- 

 portant cause is the fact that the slight inclination of the 

 strata on that side of the valley has made the matter of 

 obtaining an artesian flow much simpler, involving less 

 chances of failure and less expense. 



Seasonal Variations. Near the margin of the area of 

 flowing wells there is a decided periodical variation in pres- 

 sure or head. Just on the limiting line there are a number 

 of wells that flow during a certain portion of the year and 

 have to be pumped during the remainder of it. The varia- 

 tion in head in these wells is not accurately determined but 

 is about 4 feet. The same variation affects wells within the 



Abstract from Water Supply paper No. 240 of the United States 

 Geological Survey. 



limits of the flowing-well area, but it there shows itself as 

 a slightly increased or decreased flow, and is not so manifest 

 as in wells along the critical line. These wells, with the 

 seasonal intermissions, flow during the summer and fall and 

 do not flow for the rest of the year. As this is the season 

 of irrigation the flow of the wells is popularly said to 

 "come up with the sub." (that is, with the rise of the water 

 table due to subirrigation) ; and this is probably true, though 

 of course there is no direct connection of the ditch water 

 with the aquifer. The water in the water-carrying stratum 

 is under constant hydrostatic pressure, tending to rise to 

 the surface and pressing upward always against the confining 

 clay bed above. Any increase of weight upon this clay bed 

 is transmitted downward to the aquifer, which, being thus 

 under greater pressure, yields greater flows than before. The 

 water which is put upon the ground in irrigation adds a 

 very definite increment to the pressure upon the aquifer and 

 it is therefore true that the flow rises with the ditch water. 

 Likewise, the rainfall during the showery season adds to the 

 general pressure and helps to increase the head. The sea- 

 sonal fluctuation due to irrigation is hence closely allied in 

 principle to the tidal fluctuations in artesian wells at the 

 seashore. 



Gradual Failure of Wells. Several factors contribute to 

 cause the gradual failure of wells. Among these one of the 

 most obvious is the growth of a green alga. This lines the 

 inside of a vertical pipe down for a foot or so, probably 

 as far as light is efficacious, and by its continued growth often 

 constricts the opening so that the water is forced to a height; 

 and the considerable pressure thus exerted on the well doubt- 

 less to an appreciable extent reduces its flow. 



Another possible cause of the gradual failure of wells 

 is a reduction of the porosity of the sand bed through which 

 the water comes to the bottom of the well. This has been 

 popularly expressed as a "silting up of the water bed." It 

 seems more reasonable to suppose that the free silica, in 

 which the analyses show the water to be especially high, is 

 by the reduction of pressure at the bottom of the well in part 

 precipitated about the grains of sand in the aquifer adjacent 

 to the bottom of the well, which tends to seal the interstices 

 and to reduce the porosity of the bed. For most of their 

 long journey through the beds of granitic and volcanic sand 

 the artesian waters are undoubtedly augmenting their silica 

 content, as shown by the fact that the silicic acid in water 

 from the Rio Grande at Del Norte is 24 parts per million, 

 whereas that of various wells in the San Luis valley ranges 

 from 38 to 106 parts per million. But this fact is not in any 

 way inconsistent with the theory that some precipitation of 

 silica may take place as the waters pass from the sands 

 and gravels of the aquifer to the opening at the bottom of 

 the well tube. Decisive proof of this deposition of silica 

 would be had if secondarily enlarged grains of sand should 

 be brought up in cleaning out some old well that had slowly 

 failed. The writer, though repeatedly trying, has as yet 

 failed to obtain such material with which to test the theory. 

 But presumably the greater number of cases of gradual 

 failure of wells are due to the increase in number beyond 

 the capacity of the aquifer to furnish the full flow for each. 

 The minimum distance from one another at which wells may 

 be put down without affecting the common flow is difficult 

 to determine and depends on the size of the bore, the 

 capacity of the aquifer, and the artesian pressure. It has 

 been shown that the wells in the town of Monte Vista 

 so seriously affect one another that they have ordinarily 

 a uniform flow. The distance there between the wells is 

 from 50 to 200 feet. A mile northwest of La Jara, on 

 William Lambert's place, of two wells 150 feet apart the 

 newer well seriously affected the flow of the old well. It 

 is noticed in all the towns of the valley that the flows now 

 obtained are not so strong as the flows formerly obtained 

 at the same depth, though the adjacent wells may not seem 

 tot be affected by the sinking of a new well. The normal 

 flow of wells in towns is often so concealed by the piping 

 or restricted by partial use only that it might be seriously 

 impaired without the fact becoming apparent. Such a failure, 

 of course, may be due in part to other causes, but it is un- 

 doubtedly due mainly to the increase of wells. It seems 

 certain that the large wells may be placed as close as 440 

 yards, and reasonably sure that they may be as close as 

 220 yards, without affecting one another. Smaller wells can, 

 of course, be placed still closer without mutual injury. If 

 it is desired to have two or more large wells close together, 



