936 



THE IEEIGATION AGE. 



Geology and Water Resources of the San 

 Luis Valley, Colorado* 



By C. E. Siebenthal 



The San Luis Valley, having an area comparable to the 

 State of Connecticut, lies in the south-central part of Colo- 

 rado (fig. 1), with a narrowed southern end reaching into 

 New Mexico about 15 miles. The whole length of the valley 

 from north to south is about 150 miles and its greatest width 

 about 50 miles. The San Luis Hills, a series of basalt-capped 

 table-topped mountains extending from Antpnito in the direc- 

 tion of Fort Garland, separate the valley into two portions. 

 It is with the northern part, containing the artesian basin, 

 that we have to do. This partion of the valley is limited 

 on the east by the majestic Sangre do Cristo Range, on the 

 west by the Sawatch and Conejos ranges, and on the south 

 by the San Luis Hills. 



The salient features are the bold Sangre de Cristo Range 

 and the less abrupt Culebra Range on the east, the gentler 

 eastward-sloping Sawatch and Conejos ranges on the west, 

 the flat-topped San Luis Hills on the south, and the almost 

 flat surface of the valley itself, the trough of the valley lies 

 far east of the median axis close under the Sangre de Cristo, 

 in fact. From the trough the country rises to the foothills, 

 more steeply eastward, very gently westward, at first not 

 more than 3 to 6 feet to the mile, but gradually increasing 

 until near the foothills the rise is quite perceptible to the eye. 

 The extreme flatness of the valley is shown by the great dis- 

 tances for which the canals are constructed along straight 

 lines. The Prairie ditch and the laterals of the Farmer's 

 Union canal and the Del Norte canal run from 10 to 25 miles 

 on straight east and west lines, with branches north and 

 south at right angles. 



So nearly level is the valley floor that its essential char- 

 acter entirely escapes one traveling over it, and it is only 

 when brought out by a topographic map that it becomes clear. 

 In such a map, however, the alluvial-fan structure of the 

 valley floors is strikingly manifest. Each stream descending 

 from the steeper slopes of the mountains to the valley has 

 deposited its spreading fan of gravel and sand. Around the 

 west and south sides of Blanca Peak these alluvial fans are 

 especially prominent. They are so close together that they 

 coalesce along their lateral margins to form a steep, gravelly 

 alluvial slope skirting the foot of the mountain. The streams 

 coming down from the west range, having a much lower 

 gradient, can carry neither such heavy material nor so much 

 of it, and as a consequence have built up much flatter fans, 

 though the form of the fans is no doubt chiefly due to the fact 

 that, except for the surface veneer of gravel and sand, they 

 were deposited in water, as shown by the continuity of the 

 clay and sand beds of the Alamosa formation. However, 

 La Jara, Alamosa, and Cat creeks have built up pronounced 

 fan deltas. But the map shows at a glance that the Rio 

 Grande has built the most extensive fan of all and, for the 

 reason given above, the flattest. It is also clear that the 

 trough of the valley lies so far to the east because the en- 

 croaching fan has pushed it there, and that the sluggish char- 

 acter of San Luis Creek has resulted from the filling in of 

 its lower course by the same agency. Conejos River has built 

 a long fan partly confined between the Mogotes Mountains 

 and the San Luis Hills, and partly extending south of the 

 San Luis Hills into New Mexico, and must have discharged 

 or at least sent distributaries at times to the south of the San 

 Luis Hills. Saguache Creek has likewise a fan delta, but 

 one rather poorly developed. San Luis Creek, taking its 

 source in Poncha Pass not much above the level of the valley 

 itself, naturally could develop no fan. 



The streams in the valley proper have cut their valleys 

 to various depths, but all are shallow. The bank of the Rio 

 Grande at Monte Vista, where the oldest terrace comes to the 

 river, is 20 or 30 feet high. At Alamosa the bank is from 8 

 to 12 feet high. The other streams have banks varying from 

 a few inches to 4 or 5 feet high. 



At different places in the valley there are bluffs which 

 are not now adjacent to streams but which represent the 



'Abstract from water supply paper 240, of the Department of the 

 Interior United States Geological Survey. 



margins of abandoned courses of the streams in their wander- 

 ings over the alluvial fans and slopes after the emptying of 

 the lake which filled the valley during the deposition of the 

 Alamosa formation. One passes through Sanford, two are 

 near Henry station, and one northwest of Hooper. They 

 vary in height, but are mostly not far from 10 feet. 



Stretching from Washington Springs south to the mouth 

 of Trinchera Creek is a bluff bank which culminates opposite 

 the Hansen ranch and has for that reason been called the 

 Hansen Bluff. It here reaches a height above the Rio Grande 

 of 60 feet. The level country stretching away eastward from 

 its top represents the level of the valley bottom at the time 

 when the water cut down the divide in the San Luis Hills 

 and began to drain the lake that originally occupied the center 

 of the valley. The original course of the river was probably 

 somewhat west of its present course, but the alluvial fans 

 of the streams on the west side of the valley have continu- 

 ally pushed it eastward and caused it to undercut the bluff. 



The Rio Grande enters from the middle of the west side, 

 pursues a southeasterly course to the San Luis Hills, and 

 leaves through a defile in them. A number of tributary 

 streams, notably Conejos River and La Jara, Alamosa, and 

 Saguache creeks, flow down from the Conejos and Sawatch 

 ranges, whose more gentle slopes gives room for extensive 

 drainage areas. Arms of the valley extend for some distance 

 up the course of the Conejos, the Rio Grande, and the Sagu- 

 ache, while a long, narrow arm, extending northward to 

 Poncha Pass, the upper end being known as Homan's Park, 

 is drained by San Luis Creek. The surface configuration of 

 the valley is such that this creek should receive all the water 

 entering the valley on either side, north of the Rio Grande, 

 but as a matter of fact most of the drainage, especially that 

 of the eastern range, is lost by seepage before it reaches the 

 creek, or reaches it only in flood season. The creek itself 

 in its lower course develops a series of wet-weather ponds 

 and finally flows into the San Luis Lakes. The old overflow 

 drainage course to the Rio Grande still exists but has been 

 so blocked and concealed by incipient sand dunes as to be 

 very difficult to trace except in its general features. 



A gauging station has been maintained for many years 

 at Del Norte, where the Rio Grande enters the San Luis 

 Valley, and above the head-gate of the uppermost of the 

 valley, irrigation system. Another one was maintained for a 

 long period at Embudo, N. Mex., at the lower end of Embudo 

 Canyon, into which the river flows just before crossing the 

 Colorado-New Mexico line. From 1899 until 1904 a station 

 was also maintained at State Bridge, at the upper end of 

 Embudo Canyon. This station, situated but a few miles 

 below the cultivated area of the valley, was of great impor- 

 tance in determining the volume of water left in the river 

 after the valley irrigation, and the discontinuance of this and 

 the Embudo station is much to be regretted. 



At this point it may not be amiss to call attention to 

 some facts bearing on the interstate relations of irrigation 

 in the San Luis Valley. Agriculture and irrigation spread 

 up the Rio Grande Valley from Mexico and Texas, through 

 New Mexico to San Luis Valley in Colorado. With the 

 rapid expansion of irrigation in the San Luis Valley the water 

 of the Rio Grande was largely withdrawn in Colorado, to 

 the alleged great detriment of agriculture on the lower course 

 of the river. The question of prior rights to the use of the 

 water became a subject for interstate and international dis- 

 cussion. Major Powell in 1890, testifying before the Senate 

 Special Committee on Irrigation and Reclamation of Arid 

 Lands, said : 



Passing into New Mexico, then, the water that practically 

 heads in the high mountains of Colorado is largely, almost 

 wholly, cut off from the Rio Grande, so that no portion of 

 the water that heads in these mountains where there is great 

 precipitation will cross the line into New Mexico (in the dry 

 season). * * * In a dry season nothing can be raised 

 in the lower region and sometimes dry seasons come two 

 or three together. 



Nevertheless, Major Powell argued that it is advantageous 

 that the water in a stream be used for irrigation as near to 

 its source as possible, since there the duty of water is greatest 

 and the loss from evaporation and seepage is least. 



In the valley of the Rio Grande the greater portion of 

 the water during the season of irrigation is lost in the sands, 

 as in the valley of the Arkansas. If the water of the Rio 



