<)78 



THE IRRIGATION AGE. 



drops part of its load and builds up a little alluvial fan. 

 But a wrong impression is conveyed by alluding to it as 

 "the" former course if that is meant to imply that the 

 Rio Grande has had but two courses, that one and the 

 present one. 



Endlich outlines a former course of the Rio Grande, 

 leaving the present course of that river 3 miles above 

 Alamosa, circling north of Washington Springs, crossing 

 the railroad near Baldy station, crossing Trinchera Creek 

 south of that station, crossing the Rio Culebra about 6 

 miles west of San Luis, and rejoining the present course 

 some 15 miles below the New Mexico line. A glance at 

 the topographic map will show the utter improbability 

 of this supposition. Baldy station is 60 feet higher than 

 the point on the river where the old course is supposed 

 to leave and the divide south of Trinchera Creek is 50 

 feet higher still. 



Alluvium. 



Alluvial bottoms occur along the Rio Grande and its 

 various tributaries as well as along the other streams 

 which enter the valley but do not reach that river. The 

 width of the alluvium along the Rio Grande varies from 

 1 to 2 miles, with here and there reaches of gravelly and 

 sandy loam. The habit of the smaller streams to divide 

 into distributaries reuniting again lower down, forming 

 "sloughs" or "bayous," has a tendency to make the alluvial 

 areas of those streams wider than they would other- 

 wise be. 



Underground Waters. 



The essentials of an artesian system are few and 

 simple: 



1. An inclined stratum, as of sand, which receives 

 water in the higher portions and transmits it freely to 

 the lower portions. 



2. Relatively impervious layers, as clay or shale or 

 finer sand, which confine the water within the water- 

 bearing bed or aquifer. 



3. Resistance to lateral escape of the water from 

 the lower parts of the aquifer greater than its resistance 

 to the ascent of water in the well. This may be due to 

 either of several factors, among the commonest of which 

 are 



(a) The bending up of the beds to form the opposite 

 side of a basin. 



(b) The thinning out of the water-bearing sand bed 

 or aquifer. 



(c) Loss of porosity in the aquifer. 



(d) Frictional resistance to lateral movement within 

 the aquifer itself. 



(e) Unconformable depositional contact with less 

 pervious beds, as along lower Conejos River. 



The San Luis Valley is an almost ideal example of 

 the artesian basin The structure of the valley has been 

 already discussed, and a cross section of the valley is 

 shown in figure 3. The water occurs in beds of fine blue 

 to gray sand varying from 1 to 20 feet or more in thick- 

 ness, separated from one another by beds of blue clay 

 ranging from 1 foot to several hundred feet in thickness. 

 In the alluvial slope near the base of the mountains at 

 the margin of the valley the clay beds thin out, the lowest 

 ones usually reaching the farthest and highest up the 

 slope, thus giving the greatest pressure to the flows 

 beneath them, where they are penetrated out in the valley. 

 If all lower openings and avenues of escape in the aquifers 

 were stopped, the water would rise to the level of the 

 receiving area, or, rather, to the lowest point in that are5". 

 Thus the water in the strata penetrated at Alamosa would 

 have sufficient head or be under sufficient pressure to rise 

 as high as the margin of the basin about Monte Vista, 

 which would be about 100 feet above the surface at Ala- 

 mosa. As a matter of fact the head of the Bucher well 

 at Alamosa, the deepest in that vicinity, is not over 56 

 feet, and the greater number of wells reaching a depth of 

 about 700 feet have a head of less than 40 feet. This loss 

 of head is partly due to the drain upon the water bed 

 by other wells but is probably due chiefly to loss by 

 lateral flow where the sands of the Alamosa formation 

 come into contact with the Santa Fe formation along the 

 northwest margin of the San Luis Hills. In support of 

 this suggestion may be cited the fact that wells near the 

 mouth of Conejos River, in the very lowest portion of 



the artesian basin, instead of having the very strongest 

 pressure in the valley, as they theoretically should, have 

 on the contrary but small pressures, though very good 

 flows. 



The source of supply of the artesin water in the San 

 Luis Valley is unquestionably the mountain streams which 

 flow down across the alluvial slope. The disappearance 

 of the mountain streams soon after they reach the alluvial 

 slopes is the matter of common observation. This is 

 particularly true of the streams flowing from the Sangre 

 de Cristo Range. Very few of these streams get beyond 

 the piiions which cover the upper part of the alluvial 

 slope. The same is true of the streams on the west side 

 of the valley, though the greater number of the streams 

 there, being much longer and larger than any in the 

 Sangre de Cristo Range, have sufficient volume of water 

 to stand the loss in passing across the alluvial slope and 

 to send a considerable flow to the Rio Grande, especially 

 in the wet season. But all these streams suffer noticable 

 loss in the region of the gravel slope. 



The largest stream entering from the west is, of 

 course, the Rio Grande, and along this stream various dis- 

 charge measurements have been made with a view to 

 showing the loss by seepage of this river in various por- 

 tions of its course through the valley. As shown by 

 these gagings, the actual loss by seepage of the Rio 

 Grande between Del Norte and Monte Vista, a distance 

 of 15 miles, is 75 second-feet. No other section of the 

 river shows a loss at all comparable to this; in fact, the 

 remainder of its course for the most .part shows a gain 

 from seepage. Discharge measurements of various other 

 of the principal streams on the west side of the valley 

 have been made from time to time by the state engineer 

 of Colorado, the results of which show like losses in the 

 region of the alluvial slope. 



It has been explained on a previous page that the 

 artesian supply has the first call upon the water of tne 

 Rio Grande, inasmuch as one of the large ditches, which 

 has the earliest priority of all ,takes its water from the 

 Rio Grande below the section which supplies water to 

 the artesian basin. Since this priority must be satisfied 

 before any water is taken out above, there will always 

 be water flowing across the area which supplies the arte- 

 sian, basin when there is any water in the river at all. 

 This is largely true of the other streams as well. There- 

 fore the artesian basin will always be supplied with water, 

 and any question of failure of wells in the valley will be 

 one of mutual interference as a result of too great con- 

 centration of wells, and not one of expansion of the irri- 

 gation systems. 



In 1891 Professor Carpenter, estimating 2,000 wells 

 in the valley, with an assumed average flow of 25 gal- 

 lons per minute, found the total artesian flow to be about 

 110 second-feet. At 70 acres to the second-foot, this 

 volume of water, if all used, would irrigate 7,700 acres. 



Since that year many new deep wells have been bored 

 and older ones sunk to deeper and heavier flows, so 

 that the average flow is now greater than at that time. 

 By actual count, the wells shown on the map and plats 

 herewith number 3,234. The average flow of 1,000 meas- 

 ured and estimated 2 and 3-inch wells, excluding those 

 in towns, amounts to 40 gallons per minute. The wells 

 of the towns have in general much smaller flows than 

 those in the country, but it is believed that the large 

 flows of the wells over 3 inches in diameter, which are not 

 included in the average, will offset the small flows o'f 

 the town wells, and that the average of 40 gallons is a 

 fair one. Flowing at this average, the 3,234 wells in the 

 valley will yield a volume of water equal to 286 seconfl- 

 feet. At the rate of 70 acres to the second-foot this vol- 

 ume of water will irrigate 20,000 acres. This estimate 

 of the duty of water is probably a minimum. On the 

 average it is perhaps 25 per cent too small, so that 25,000 

 acres is nearer a true estimate. With adequate storage, 

 this volume of water could be made to irrigate two or 

 three times as great an acreage. 



It is practically impossible to estimate the acreage 

 actually irrigated from artesian wells unless a painstaking 

 census be made with tht object in view. Even so, it wouM 

 be impossible to get accurate figures, inasmuch as many 

 persons use the flow of the wells in connection with the 



