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j J CONDITIONS ON IRRIGATED SECTIONS OF THE RIO GRANDE IN COLORADO by D. H. MATHIAS , . , I Special Deputy State Engineer Department of Water Resources Monte 'Vista, Colorado With respect to the usage of water, the only area affected by the Rio Grande is that pmtion of Cole- fado known as the San Luis Valley. Much of the terdtmy adjacent to the nmthem reaches of the Rio Grande in the eady 1700's was the stfOnghold of the Comanche Indians. The first recmded enuance of white man into the area was made by Juan Bautista de Anze who led the Spaniards against these Indians in the Green Horn Mountains, now the Wet Mountains. The expe- dition kept to "the east of the Rio Grande del Norte on its northern march, and on its return to Santa Fe crossed the river near present Del Norte, Colo. In January 1807 the famous explmer Zebulon Pike constructed a large fort on the lower Conejos River. History tells us that Pike had occupied his pretentious battlement less than a month when his garrison was forced to accompany a superior Span- ish force into Sante Fe, N. Mex. In 1842 a considerable part of Conejos County in the southem end of the San Luis Valley was deeded by the Mexican Government to four of its prominent citizens. As a result of the war with Mexico, the area became a part of the United States in 1848. This settlement in Conejos was the beginning of the town of San Luis, the oldest in Colorado; its establishment led to the construction of ditches for irrigation and the issuance of the first court decree pertaining to irrigation in Colorado (dated April 10, 1852), and marked the beginning of 100 years of organized and continuous irrigation in the San Luis Valley. The San Luis Valley in the south-central part of Colofado is a large plain appfOximately 100 miles from north to south and SO miles from east to west, almost completely surrounded by mountains. It is in the shape of a large hmseshoe with the open end to the south, this portion being in New Mexico. The Sangre de Cdsto Range forms the eastem and northern boundaries, reaching an altitude of over 14,000 feet. The westem range includes the Sa- guache, La Garita, San Juan, and Conejos Mountains, and forms the Continental Divide. The Continental Divide, a natural wall, extends from Mexico into Canada, forming a barrier to the moisture-laden clouds from the Pacific Ocean, and causing condensation with heavy precipitation in the mountains but very light precipitation in the en- closed valley and on the vast plains east of the Rocky Mountains. From this vast snowshed come numerous streams. The Rio Grande rises in the San Juan Mountains and flows in a southeasterly direction between the Conejos Mountains and La Garita Hills, entering the San Luis Valley on the west. It continues then in a southeasterly direction to Alamosa, where it turns in a southerly course for nearly 40 miles and passes thfOugh a deep cut in the San Luis Hills and enters New Mexico. Adjacent to or concentric with this mountain lies a continuous. fringe of foothills or border strip, extensive in area in places and lying above the valley flom. All streams entedng the valley must cross this bordering alluvial slope, the texture of which is conducive to high percolation, supplying water in large quantities to the vast ground. water reservoir underlying the entire valley. It is this pervious border strip, lying above and funneling into many upturned impervious strata, which makes the San Luis Valley unique, with all the essentials of an ideal artesian system rarely found in the United States. The valley floor is large in area, comprising some 4,500 square miles or about 3 million acres; the ad- jacent mountainous area which forms the snowshed is equally large. As stated above, it is ffOm this common source that the streams are fed and the vast underground lake is continuously recharged. Accmding to the National Resources Committee Repmt of 1938, the total mountain runoff area above the 8,000-foot contour is about 4,000 square miles. The entire flom of the San Luis Valley is under- lain by a body of unconfined water of shallow depth, locally known as "sub" water. Beneath the shallow ground waters, and separated from them by confining beds, lie large bodies of artesian water, occupying numerous strata in the valley fill. The artesian water has been developed extensively for domestic and irrigation purposes over a period of more than 70 years; more than 6,000 flowing wells are now in use. On the basis of the artesian invent my in 1936, if all the wells in the valley had been allowed to flow unrestrictedly their potential annual discharge would have been about 142,000 acre-feet. The total dis- O':):::~li3 9